Re: [PATCH v3 1/2] drm/virtio: Add window server support

[Tomeu Vizoso]

[Tomasz wants to comment, adding him to CC]

On 2/5/18 9:19 AM, Tomeu Vizoso wrote:

On 1 February 2018 at 17:36, Gerd Hoffmann  wrote:

Hi,

Sorry for joining the party late. Had a broken finger and was
offline for a bunch of weeks (and a buif backlog afterwards ...).


Hi, no problem, hope it's fine now.


This is to allow clients running within VMs to be able to
communicate with a compositor in the host. Clients will use the
communication protocol that the compositor supports, and virtio-gpu
will assist with making buffers available in both sides, and
copying content as needed.


Why not use virtio-vsock to run the wayland protocol? I don't like
the idea to duplicate something with very simliar functionality in 
virtio-gpu.


The reason for abandoning that approach was the type of objects that
could be shared via virtio-vsock would be extremely limited. Besides
that being potentially confusing to users, it would mean from the
implementation side that either virtio-vsock would gain a dependency on
the drm subsystem, or an appropriate abstraction for shareable buffers
would need to be added for little gain.

Another factor that was taken into account was that the complexity
required for implementing passing protocol data around was very small
when compared with the buffer sharing mechanism.

It is expected that a service in the guest will act as a proxy, 
interacting with virtio-gpu to support unmodified clients.


If you have a guest proxy anyway using virtio-sock for the protocol 
stream and virtio-gpu for buffer sharing (and some day 3d rendering

too) should work fine I think.


If I understand correctly your proposal, virtio-gpu would be used for
creating buffers that could be shared across domains, but something
equivalent to SCM_RIGHTS would still be needed in virtio-vsock?

If so, that's what was planned initially, with the concern being that we
would be adding a bunch of complexity to virtio-vsock that would be only
used in this specific use case. Then we would also need to figure out
how virtio-vsock would be able to work with buffers from virtio-gpu
(either direct dependency or a new abstraction).

If the mechanics of passing presentation data were very complex, I think
this approach would have more merit. But as you can see from the code,
it isn't that bad.


When the client notifies the compositor that it can read from that

buffer,

the proxy should copy the contents from the SHM region to the
virtio-gpu resource and call DRM_VIRTGPU_TRANSFER_TO_HOST.


What is the plan for the host side? I see basically two options. Either 
implement the host wayland proxy directly in qemu. Or

implement it as separate process, which then needs some help from
qemu to get access to the buffers. The later would allow qemu running
independant from the desktop session.


Regarding synchronizing buffers, this will stop becoming needed in
subsequent commits as all shared memory is allocated in the host and
mapped to the guest via KVM_SET_USER_MEMORY_REGION.

This is already the case for buffers passed from the compositor to the
clients (see patch 2/2), and I'm working on the equivalent for buffers
from the guest to the host (clients still have to create buffers with
DRM_VIRTGPU_RESOURCE_CREATE but they will be only backend by host memory
so no calls to DRM_VIRTGPU_TRANSFER_TO_HOST are needed).

But in the case that we still need a proxy for some reason on the host
side, I think it would be better to have it in the same process where
virtio-gpu is implemented. In crosvm's case it would be in a process
separate from the main VMM, as device processes are isolated from each
other with minijail (see
https://chromium.googlesource.com/chromiumos/platform/crosvm/ ).

Regards,

Tomeu


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Re: [PATCH v3 1/2] drm/virtio: Add window server support

[Gerd Hoffmann]

> > Yes.
> 
> Would it make sense for virtio-gpu to map buffers to the guest via PCI BARs?
> So we can use a single drm driver for both 2d and 3d.

Should be doable.

I'm wondering two things though:

(1) Will shmem actually help avoiding a copy?

virtio-gpu with virgl will (even if the guest doesn't use opengl) store
the resources in gpu memory.  So the VIRTIO_GPU_CMD_TRANSFER_TO_HOST_2D
copy goes from guest memory directly to gpu memory, and if we export
that as dma-buf and pass it to the wayland server it should be able to
render it without doing another copy.

How does the wl_shm_pool workflow look like inside the wayland server?
Can it ask the gpu to render directly from the pool?  Or is a copy to
gpu memory needed here?  If the latter we would effectively trade one
copy for another ...

(2) Could we handle the mapping without needing shmem?

Possibly we could extend the vgem driver.  So we pass in a iov (which
qemu gets from guest via VIRTIO_GPU_CMD_RESOURCE_ATTACH_BACKING), get
back a drm object.  Which effectively creates drm objects on the host
which match the drm object in the guest (both backed by the same set of
physical pages).

cheers,
  Gerd

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Re: [PATCH v3 1/2] drm/virtio: Add window server support

[Tomeu Vizoso]

On 02/12/2018 12:45 PM, Gerd Hoffmann wrote: 4. QEMU pops 
data+buffers from the virtqueue, looks up shmem FD for each

 resource, sends data + FDs to the compositor with SCM_RIGHTS
>>>
>>> BTW: Is there a 1:1 relationship between buffers and shmem blocks?  Or
>>> does the wayland protocol allow for offsets in buffer meta data, so you
>>> can place multiple buffers in a single shmem block?
>>
>> The latter:
>> 
https://wayland.freedesktop.org/docs/html/apa.html#protocol-spec-wl_shm_pool

>
> Ah, good, that makes it alot easier.
>
> So, yes, using ivshmem would be one option.  Tricky part here is the
> buffer management though.  It's just a raw piece of memory.  The guest
> proxy could mmap the pci bar and manage it.  But then it is again either
> unmodified guest + copying the data, or modified client (which requests
> buffers from guest proxy) for zero-copy.

What if at VIRTIO_GPU_CMD_RESOURCE_CREATE_2D time we created a ivshmem 
device to back that resource. The ivshmem device would in turn be backed 
by a hostmem device that wraps a shmem FD.


The guest client can then export that resource/BO and pass the FD to the 
guest proxy. The guest proxy would import it and put the resource_id in 
the equivalent message in our protocol extension.


QEMU would get that resource id from vsock, look up which hostmem device 
is associated with that resource, and pass its FD to the compositor.


> We also need a solution for the keymap shmem block.  I guess the keymap
> doesn't change all that often, so maybe it is easiest to just copy it
> over (host proxy -> guest proxy) instead of trying to map the host shmem
> into the guest?

Not sure if that would be much simpler than creating a ivshmem+hostmem 
combo that wraps the incoming shmem FD and then having virtio-gpu create 
a BO that imports it.


Regards,

Tomeu
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Re: [PATCH v3 1/2] drm/virtio: Add window server support

[Tomeu Vizoso]

On 02/12/2018 12:45 PM, Gerd Hoffmann wrote:

   Hi,


(a) software rendering: client allocates shared memory buffer, renders
into it, then passes a file handle for that shmem block together
with some meta data (size, format, ...) to the wayland server.

(b) gpu rendering: client opens a render node, allocates a buffer,
asks the cpu to renders into it, exports the buffer as dma-buf
(DRM_IOCTL_PRIME_HANDLE_TO_FD), passes this to the wayland server
(again including meta data of course).

Is that correct?


Both are correct descriptions of typical behaviors. But it isn't spec'ed
anywhere who has to do the buffer allocation.


Well, according to Pekka's reply it is spec'ed that way, for the
existing buffer types.  So for server allocated buffers you need
(a) a wayland protocol extension and (b) support for the extension
in the clients.


That's to say that if we cannot come up with a zero-copy solution for
unmodified clients, we should at least support zero-copy for cooperative
clients.


"cooperative clients" == "client which has support for the wayland
protocol extension", correct?


Guess it could be that, but I was rather thinking of clients that would 
allocate the buffer for wl_shm_pool with DRM_VIRTGPU_RESOURCE_CREATE or 
equivalent. Then that buffer would be exported and the fd passed using 
the standard wl_shm protocol.



4. QEMU maps that buffer to the guest's address space
(KVM_SET_USER_MEMORY_REGION), passes the guest PFN to the virtio driver


That part is problematic.  The host can't simply allocate something in
the physical address space, because most physical address space
management is done by the guest.  All pci bars are mapped by the guest
firmware for example (or by the guest OS in case of hotplug).


How can KVM_SET_USER_MEMORY_REGION ever be safely used then? I would have
expected that callers of that ioctl have enough knowledge to be able to
choose a physical address that won't conflict with the guest's kernel.


Depends on the kind of region.  Guest RAM is allocated and mapped by
qemu, guest firmware can query qemu about RAM mappings using a special
interface, then create a e820 memory map for the guest os.  PCI device
bars are mapped according to the pci config space registers, which in
turn are initialized by the guest firmware, so it is basically in the
guests hand where they show up.


I see that the ivshmem device in QEMU registers the memory region in BAR 2
of a PCI device instead. Would that be better in your opinion?


Yes.


Would it make sense for virtio-gpu to map buffers to the guest via PCI 
BARs? So we can use a single drm driver for both 2d and 3d.



4. QEMU pops data+buffers from the virtqueue, looks up shmem FD for each
resource, sends data + FDs to the compositor with SCM_RIGHTS


BTW: Is there a 1:1 relationship between buffers and shmem blocks?  Or
does the wayland protocol allow for offsets in buffer meta data, so you
can place multiple buffers in a single shmem block?


The latter:
https://wayland.freedesktop.org/docs/html/apa.html#protocol-spec-wl_shm_pool


Ah, good, that makes it alot easier.

So, yes, using ivshmem would be one option.  Tricky part here is the
buffer management though.  It's just a raw piece of memory.  The guest
proxy could mmap the pci bar and manage it.  But then it is again either
unmodified guest + copying the data, or modified client (which requests
buffers from guest proxy) for zero-copy.

Another idea would be extending stdvga.  Basically qemu would have to
use shmem as backing storage for vga memory instead of anonymous memory,
so it would be very  simliar to ivshmem on the host side.  But on the
guest side we have a drm driver for it (bochs-drm).  So clients can
allocate dumb drm buffers for software rendering, and the buffer would
already be backed by a host shmem segment.  Given that wayland already
supports drm buffers for 3d rendering that could work without extending
the wayland protocol.  The client proxy would have to translate the drm
buffer into an pci bar offset and pass it to the host side.  The host
proxy could register the pci bar as wl_shm_pool, then just pass through
the offset to reference the individual buffers.

Drawback of both approaches would be that software rendering and gpu
rendering would use quite different code paths.


Yeah, would be great if we could find a way to avoid that.


We also need a solution for the keymap shmem block.  I guess the keymap
doesn't change all that often, so maybe it is easiest to just copy it
over (host proxy -> guest proxy) instead of trying to map the host shmem
into the guest?


I think that should be fine for now. Something similar will have to 
happen for the clipboard, which currently uses pipes to exchange data.


Thanks,

Tomeu
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Re: [PATCH v3 1/2] drm/virtio: Add window server support

[Pekka Paalanen]

On Mon, 12 Feb 2018 12:45:40 +0100
Gerd Hoffmann  wrote:

>   Hi,
> 
> > >(a) software rendering: client allocates shared memory buffer, renders
> > >into it, then passes a file handle for that shmem block together
> > >with some meta data (size, format, ...) to the wayland server.
> > > 
> > >(b) gpu rendering: client opens a render node, allocates a buffer,
> > >asks the cpu to renders into it, exports the buffer as dma-buf
> > >(DRM_IOCTL_PRIME_HANDLE_TO_FD), passes this to the wayland server
> > >(again including meta data of course).
> > > 
> > > Is that correct?  
> > 
> > Both are correct descriptions of typical behaviors. But it isn't spec'ed
> > anywhere who has to do the buffer allocation.  
> 
> Well, according to Pekka's reply it is spec'ed that way, for the
> existing buffer types.  So for server allocated buffers you need
> (a) a wayland protocol extension and (b) support for the extension
> in the clients.

Correct. Or simply a libEGL that uses such Wayland extension behind
everyone's back. I believe such things did at least exist, but are
probably not relevant for this discussion.

(If there is a standard library, like libEGL, loaded and used by both a
server and a client, that library can advertise custom private Wayland
protocol extensions and the client side can take advantage of them,
both without needing any code changes on either the server or the
client.)

> We also need a solution for the keymap shmem block.  I guess the keymap
> doesn't change all that often, so maybe it is easiest to just copy it
> over (host proxy -> guest proxy) instead of trying to map the host shmem
> into the guest?

Yes, I believe that would be a perfectly valid solution for that
particular case.


Thanks,
pq


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Re: [PATCH v3 1/2] drm/virtio: Add window server support

[Gerd Hoffmann]

> > I was more thinking about a struct containing enough info to allow the
> > proxy on the host side find the buffer, something like:
> > 
> > struct {
> >enum type { stdvga, virtio-cpu, ... }
> >pcislot device;
> >union {
> >   int stdvga_pcibar_offset;
> >   int virtio_gpu_resource_id;
> >}
> > }
> > 
> > So when the guest proxy gets a message with a fd referencing a buffer it
> > would have to figure where the buffer is, rewrite the message into the
> > struct above for the host proxy.  The host proxy would rewrite the
> > message again for the server.
> 
> What I don't understand yet is how we can keep the buffer descriptions
> together with the protocol data that references them.
> 
> With SCM_RIGHTS, the FDs are placed in the ancillary data that "travels"
> together with the protocol data that references them.

Place the buffer description into the wayland extension protocol messages?

i.e. have some wl_virt_proxy protocol extension.  Then, for the stdvga case:

  (1) client sends wl_drm/create_prime_buffer request to the guest proxy
  (2) guest proxy rewrites this into wl_virt_proxy/create_buffer, or
  maybe a create_stdvga_buffer request, carrying all the information
  listed above, and sends it to the host proxy.
  (3) host proxy rewrites it again into a wl_shm_pool/create_buffer and
  forwards it to the server.

cheers,
  Gerd

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Re: [PATCH v3 1/2] drm/virtio: Add window server support

[Tomeu Vizoso]

On 02/12/2018 03:27 PM, Gerd Hoffmann wrote:

On Mon, Feb 12, 2018 at 03:00:24PM +0100, Tomeu Vizoso wrote:

On 02/12/2018 12:52 PM, Gerd Hoffmann wrote:

Hi,


can we reach agreement on whether vsock should be involved in this?


I think the best approach would be to have guest proxy and host proxy
use vsock for the wayland protocol.  Use a wayland protocol extension to
reference the buffers in stdvga / ivshmem / virtio-gpu.  Only the two
proxies need to understand the extension, the client <=> guest proxy and
host proxy <=> server communication would be standard wayland protocol.


Thanks for the ideas. What I haven't understood yet is how you see the
actual passing of buffers via vsock. Are you thinking of using ancillary
data to pass FDs, or something else?


I was more thinking about a struct containing enough info to allow the
proxy on the host side find the buffer, something like:

struct {
   enum type { stdvga, virtio-cpu, ... }
   pcislot device;
   union {
  int stdvga_pcibar_offset;
  int virtio_gpu_resource_id;
   }
}

So when the guest proxy gets a message with a fd referencing a buffer it
would have to figure where the buffer is, rewrite the message into the
struct above for the host proxy.  The host proxy would rewrite the
message again for the server.


What I don't understand yet is how we can keep the buffer descriptions 
together with the protocol data that references them.


With SCM_RIGHTS, the FDs are placed in the ancillary data that "travels" 
together with the protocol data that references them.


With the present series, the DRM_IOCTL_VIRTGPU_WINSRV_TX ioctl struct has 
a field for the protocol data and an array of FDs.


How are you proposing to pass instances of that struct from above along 
the protocol data that refers to them?


Thanks,

Tomeu
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Re: [PATCH v3 1/2] drm/virtio: Add window server support

[Gerd Hoffmann]

On Mon, Feb 12, 2018 at 03:00:24PM +0100, Tomeu Vizoso wrote:
> On 02/12/2018 12:52 PM, Gerd Hoffmann wrote:
> >Hi,
> > 
> > > can we reach agreement on whether vsock should be involved in this?
> > 
> > I think the best approach would be to have guest proxy and host proxy
> > use vsock for the wayland protocol.  Use a wayland protocol extension to
> > reference the buffers in stdvga / ivshmem / virtio-gpu.  Only the two
> > proxies need to understand the extension, the client <=> guest proxy and
> > host proxy <=> server communication would be standard wayland protocol.
> 
> Thanks for the ideas. What I haven't understood yet is how you see the
> actual passing of buffers via vsock. Are you thinking of using ancillary
> data to pass FDs, or something else?

I was more thinking about a struct containing enough info to allow the
proxy on the host side find the buffer, something like:

   struct {
  enum type { stdvga, virtio-cpu, ... }
  pcislot device;
  union {
 int stdvga_pcibar_offset;
 int virtio_gpu_resource_id;
  }
   }

So when the guest proxy gets a message with a fd referencing a buffer it
would have to figure where the buffer is, rewrite the message into the
struct above for the host proxy.  The host proxy would rewrite the
message again for the server.

cheers,
  Gerd

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Re: [PATCH v3 1/2] drm/virtio: Add window server support

[Tomeu Vizoso]

On 02/12/2018 12:52 PM, Gerd Hoffmann wrote:

   Hi,


can we reach agreement on whether vsock should be involved in this?


I think the best approach would be to have guest proxy and host proxy
use vsock for the wayland protocol.  Use a wayland protocol extension to
reference the buffers in stdvga / ivshmem / virtio-gpu.  Only the two
proxies need to understand the extension, the client <=> guest proxy and
host proxy <=> server communication would be standard wayland protocol.


Thanks for the ideas. What I haven't understood yet is how you see the 
actual passing of buffers via vsock. Are you thinking of using ancillary 
data to pass FDs, or something else?


Thanks,

Tomeu
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Re: [PATCH v3 1/2] drm/virtio: Add window server support

[Gerd Hoffmann]

  Hi,

> can we reach agreement on whether vsock should be involved in this?

I think the best approach would be to have guest proxy and host proxy
use vsock for the wayland protocol.  Use a wayland protocol extension to
reference the buffers in stdvga / ivshmem / virtio-gpu.  Only the two
proxies need to understand the extension, the client <=> guest proxy and
host proxy <=> server communication would be standard wayland protocol.

cheers,
  Gerd

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Re: [PATCH v3 1/2] drm/virtio: Add window server support

[Gerd Hoffmann]

  Hi,

> >(a) software rendering: client allocates shared memory buffer, renders
> >into it, then passes a file handle for that shmem block together
> >with some meta data (size, format, ...) to the wayland server.
> > 
> >(b) gpu rendering: client opens a render node, allocates a buffer,
> >asks the cpu to renders into it, exports the buffer as dma-buf
> >(DRM_IOCTL_PRIME_HANDLE_TO_FD), passes this to the wayland server
> >(again including meta data of course).
> > 
> > Is that correct?
> 
> Both are correct descriptions of typical behaviors. But it isn't spec'ed
> anywhere who has to do the buffer allocation.

Well, according to Pekka's reply it is spec'ed that way, for the
existing buffer types.  So for server allocated buffers you need
(a) a wayland protocol extension and (b) support for the extension
in the clients.

> That's to say that if we cannot come up with a zero-copy solution for
> unmodified clients, we should at least support zero-copy for cooperative
> clients.

"cooperative clients" == "client which has support for the wayland
protocol extension", correct?

> > > Creation of shareable buffer by guest
> > > -
> > > 
> > > 1. Client requests virtio driver to create a buffer suitable for sharing
> > > with host (DRM_VIRTGPU_RESOURCE_CREATE)
> > 
> > client or guest proxy?
> 
> As per the above, the GUI toolkit could have been modified so the client
> directly creates a shareable buffer, and renders directly to it without any
> extra copies.
> 
> If clients cannot be modified, then it's the guest proxy what has to create
> the shareable buffer and keep it in sync with the client's non-shareable
> buffer at the right times, by intercepting wl_surface.commit messages and
> copying buffer contents.

Ok.

> > > 4. QEMU maps that buffer to the guest's address space
> > > (KVM_SET_USER_MEMORY_REGION), passes the guest PFN to the virtio driver
> > 
> > That part is problematic.  The host can't simply allocate something in
> > the physical address space, because most physical address space
> > management is done by the guest.  All pci bars are mapped by the guest
> > firmware for example (or by the guest OS in case of hotplug).
> 
> How can KVM_SET_USER_MEMORY_REGION ever be safely used then? I would have
> expected that callers of that ioctl have enough knowledge to be able to
> choose a physical address that won't conflict with the guest's kernel.

Depends on the kind of region.  Guest RAM is allocated and mapped by
qemu, guest firmware can query qemu about RAM mappings using a special
interface, then create a e820 memory map for the guest os.  PCI device
bars are mapped according to the pci config space registers, which in
turn are initialized by the guest firmware, so it is basically in the
guests hand where they show up.

> I see that the ivshmem device in QEMU registers the memory region in BAR 2
> of a PCI device instead. Would that be better in your opinion?

Yes.

> > > 4. QEMU pops data+buffers from the virtqueue, looks up shmem FD for each
> > > resource, sends data + FDs to the compositor with SCM_RIGHTS
> > 
> > BTW: Is there a 1:1 relationship between buffers and shmem blocks?  Or
> > does the wayland protocol allow for offsets in buffer meta data, so you
> > can place multiple buffers in a single shmem block?
> 
> The latter:
> https://wayland.freedesktop.org/docs/html/apa.html#protocol-spec-wl_shm_pool

Ah, good, that makes it alot easier.

So, yes, using ivshmem would be one option.  Tricky part here is the
buffer management though.  It's just a raw piece of memory.  The guest
proxy could mmap the pci bar and manage it.  But then it is again either
unmodified guest + copying the data, or modified client (which requests
buffers from guest proxy) for zero-copy.

Another idea would be extending stdvga.  Basically qemu would have to
use shmem as backing storage for vga memory instead of anonymous memory,
so it would be very  simliar to ivshmem on the host side.  But on the
guest side we have a drm driver for it (bochs-drm).  So clients can
allocate dumb drm buffers for software rendering, and the buffer would
already be backed by a host shmem segment.  Given that wayland already
supports drm buffers for 3d rendering that could work without extending
the wayland protocol.  The client proxy would have to translate the drm
buffer into an pci bar offset and pass it to the host side.  The host
proxy could register the pci bar as wl_shm_pool, then just pass through
the offset to reference the individual buffers.

Drawback of both approaches would be that software rendering and gpu
rendering would use quite different code paths.

We also need a solution for the keymap shmem block.  I guess the keymap
doesn't change all that often, so maybe it is easiest to just copy it
over (host proxy -> guest proxy) instead of trying to map the host shmem
into the guest?

cheers,
  Gerd

Re: [PATCH v3 1/2] drm/virtio: Add window server support

[Tomeu Vizoso]

Hi Gerd and Stefan,

can we reach agreement on whether vsock should be involved in this?

Thanks,

Tomeu

On 02/07/2018 10:49 AM, Tomeu Vizoso wrote:

On 02/06/2018 03:23 PM, Gerd Hoffmann wrote:

   Hi,


Hmm?  I'm assuming the wayland client (in the guest) talks to the
wayland proxy, using the wayland protocol, like it would talk to a
wayland display server.  Buffers must be passed from client to
server/proxy somehow, probably using fd passing, so where is the
problem?

Or did I misunderstand the role of the proxy?


Hi Gerd,

it's starting to look to me that we're talking a bit past the other, so I
have pasted below a few words describing my current plan regarding the 
3 key

scenarios that I'm addressing.


You are describing the details, but I'm missing the big picture ...

So, virtualization aside, how do buffers work in wayland?  As far I know
it goes like this:

   (a) software rendering: client allocates shared memory buffer, renders
   into it, then passes a file handle for that shmem block together
   with some meta data (size, format, ...) to the wayland server.

   (b) gpu rendering: client opens a render node, allocates a buffer,
   asks the cpu to renders into it, exports the buffer as dma-buf
   (DRM_IOCTL_PRIME_HANDLE_TO_FD), passes this to the wayland server
   (again including meta data of course).

Is that correct?


Both are correct descriptions of typical behaviors. But it isn't spec'ed 
anywhere who has to do the buffer allocation.


In practical terms, the buffer allocation happens in either the 2D GUI 
toolkit (gtk+, for example), or the EGL implementation. Someone using 
this in a real product would most probably be interested in avoiding any 
extra copies and make sure that both allocate buffers via virtio-gpu, for 
example.


Depending on the use case, they could be also interested in supporting 
unmodified clients with an extra copy per buffer presentation.


That's to say that if we cannot come up with a zero-copy solution for 
unmodified clients, we should at least support zero-copy for cooperative 
clients.



Now, with virtualization added to the mix it becomes a bit more
complicated.  Client and server are unmodified.  The client talks to the
guest proxy (wayland protocol).  The guest proxy talks to the host proxy
(protocol to be defined). The host proxy talks to the server (wayland
protocol).

Buffers must be managed along the way, and we want avoid copying around
the buffers.  The host proxy could be implemented directly in qemu, or
as separate process which cooperates with qemu for buffer management.

Fine so far?


Yep.

I really think that whatever we come up with needs to support 3D 
clients as

well.


Lets start with 3d clients, I think these are easier.  They simply use
virtio-gpu for 3d rendering as usual.  When they are done the rendered
buffer already lives in a host drm buffer (because virgl runs the actual
rendering on the host gpu).  So the client passes the dma-buf to the
guest proxy, the guest proxy imports it to look up the resource-id,
passes the resource-id to the host proxy, the host proxy looks up the
drm buffer and exports it as dma-buf, then passes it to the server.
Done, without any extra data copies.


Yep.


Creation of shareable buffer by guest
-

1. Client requests virtio driver to create a buffer suitable for sharing
with host (DRM_VIRTGPU_RESOURCE_CREATE)


client or guest proxy?


As per the above, the GUI toolkit could have been modified so the client 
directly creates a shareable buffer, and renders directly to it without 
any extra copies.


If clients cannot be modified, then it's the guest proxy what has to 
create the shareable buffer and keep it in sync with the client's 
non-shareable buffer at the right times, by intercepting 
wl_surface.commit messages and copying buffer contents.



4. QEMU maps that buffer to the guest's address space
(KVM_SET_USER_MEMORY_REGION), passes the guest PFN to the virtio driver


That part is problematic.  The host can't simply allocate something in
the physical address space, because most physical address space
management is done by the guest.  All pci bars are mapped by the guest
firmware for example (or by the guest OS in case of hotplug).


How can KVM_SET_USER_MEMORY_REGION ever be safely used then? I would have 
expected that callers of that ioctl have enough knowledge to be able to 
choose a physical address that won't conflict with the guest's kernel.


I see that the ivshmem device in QEMU registers the memory region in BAR 
2 of a PCI device instead. Would that be better in your opinion?



4. QEMU pops data+buffers from the virtqueue, looks up shmem FD for each
resource, sends data + FDs to the compositor with SCM_RIGHTS


BTW: Is there a 1:1 relationship between buffers and shmem blocks?  Or
does the wayland protocol allow for offsets in buffer meta data, so you
can place multiple buffers in a single shmem block?


The 

Re: [PATCH v3 1/2] drm/virtio: Add window server support

[Tomeu Vizoso]

On 02/06/2018 03:23 PM, Gerd Hoffmann wrote:

   Hi,


Hmm?  I'm assuming the wayland client (in the guest) talks to the
wayland proxy, using the wayland protocol, like it would talk to a
wayland display server.  Buffers must be passed from client to
server/proxy somehow, probably using fd passing, so where is the
problem?

Or did I misunderstand the role of the proxy?


Hi Gerd,

it's starting to look to me that we're talking a bit past the other, so I
have pasted below a few words describing my current plan regarding the 3 key
scenarios that I'm addressing.


You are describing the details, but I'm missing the big picture ...

So, virtualization aside, how do buffers work in wayland?  As far I know
it goes like this:

   (a) software rendering: client allocates shared memory buffer, renders
   into it, then passes a file handle for that shmem block together
   with some meta data (size, format, ...) to the wayland server.

   (b) gpu rendering: client opens a render node, allocates a buffer,
   asks the cpu to renders into it, exports the buffer as dma-buf
   (DRM_IOCTL_PRIME_HANDLE_TO_FD), passes this to the wayland server
   (again including meta data of course).

Is that correct?


Both are correct descriptions of typical behaviors. But it isn't spec'ed 
anywhere who has to do the buffer allocation.


In practical terms, the buffer allocation happens in either the 2D GUI 
toolkit (gtk+, for example), or the EGL implementation. Someone using 
this in a real product would most probably be interested in avoiding any 
extra copies and make sure that both allocate buffers via virtio-gpu, for 
example.


Depending on the use case, they could be also interested in supporting 
unmodified clients with an extra copy per buffer presentation.


That's to say that if we cannot come up with a zero-copy solution for 
unmodified clients, we should at least support zero-copy for cooperative 
clients.



Now, with virtualization added to the mix it becomes a bit more
complicated.  Client and server are unmodified.  The client talks to the
guest proxy (wayland protocol).  The guest proxy talks to the host proxy
(protocol to be defined). The host proxy talks to the server (wayland
protocol).

Buffers must be managed along the way, and we want avoid copying around
the buffers.  The host proxy could be implemented directly in qemu, or
as separate process which cooperates with qemu for buffer management.

Fine so far?


Yep.


I really think that whatever we come up with needs to support 3D clients as
well.


Lets start with 3d clients, I think these are easier.  They simply use
virtio-gpu for 3d rendering as usual.  When they are done the rendered
buffer already lives in a host drm buffer (because virgl runs the actual
rendering on the host gpu).  So the client passes the dma-buf to the
guest proxy, the guest proxy imports it to look up the resource-id,
passes the resource-id to the host proxy, the host proxy looks up the
drm buffer and exports it as dma-buf, then passes it to the server.
Done, without any extra data copies.


Yep.


Creation of shareable buffer by guest
-

1. Client requests virtio driver to create a buffer suitable for sharing
with host (DRM_VIRTGPU_RESOURCE_CREATE)


client or guest proxy?


As per the above, the GUI toolkit could have been modified so the client 
directly creates a shareable buffer, and renders directly to it without 
any extra copies.


If clients cannot be modified, then it's the guest proxy what has to 
create the shareable buffer and keep it in sync with the client's 
non-shareable buffer at the right times, by intercepting 
wl_surface.commit messages and copying buffer contents.



4. QEMU maps that buffer to the guest's address space
(KVM_SET_USER_MEMORY_REGION), passes the guest PFN to the virtio driver


That part is problematic.  The host can't simply allocate something in
the physical address space, because most physical address space
management is done by the guest.  All pci bars are mapped by the guest
firmware for example (or by the guest OS in case of hotplug).


How can KVM_SET_USER_MEMORY_REGION ever be safely used then? I would have 
expected that callers of that ioctl have enough knowledge to be able to 
choose a physical address that won't conflict with the guest's kernel.


I see that the ivshmem device in QEMU registers the memory region in BAR 
2 of a PCI device instead. Would that be better in your opinion?



4. QEMU pops data+buffers from the virtqueue, looks up shmem FD for each
resource, sends data + FDs to the compositor with SCM_RIGHTS


BTW: Is there a 1:1 relationship between buffers and shmem blocks?  Or
does the wayland protocol allow for offsets in buffer meta data, so you
can place multiple buffers in a single shmem block?


The latter: 
https://wayland.freedesktop.org/docs/html/apa.html#protocol-spec-wl_shm_pool


Regards,

Tomeu
___

Re: [PATCH v3 1/2] drm/virtio: Add window server support

[Tomeu Vizoso]

On 02/07/2018 02:09 AM, Michael S. Tsirkin wrote:

On Tue, Feb 06, 2018 at 03:23:02PM +0100, Gerd Hoffmann wrote:

Creation of shareable buffer by guest
-

1. Client requests virtio driver to create a buffer suitable for sharing
with host (DRM_VIRTGPU_RESOURCE_CREATE)


client or guest proxy?


4. QEMU maps that buffer to the guest's address space
(KVM_SET_USER_MEMORY_REGION), passes the guest PFN to the virtio driver


That part is problematic.  The host can't simply allocate something in
the physical address space, because most physical address space
management is done by the guest.  All pci bars are mapped by the guest
firmware for example (or by the guest OS in case of hotplug).


4. QEMU pops data+buffers from the virtqueue, looks up shmem FD for each
resource, sends data + FDs to the compositor with SCM_RIGHTS


If you squint hard, this sounds a bit like a use-case for vhost-user-gpu, does 
it not?


Can you extend on what makes you think that?

As an aside, crosvm runs the virtio-gpu device in a separate, jailed
process, among other virtual devices.

https://chromium.googlesource.com/chromiumos/platform/crosvm/

Regards,

Tomeu
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Re: [PATCH v3 1/2] drm/virtio: Add window server support

[Michael S. Tsirkin]

On Tue, Feb 06, 2018 at 03:23:02PM +0100, Gerd Hoffmann wrote:
> > Creation of shareable buffer by guest
> > -
> > 
> > 1. Client requests virtio driver to create a buffer suitable for sharing
> > with host (DRM_VIRTGPU_RESOURCE_CREATE)
> 
> client or guest proxy?
> 
> > 4. QEMU maps that buffer to the guest's address space
> > (KVM_SET_USER_MEMORY_REGION), passes the guest PFN to the virtio driver
> 
> That part is problematic.  The host can't simply allocate something in
> the physical address space, because most physical address space
> management is done by the guest.  All pci bars are mapped by the guest
> firmware for example (or by the guest OS in case of hotplug).
> 
> > 4. QEMU pops data+buffers from the virtqueue, looks up shmem FD for each
> > resource, sends data + FDs to the compositor with SCM_RIGHTS

If you squint hard, this sounds a bit like a use-case for vhost-user-gpu, does 
it not?

> BTW: Is there a 1:1 relationship between buffers and shmem blocks?  Or
> does the wayland protocol allow for offsets in buffer meta data, so you
> can place multiple buffers in a single shmem block?
> 
> cheers,
>   Gerd
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Re: [PATCH v3 1/2] drm/virtio: Add window server support

[Pekka Paalanen]

On Mon, 5 Feb 2018 17:03:22 +0100
Gerd Hoffmann  wrote:

> On Mon, Feb 05, 2018 at 03:46:17PM +0100, Tomeu Vizoso wrote:
> > On 02/05/2018 01:20 PM, Gerd Hoffmann wrote:  
> > >Hi,
> > >   
> 
> Hmm.  I allways assumed the wayland client allocates the buffers, not
> the server.  Is that wrong?

Hi Gerd,

a fly-by comment here:

The standard operation mode on Wayland indeed is that the client
allocates any pixel buffers. It is not the whole story though.

Server allocated buffers passed to a client also exist:
- core protocol uses this to pass keymaps to clients
- people are not forbidden from writing Wayland extensions that do this
  for whatever reason

The latter server-allocated case could probably be overlooked, but the
keymap case not really. Furthermore, copy and drag protocol
pass pipe file descriptors via Wayland to establish client-to-client
pipes.


Thanks,
pq


pgpQxF06nhdFw.pgp
Description: OpenPGP digital signature
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Re: [PATCH v3 1/2] drm/virtio: Add window server support

[Gerd Hoffmann]

  Hi,

> > Hmm?  I'm assuming the wayland client (in the guest) talks to the
> > wayland proxy, using the wayland protocol, like it would talk to a
> > wayland display server.  Buffers must be passed from client to
> > server/proxy somehow, probably using fd passing, so where is the
> > problem?
> > 
> > Or did I misunderstand the role of the proxy?
> 
> Hi Gerd,
> 
> it's starting to look to me that we're talking a bit past the other, so I
> have pasted below a few words describing my current plan regarding the 3 key
> scenarios that I'm addressing.

You are describing the details, but I'm missing the big picture ...

So, virtualization aside, how do buffers work in wayland?  As far I know
it goes like this:

  (a) software rendering: client allocates shared memory buffer, renders
  into it, then passes a file handle for that shmem block together
  with some meta data (size, format, ...) to the wayland server.

  (b) gpu rendering: client opens a render node, allocates a buffer,
  asks the cpu to renders into it, exports the buffer as dma-buf
  (DRM_IOCTL_PRIME_HANDLE_TO_FD), passes this to the wayland server
  (again including meta data of course).

Is that correct?

Now, with virtualization added to the mix it becomes a bit more
complicated.  Client and server are unmodified.  The client talks to the
guest proxy (wayland protocol).  The guest proxy talks to the host proxy
(protocol to be defined). The host proxy talks to the server (wayland
protocol).

Buffers must be managed along the way, and we want avoid copying around
the buffers.  The host proxy could be implemented directly in qemu, or
as separate process which cooperates with qemu for buffer management.

Fine so far?

> I really think that whatever we come up with needs to support 3D clients as
> well.

Lets start with 3d clients, I think these are easier.  They simply use
virtio-gpu for 3d rendering as usual.  When they are done the rendered
buffer already lives in a host drm buffer (because virgl runs the actual
rendering on the host gpu).  So the client passes the dma-buf to the
guest proxy, the guest proxy imports it to look up the resource-id,
passes the resource-id to the host proxy, the host proxy looks up the
drm buffer and exports it as dma-buf, then passes it to the server.
Done, without any extra data copies.

> Creation of shareable buffer by guest
> -
> 
> 1. Client requests virtio driver to create a buffer suitable for sharing
> with host (DRM_VIRTGPU_RESOURCE_CREATE)

client or guest proxy?

> 4. QEMU maps that buffer to the guest's address space
> (KVM_SET_USER_MEMORY_REGION), passes the guest PFN to the virtio driver

That part is problematic.  The host can't simply allocate something in
the physical address space, because most physical address space
management is done by the guest.  All pci bars are mapped by the guest
firmware for example (or by the guest OS in case of hotplug).

> 4. QEMU pops data+buffers from the virtqueue, looks up shmem FD for each
> resource, sends data + FDs to the compositor with SCM_RIGHTS

BTW: Is there a 1:1 relationship between buffers and shmem blocks?  Or
does the wayland protocol allow for offsets in buffer meta data, so you
can place multiple buffers in a single shmem block?

cheers,
  Gerd

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Re: [PATCH v3 1/2] drm/virtio: Add window server support

[Tomeu Vizoso]

On 02/05/2018 05:03 PM, Gerd Hoffmann wrote:

On Mon, Feb 05, 2018 at 03:46:17PM +0100, Tomeu Vizoso wrote:

On 02/05/2018 01:20 PM, Gerd Hoffmann wrote:

Hi,


Why not use virtio-vsock to run the wayland protocol? I don't like
the idea to duplicate something with very simliar functionality in
virtio-gpu.


The reason for abandoning that approach was the type of objects that
could be shared via virtio-vsock would be extremely limited. Besides
that being potentially confusing to users, it would mean from the
implementation side that either virtio-vsock would gain a dependency on
the drm subsystem, or an appropriate abstraction for shareable buffers
would need to be added for little gain.


Well, no.  The idea is that virtio-vsock and virtio-gpu are used largely
as-is, without knowing about each other.  The guest wayland proxy which
does the buffer management talks to both devices.


Note that the proxy won't know anything about buffers if clients opt-in for
zero-copy support (they allocate the buffers in a way that allows for
sharing with the host).


Hmm?  I'm assuming the wayland client (in the guest) talks to the
wayland proxy, using the wayland protocol, like it would talk to a
wayland display server.  Buffers must be passed from client to
server/proxy somehow, probably using fd passing, so where is the
problem?

Or did I misunderstand the role of the proxy?


Hi Gerd,

it's starting to look to me that we're talking a bit past the other, so 
I have pasted below a few words describing my current plan regarding the 
3 key scenarios that I'm addressing.


I mention below KVM_SET_USER_MEMORY_REGION, but I guess we can discuss 
alternatives such as the one you are proposing using PCI BARs at a later 
stage.


I really think that whatever we come up with needs to support 3D clients 
as well.



Creation of shareable buffer by guest
-

1. Client requests virtio driver to create a buffer suitable for sharing 
with host (DRM_VIRTGPU_RESOURCE_CREATE)


2. Virtio driver creates a new resource ID and passes the request to 
QEMU (VIRTIO_GPU_CMD_RESOURCE_CREATE_2D)


3. QEMU creates a shmem file (for example with mkostemp), associates 
that FD with the ID of this resource


4. QEMU maps that buffer to the guest's address space 
(KVM_SET_USER_MEMORY_REGION), passes the guest PFN to the virtio driver


5. DRM_VIRTGPU_RESOURCE_CREATE returns the resource id just created

6. Client mmaps it with DRM_IOCTL_VIRTGPU_MAP+mmap and can render to it

7. Gets a FD with DRM_IOCTL_PRIME_HANDLE_TO_FD that can be sent around


Send of shareable buffer by guest
-

1. Client sends the host a message that refers to this buffer, passing 
the FD using SCM_RIGHTS


2. Guest proxy passes the message (serialized data + FDs) on to the 
virtio driver responsable for winsrv support


3. virtio driver puts the data and the resource ids corresponding to the 
FDs in a virtqueue, kicks it


4. QEMU pops data+buffers from the virtqueue, looks up shmem FD for each 
resource, sends data + FDs to the compositor with SCM_RIGHTS



Reception of buffer from the compositor
-

1. QEMU reads from the socket and gets a FD via SCM_RIGHTS

2. QEMU mmaps the FD and maps the resulting pointer to the guest via 
KVM_SET_USER_MEMORY_REGION


3. QEMU sends the guest PFN along the presentation data to the virtio 
driver (VIRTIO_GPU_CMD_WINSRV_RX)


4. Virtio driver wraps a FD around that PFN, puts it in a queue

5. Guest proxy calls DRM_IOCTL_VIRTGPU_WINSRV_RX and gets data plus that FD

6. Guest proxy sends that data + FD to the client via SCM_RIGHTS

7. Client gets FD, mmaps it and reads the data from the compositor



Thanks,

Tomeu



If you have a guest proxy anyway using virtio-sock for the protocol
stream and virtio-gpu for buffer sharing (and some day 3d rendering
too) should work fine I think.


If I understand correctly your proposal, virtio-gpu would be used for
creating buffers that could be shared across domains, but something
equivalent to SCM_RIGHTS would still be needed in virtio-vsock?


Yes, the proxy would send a reference to the buffer over virtio-vsock.
I was more thinking about a struct specifying something like
"ressource-id 42 on virtio-gpu-pci device in slot 1:23.0" instead of
using SCM_RIGHTS.


Can you extend on this? I'm having trouble figuring out how this could work
in a way that keeps protocol data together with the resources it refers to.


Don't know much about the wayland protocol.  Assuming you are passing
buffers as file handles, together with some information what kind of
buffer this is (sysv shm, dma-buf, ...).

We have a proxy on both ends.  One running in the guest, one on the host
(be it qemu or some external one).  So these two have to agree on how to
pass buffers from one to the other.  One way would be to have them talk
a simple meta protocol to each other, with "here comes a 

Re: [PATCH v3 1/2] drm/virtio: Add window server support

[Gerd Hoffmann]

On Mon, Feb 05, 2018 at 03:46:17PM +0100, Tomeu Vizoso wrote:
> On 02/05/2018 01:20 PM, Gerd Hoffmann wrote:
> >Hi,
> > 
> > > > Why not use virtio-vsock to run the wayland protocol? I don't like
> > > > the idea to duplicate something with very simliar functionality in
> > > > virtio-gpu.
> > > 
> > > The reason for abandoning that approach was the type of objects that
> > > could be shared via virtio-vsock would be extremely limited. Besides
> > > that being potentially confusing to users, it would mean from the
> > > implementation side that either virtio-vsock would gain a dependency on
> > > the drm subsystem, or an appropriate abstraction for shareable buffers
> > > would need to be added for little gain.
> > 
> > Well, no.  The idea is that virtio-vsock and virtio-gpu are used largely
> > as-is, without knowing about each other.  The guest wayland proxy which
> > does the buffer management talks to both devices.
> 
> Note that the proxy won't know anything about buffers if clients opt-in for
> zero-copy support (they allocate the buffers in a way that allows for
> sharing with the host).

Hmm?  I'm assuming the wayland client (in the guest) talks to the
wayland proxy, using the wayland protocol, like it would talk to a
wayland display server.  Buffers must be passed from client to
server/proxy somehow, probably using fd passing, so where is the
problem?

Or did I misunderstand the role of the proxy?

> > > > If you have a guest proxy anyway using virtio-sock for the protocol
> > > > stream and virtio-gpu for buffer sharing (and some day 3d rendering
> > > > too) should work fine I think.
> > > 
> > > If I understand correctly your proposal, virtio-gpu would be used for
> > > creating buffers that could be shared across domains, but something
> > > equivalent to SCM_RIGHTS would still be needed in virtio-vsock?
> > 
> > Yes, the proxy would send a reference to the buffer over virtio-vsock.
> > I was more thinking about a struct specifying something like
> > "ressource-id 42 on virtio-gpu-pci device in slot 1:23.0" instead of
> > using SCM_RIGHTS.
> 
> Can you extend on this? I'm having trouble figuring out how this could work
> in a way that keeps protocol data together with the resources it refers to.

Don't know much about the wayland protocol.  Assuming you are passing
buffers as file handles, together with some information what kind of
buffer this is (sysv shm, dma-buf, ...).

We have a proxy on both ends.  One running in the guest, one on the host
(be it qemu or some external one).  So these two have to agree on how to
pass buffers from one to the other.  One way would be to have them talk
a simple meta protocol to each other, with "here comes a chunk wayland
protocol to pass along" and "here is a buffer mgmt message".  Possibly
it is better to extend the wayland protocol to also cover this new kind
of buffer, so you don't need the meta protocol.

The proxies would talk normal wayland protocol to the client (in the
guest) and the server (on the host).  They will have to transform the
buffer into something they can pass along using the wayland protocol.

> > > > What is the plan for the host side? I see basically two options. Either
> > > > implement the host wayland proxy directly in qemu. Or
> > > > implement it as separate process, which then needs some help from
> > > > qemu to get access to the buffers. The later would allow qemu running
> > > > independant from the desktop session.
> > > 
> > > Regarding synchronizing buffers, this will stop becoming needed in
> > > subsequent commits as all shared memory is allocated in the host and
> > > mapped to the guest via KVM_SET_USER_MEMORY_REGION.
> > 
> > --verbose please.  The qemu patches linked from the cover letter not
> > exactly helpful in understanding how all this is supposed to work.
> 
> A client will allocate a buffer with DRM_VIRTGPU_RESOURCE_CREATE, export it
> and pass the FD to the compositor (via the proxy).
> 
> During resource creation, QEMU would allocate a shmem buffer and map it into
> the guest with KVM_SET_USER_MEMORY_REGION.

So the buffer magically shows up somewhere in the physical address space
of the guest?  That kind if magic usually isn't a very good idea.

> When a FD comes from the compositor, QEMU mmaps it and maps that virtual
> address to the guest via KVM_SET_USER_MEMORY_REGION.
> 
> When the guest proxy reads from the winsrv socket, it will get a FD that
> wraps the buffer referenced above.
> 
> When the client reads from the guest proxy, it would get a FD that
> references that same buffer and would mmap it. At that point, the client is
> reading from the same physical pages where the compositor wrote to.

Hmm.  I allways assumed the wayland client allocates the buffers, not
the server.  Is that wrong?

What is your plan for 3d acceleration support?

> To be clear, I'm not against solving this via some form of restricted FD
> passing in virtio-vsock, but Stefan (added to CC) thought that it would be
> 

Re: [PATCH v3 1/2] drm/virtio: Add window server support

[Tomeu Vizoso]

On 02/05/2018 01:20 PM, Gerd Hoffmann wrote:

   Hi,


Why not use virtio-vsock to run the wayland protocol? I don't like
the idea to duplicate something with very simliar functionality in
virtio-gpu.


The reason for abandoning that approach was the type of objects that
could be shared via virtio-vsock would be extremely limited. Besides
that being potentially confusing to users, it would mean from the
implementation side that either virtio-vsock would gain a dependency on
the drm subsystem, or an appropriate abstraction for shareable buffers
would need to be added for little gain.


Well, no.  The idea is that virtio-vsock and virtio-gpu are used largely
as-is, without knowing about each other.  The guest wayland proxy which
does the buffer management talks to both devices.


Note that the proxy won't know anything about buffers if clients opt-in 
for zero-copy support (they allocate the buffers in a way that allows 
for sharing with the host).



If you have a guest proxy anyway using virtio-sock for the protocol
stream and virtio-gpu for buffer sharing (and some day 3d rendering
too) should work fine I think.


If I understand correctly your proposal, virtio-gpu would be used for
creating buffers that could be shared across domains, but something
equivalent to SCM_RIGHTS would still be needed in virtio-vsock?


Yes, the proxy would send a reference to the buffer over virtio-vsock.
I was more thinking about a struct specifying something like
"ressource-id 42 on virtio-gpu-pci device in slot 1:23.0" instead of
using SCM_RIGHTS.


Can you extend on this? I'm having trouble figuring out how this could 
work in a way that keeps protocol data together with the resources it 
refers to.



If the mechanics of passing presentation data were very complex, I think
this approach would have more merit. But as you can see from the code,
it isn't that bad.


Well, the devil is in the details.  If you have multiple connections you
don't want one being able to stall the others for example.  There are
reasons took quite a while to bring virtio-vsock to the state where it
is today.


Yes, but at the same time there are use cases that virtio-vsock has to 
support but aren't important in this scenario.



What is the plan for the host side? I see basically two options. Either
implement the host wayland proxy directly in qemu. Or
implement it as separate process, which then needs some help from
qemu to get access to the buffers. The later would allow qemu running
independant from the desktop session.


Regarding synchronizing buffers, this will stop becoming needed in
subsequent commits as all shared memory is allocated in the host and
mapped to the guest via KVM_SET_USER_MEMORY_REGION.


--verbose please.  The qemu patches linked from the cover letter not
exactly helpful in understanding how all this is supposed to work.


A client will allocate a buffer with DRM_VIRTGPU_RESOURCE_CREATE, export 
it and pass the FD to the compositor (via the proxy).


During resource creation, QEMU would allocate a shmem buffer and map it 
into the guest with KVM_SET_USER_MEMORY_REGION.


The client would mmap that resource and render to it. Because it's 
backed by host memory, the compositor would be able to read it without 
any further copies.



This is already the case for buffers passed from the compositor to the
clients (see patch 2/2), and I'm working on the equivalent for buffers
from the guest to the host (clients still have to create buffers with
DRM_VIRTGPU_RESOURCE_CREATE but they will be only backend by host memory
so no calls to DRM_VIRTGPU_TRANSFER_TO_HOST are needed).


Same here.  --verbose please.


When a FD comes from the compositor, QEMU mmaps it and maps that virtual 
address to the guest via KVM_SET_USER_MEMORY_REGION.


When the guest proxy reads from the winsrv socket, it will get a FD that 
wraps the buffer referenced above.


When the client reads from the guest proxy, it would get a FD that 
references that same buffer and would mmap it. At that point, the client 
is reading from the same physical pages where the compositor wrote to.


To be clear, I'm not against solving this via some form of restricted FD 
passing in virtio-vsock, but Stefan (added to CC) thought that it would 
be cleaner to do it all within virtio-gpu. This is the thread where it 
was discussed:


https://lkml.kernel.org/r/<2d73a3e1-af70-83a1-0e84-98b5932ea...@collabora.com>

Thanks,

Tomeu
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Re: [PATCH v3 1/2] drm/virtio: Add window server support

[Gerd Hoffmann]

  Hi,

> > Why not use virtio-vsock to run the wayland protocol? I don't like
> > the idea to duplicate something with very simliar functionality in
> > virtio-gpu.
> 
> The reason for abandoning that approach was the type of objects that
> could be shared via virtio-vsock would be extremely limited. Besides
> that being potentially confusing to users, it would mean from the
> implementation side that either virtio-vsock would gain a dependency on
> the drm subsystem, or an appropriate abstraction for shareable buffers
> would need to be added for little gain.

Well, no.  The idea is that virtio-vsock and virtio-gpu are used largely
as-is, without knowing about each other.  The guest wayland proxy which
does the buffer management talks to both devices.

> > If you have a guest proxy anyway using virtio-sock for the protocol
> > stream and virtio-gpu for buffer sharing (and some day 3d rendering
> > too) should work fine I think.
> 
> If I understand correctly your proposal, virtio-gpu would be used for
> creating buffers that could be shared across domains, but something
> equivalent to SCM_RIGHTS would still be needed in virtio-vsock?

Yes, the proxy would send a reference to the buffer over virtio-vsock.
I was more thinking about a struct specifying something like
"ressource-id 42 on virtio-gpu-pci device in slot 1:23.0" instead of
using SCM_RIGHTS.

> If the mechanics of passing presentation data were very complex, I think
> this approach would have more merit. But as you can see from the code,
> it isn't that bad.

Well, the devil is in the details.  If you have multiple connections you
don't want one being able to stall the others for example.  There are
reasons took quite a while to bring virtio-vsock to the state where it
is today.

> > What is the plan for the host side? I see basically two options. Either
> > implement the host wayland proxy directly in qemu. Or
> > implement it as separate process, which then needs some help from
> > qemu to get access to the buffers. The later would allow qemu running
> > independant from the desktop session.
> 
> Regarding synchronizing buffers, this will stop becoming needed in
> subsequent commits as all shared memory is allocated in the host and
> mapped to the guest via KVM_SET_USER_MEMORY_REGION.

--verbose please.  The qemu patches linked from the cover letter not
exactly helpful in understanding how all this is supposed to work.

> This is already the case for buffers passed from the compositor to the
> clients (see patch 2/2), and I'm working on the equivalent for buffers
> from the guest to the host (clients still have to create buffers with
> DRM_VIRTGPU_RESOURCE_CREATE but they will be only backend by host memory
> so no calls to DRM_VIRTGPU_TRANSFER_TO_HOST are needed).

Same here.  --verbose please.

cheers,
  Gerd

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Re: [PATCH v3 1/2] drm/virtio: Add window server support

[Tomeu Vizoso]

On 1 February 2018 at 17:36, Gerd Hoffmann  wrote:

Hi,

Sorry for joining the party late. Had a broken finger and was
offline for a bunch of weeks (and a buif backlog afterwards ...).


Hi, no problem, hope it's fine now.


This is to allow clients running within VMs to be able to
communicate with a compositor in the host. Clients will use the
communication protocol that the compositor supports, and virtio-gpu
will assist with making buffers available in both sides, and
copying content as needed.


Why not use virtio-vsock to run the wayland protocol? I don't like
the idea to duplicate something with very simliar functionality in 
virtio-gpu.


The reason for abandoning that approach was the type of objects that
could be shared via virtio-vsock would be extremely limited. Besides
that being potentially confusing to users, it would mean from the
implementation side that either virtio-vsock would gain a dependency on
the drm subsystem, or an appropriate abstraction for shareable buffers
would need to be added for little gain.

Another factor that was taken into account was that the complexity
required for implementing passing protocol data around was very small
when compared with the buffer sharing mechanism.

It is expected that a service in the guest will act as a proxy, 
interacting with virtio-gpu to support unmodified clients.


If you have a guest proxy anyway using virtio-sock for the protocol 
stream and virtio-gpu for buffer sharing (and some day 3d rendering

too) should work fine I think.


If I understand correctly your proposal, virtio-gpu would be used for
creating buffers that could be shared across domains, but something
equivalent to SCM_RIGHTS would still be needed in virtio-vsock?

If so, that's what was planned initially, with the concern being that we
would be adding a bunch of complexity to virtio-vsock that would be only
used in this specific use case. Then we would also need to figure out
how virtio-vsock would be able to work with buffers from virtio-gpu
(either direct dependency or a new abstraction).

If the mechanics of passing presentation data were very complex, I think
this approach would have more merit. But as you can see from the code,
it isn't that bad.


When the client notifies the compositor that it can read from that

buffer,

the proxy should copy the contents from the SHM region to the
virtio-gpu resource and call DRM_VIRTGPU_TRANSFER_TO_HOST.


What is the plan for the host side? I see basically two options. 
Either implement the host wayland proxy directly in qemu. Or

implement it as separate process, which then needs some help from
qemu to get access to the buffers. The later would allow qemu running
independant from the desktop session.


Regarding synchronizing buffers, this will stop becoming needed in
subsequent commits as all shared memory is allocated in the host and
mapped to the guest via KVM_SET_USER_MEMORY_REGION.

This is already the case for buffers passed from the compositor to the
clients (see patch 2/2), and I'm working on the equivalent for buffers
from the guest to the host (clients still have to create buffers with
DRM_VIRTGPU_RESOURCE_CREATE but they will be only backend by host memory
so no calls to DRM_VIRTGPU_TRANSFER_TO_HOST are needed).

But in the case that we still need a proxy for some reason on the host
side, I think it would be better to have it in the same process where
virtio-gpu is implemented. In crosvm's case it would be in a process
separate from the main VMM, as device processes are isolated from each
other with minijail (see
https://chromium.googlesource.com/chromiumos/platform/crosvm/ ).

Regards,

Tomeu
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Re: [PATCH v3 1/2] drm/virtio: Add window server support

[Gerd Hoffmann]

  Hi,

Sorry for joining the party late.  Had a broken finger and was offline
for a bunch of weeks (and a buif backlog afterwards ...).

> This is to allow clients running within VMs to be able to communicate
> with a compositor in the host. Clients will use the communication
> protocol that the compositor supports, and virtio-gpu will assist with
> making buffers available in both sides, and copying content as needed.

Why not use virtio-vsock to run the wayland protocol?  I don't like the
idea to duplicate something with very simliar functionality in
virtio-gpu.

> It is expected that a service in the guest will act as a proxy,
> interacting with virtio-gpu to support unmodified clients.

If you have a guest proxy anyway using virtio-sock for the protocol
stream and virtio-gpu for buffer sharing (and some day 3d rendering too)
should work fine I think.

> When the client notifies the compositor that it can read from that buffer,
> the proxy should copy the contents from the SHM region to the virtio-gpu
> resource and call DRM_VIRTGPU_TRANSFER_TO_HOST.

What is the plan for the host side?  I see basically two options.
Either implement the host wayland proxy directly in qemu.  Or implement
it as separate process, which then needs some help from qemu to get
access to the buffers.  The later would allow qemu running independant
from the desktop session.

cheers,
  Gerd

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[PATCH v3 1/2] drm/virtio: Add window server support

[Tomeu Vizoso]

This is to allow clients running within VMs to be able to communicate
with a compositor in the host. Clients will use the communication
protocol that the compositor supports, and virtio-gpu will assist with
making buffers available in both sides, and copying content as needed.

It is expected that a service in the guest will act as a proxy,
interacting with virtio-gpu to support unmodified clients. For some
features of the protocol, the hypervisor might have to intervene and
also parse the protocol data to properly bridge resources. The following
IOCTLs have been added to this effect:

*_WINSRV_CONNECT: Opens a connection to the compositor in the host,
returns a FD that represents this connection and on which the following
IOCTLs can be used. Callers are expected to poll this FD for new
messages from the compositor.

*_WINSRV_TX: Asks the hypervisor to forward a message to the compositor

*_WINSRV_RX: Returns all queued messages

Alongside protocol data that is opaque to the kernel, the client can
send file descriptors that reference GEM buffers allocated by
virtio-gpu. The guest proxy is expected to figure out when a client is
passing a FD that refers to shared memory in the guest and allocate a
virtio-gpu buffer of the same size with DRM_VIRTGPU_RESOURCE_CREATE.

When the client notifies the compositor that it can read from that buffer,
the proxy should copy the contents from the SHM region to the virtio-gpu
resource and call DRM_VIRTGPU_TRANSFER_TO_HOST.

This has been tested with Wayland clients that make use of wl_shm to
pass buffers to the compositor, but is expected to work similarly for X
clients that make use of MIT-SHM with FD passing.

v2: * Add padding to two virtio command structs
* Properly cast two __user pointers (kbuild test robot)

v3: * Handle absence of winsrv support in QEMU (Dave Airlie)

Signed-off-by: Tomeu Vizoso 
---

 drivers/gpu/drm/virtio/virtgpu_drv.c   |   1 +
 drivers/gpu/drm/virtio/virtgpu_drv.h   |  39 -
 drivers/gpu/drm/virtio/virtgpu_ioctl.c | 165 +++
 drivers/gpu/drm/virtio/virtgpu_kms.c   |  66 ++--
 drivers/gpu/drm/virtio/virtgpu_vq.c| 285 -
 include/uapi/drm/virtgpu_drm.h |  29 
 include/uapi/linux/virtio_gpu.h|  43 +
 7 files changed, 613 insertions(+), 15 deletions(-)

diff --git a/drivers/gpu/drm/virtio/virtgpu_drv.c 
b/drivers/gpu/drm/virtio/virtgpu_drv.c
index 49a3d8d5a249..a528ddedd09f 100644
--- a/drivers/gpu/drm/virtio/virtgpu_drv.c
+++ b/drivers/gpu/drm/virtio/virtgpu_drv.c
@@ -79,6 +79,7 @@ static unsigned int features[] = {
 */
VIRTIO_GPU_F_VIRGL,
 #endif
+   VIRTIO_GPU_F_WINSRV,
 };
 static struct virtio_driver virtio_gpu_driver = {
.feature_table = features,
diff --git a/drivers/gpu/drm/virtio/virtgpu_drv.h 
b/drivers/gpu/drm/virtio/virtgpu_drv.h
index da2fb585fea4..268b386e1232 100644
--- a/drivers/gpu/drm/virtio/virtgpu_drv.h
+++ b/drivers/gpu/drm/virtio/virtgpu_drv.h
@@ -178,6 +178,8 @@ struct virtio_gpu_device {
 
struct virtio_gpu_queue ctrlq;
struct virtio_gpu_queue cursorq;
+   struct virtio_gpu_queue winsrv_rxq;
+   struct virtio_gpu_queue winsrv_txq;
struct kmem_cache *vbufs;
bool vqs_ready;
 
@@ -205,10 +207,32 @@ struct virtio_gpu_device {
 
 struct virtio_gpu_fpriv {
uint32_t ctx_id;
+
+   struct list_head winsrv_conns; /* list of virtio_gpu_winsrv_conn */
+   spinlock_t winsrv_lock;
+};
+
+struct virtio_gpu_winsrv_rx_qentry {
+   struct virtio_gpu_winsrv_rx *cmd;
+   struct list_head next;
+};
+
+struct virtio_gpu_winsrv_conn {
+   struct virtio_gpu_device *vgdev;
+
+   spinlock_t lock;
+
+   int fd;
+   struct drm_file *drm_file;
+
+   struct list_head cmdq; /* queue of virtio_gpu_winsrv_rx_qentry */
+   wait_queue_head_t cmdwq;
+
+   struct list_head next;
 };
 
 /* virtio_ioctl.c */
-#define DRM_VIRTIO_NUM_IOCTLS 10
+#define DRM_VIRTIO_NUM_IOCTLS 11
 extern struct drm_ioctl_desc virtio_gpu_ioctls[DRM_VIRTIO_NUM_IOCTLS];
 
 /* virtio_kms.c */
@@ -318,9 +342,22 @@ virtio_gpu_cmd_resource_create_3d(struct virtio_gpu_device 
*vgdev,
 void virtio_gpu_ctrl_ack(struct virtqueue *vq);
 void virtio_gpu_cursor_ack(struct virtqueue *vq);
 void virtio_gpu_fence_ack(struct virtqueue *vq);
+void virtio_gpu_winsrv_tx_ack(struct virtqueue *vq);
+void virtio_gpu_winsrv_rx_read(struct virtqueue *vq);
 void virtio_gpu_dequeue_ctrl_func(struct work_struct *work);
 void virtio_gpu_dequeue_cursor_func(struct work_struct *work);
+void virtio_gpu_dequeue_winsrv_rx_func(struct work_struct *work);
+void virtio_gpu_dequeue_winsrv_tx_func(struct work_struct *work);
 void virtio_gpu_dequeue_fence_func(struct work_struct *work);
+void virtio_gpu_fill_winsrv_rx(struct virtio_gpu_device *vgdev);
+void virtio_gpu_queue_winsrv_rx_in(struct virtio_gpu_device *vgdev,
+  struct virtio_gpu_winsrv_rx *cmd);
+int