On 7/10/23 05:51, Jason Wang wrote: > On Fri, Jul 7, 2023 at 7:21 PM Ilya Maximets <i.maxim...@ovn.org> wrote: >> >> On 7/7/23 03:43, Jason Wang wrote: >>> On Fri, Jul 7, 2023 at 3:08 AM Stefan Hajnoczi <stefa...@gmail.com> wrote: >>>> >>>> On Wed, 5 Jul 2023 at 02:02, Jason Wang <jasow...@redhat.com> wrote: >>>>> >>>>> On Mon, Jul 3, 2023 at 5:03 PM Stefan Hajnoczi <stefa...@gmail.com> wrote: >>>>>> >>>>>> On Fri, 30 Jun 2023 at 09:41, Jason Wang <jasow...@redhat.com> wrote: >>>>>>> >>>>>>> On Thu, Jun 29, 2023 at 8:36 PM Stefan Hajnoczi <stefa...@gmail.com> >>>>>>> wrote: >>>>>>>> >>>>>>>> On Thu, 29 Jun 2023 at 07:26, Jason Wang <jasow...@redhat.com> wrote: >>>>>>>>> >>>>>>>>> On Wed, Jun 28, 2023 at 4:25 PM Stefan Hajnoczi <stefa...@gmail.com> >>>>>>>>> wrote: >>>>>>>>>> >>>>>>>>>> On Wed, 28 Jun 2023 at 10:19, Jason Wang <jasow...@redhat.com> wrote: >>>>>>>>>>> >>>>>>>>>>> On Wed, Jun 28, 2023 at 4:15 PM Stefan Hajnoczi >>>>>>>>>>> <stefa...@gmail.com> wrote: >>>>>>>>>>>> >>>>>>>>>>>> On Wed, 28 Jun 2023 at 09:59, Jason Wang <jasow...@redhat.com> >>>>>>>>>>>> wrote: >>>>>>>>>>>>> >>>>>>>>>>>>> On Wed, Jun 28, 2023 at 3:46 PM Stefan Hajnoczi >>>>>>>>>>>>> <stefa...@gmail.com> wrote: >>>>>>>>>>>>>> >>>>>>>>>>>>>> On Wed, 28 Jun 2023 at 05:28, Jason Wang <jasow...@redhat.com> >>>>>>>>>>>>>> wrote: >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> On Wed, Jun 28, 2023 at 6:45 AM Ilya Maximets >>>>>>>>>>>>>>> <i.maxim...@ovn.org> wrote: >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> On 6/27/23 04:54, Jason Wang wrote: >>>>>>>>>>>>>>>>> On Mon, Jun 26, 2023 at 9:17 PM Ilya Maximets >>>>>>>>>>>>>>>>> <i.maxim...@ovn.org> wrote: >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> On 6/26/23 08:32, Jason Wang wrote: >>>>>>>>>>>>>>>>>>> On Sun, Jun 25, 2023 at 3:06 PM Jason Wang >>>>>>>>>>>>>>>>>>> <jasow...@redhat.com> wrote: >>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>> On Fri, Jun 23, 2023 at 5:58 AM Ilya Maximets >>>>>>>>>>>>>>>>>>>> <i.maxim...@ovn.org> wrote: >>>>>>>>>>>>>>>>>> It is noticeably more performant than a tap with vhost=on in >>>>>>>>>>>>>>>>>> terms of PPS. >>>>>>>>>>>>>>>>>> So, that might be one case. Taking into account that just >>>>>>>>>>>>>>>>>> rcu lock and >>>>>>>>>>>>>>>>>> unlock in virtio-net code takes more time than a packet >>>>>>>>>>>>>>>>>> copy, some batching >>>>>>>>>>>>>>>>>> on QEMU side should improve performance significantly. And >>>>>>>>>>>>>>>>>> it shouldn't be >>>>>>>>>>>>>>>>>> too hard to implement. >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> Performance over virtual interfaces may potentially be >>>>>>>>>>>>>>>>>> improved by creating >>>>>>>>>>>>>>>>>> a kernel thread for async Tx. Similarly to what io_uring >>>>>>>>>>>>>>>>>> allows. Currently >>>>>>>>>>>>>>>>>> Tx on non-zero-copy interfaces is synchronous, and that >>>>>>>>>>>>>>>>>> doesn't allow to >>>>>>>>>>>>>>>>>> scale well. >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> Interestingly, actually, there are a lot of "duplication" >>>>>>>>>>>>>>>>> between >>>>>>>>>>>>>>>>> io_uring and AF_XDP: >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> 1) both have similar memory model (user register) >>>>>>>>>>>>>>>>> 2) both use ring for communication >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> I wonder if we can let io_uring talks directly to AF_XDP. >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> Well, if we submit poll() in QEMU main loop via io_uring, then >>>>>>>>>>>>>>>> we can >>>>>>>>>>>>>>>> avoid cost of the synchronous Tx for non-zero-copy modes, i.e. >>>>>>>>>>>>>>>> for >>>>>>>>>>>>>>>> virtual interfaces. io_uring thread in the kernel will be >>>>>>>>>>>>>>>> able to >>>>>>>>>>>>>>>> perform transmission for us. >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> It would be nice if we can use iothread/vhost other than the >>>>>>>>>>>>>>> main loop >>>>>>>>>>>>>>> even if io_uring can use kthreads. We can avoid the memory >>>>>>>>>>>>>>> translation >>>>>>>>>>>>>>> cost. >>>>>>>>>>>>>> >>>>>>>>>>>>>> The QEMU event loop (AioContext) has io_uring code >>>>>>>>>>>>>> (utils/fdmon-io_uring.c) but it's disabled at the moment. I'm >>>>>>>>>>>>>> working >>>>>>>>>>>>>> on patches to re-enable it and will probably send them in July. >>>>>>>>>>>>>> The >>>>>>>>>>>>>> patches also add an API to submit arbitrary io_uring operations >>>>>>>>>>>>>> so >>>>>>>>>>>>>> that you can do stuff besides file descriptor monitoring. Both >>>>>>>>>>>>>> the >>>>>>>>>>>>>> main loop and IOThreads will be able to use io_uring on Linux >>>>>>>>>>>>>> hosts. >>>>>>>>>>>>> >>>>>>>>>>>>> Just to make sure I understand. If we still need a copy from >>>>>>>>>>>>> guest to >>>>>>>>>>>>> io_uring buffer, we still need to go via memory API for GPA which >>>>>>>>>>>>> seems expensive. >>>>>>>>>>>>> >>>>>>>>>>>>> Vhost seems to be a shortcut for this. >>>>>>>>>>>> >>>>>>>>>>>> I'm not sure how exactly you're thinking of using io_uring. >>>>>>>>>>>> >>>>>>>>>>>> Simply using io_uring for the event loop (file descriptor >>>>>>>>>>>> monitoring) >>>>>>>>>>>> doesn't involve an extra buffer, but the packet payload still >>>>>>>>>>>> needs to >>>>>>>>>>>> reside in AF_XDP umem, so there is a copy between guest memory and >>>>>>>>>>>> umem. >>>>>>>>>>> >>>>>>>>>>> So there would be a translation from GPA to HVA (unless io_uring >>>>>>>>>>> support 2 stages) which needs to go via qemu memory core. And this >>>>>>>>>>> part seems to be very expensive according to my test in the past. >>>>>>>>>> >>>>>>>>>> Yes, but in the current approach where AF_XDP is implemented as a >>>>>>>>>> QEMU >>>>>>>>>> netdev, there is already QEMU device emulation (e.g. virtio-net) >>>>>>>>>> happening. So the GPA to HVA translation will happen anyway in device >>>>>>>>>> emulation. >>>>>>>>> >>>>>>>>> Just to make sure we're on the same page. >>>>>>>>> >>>>>>>>> I meant, AF_XDP can do more than e.g 10Mpps. So if we still use the >>>>>>>>> QEMU netdev, it would be very hard to achieve that if we stick to >>>>>>>>> using the Qemu memory core translations which need to take care about >>>>>>>>> too much extra stuff. That's why I suggest using vhost in io threads >>>>>>>>> which only cares about ram so the translation could be very fast. >>>>>>>> >>>>>>>> What does using "vhost in io threads" mean? >>>>>>> >>>>>>> It means a vhost userspace dataplane that is implemented via io threads. >>>>>> >>>>>> AFAIK this does not exist today. QEMU's built-in devices that use >>>>>> IOThreads don't use vhost code. QEMU vhost code is for vhost kernel, >>>>>> vhost-user, or vDPA but not built-in devices that use IOThreads. The >>>>>> built-in devices implement VirtioDeviceClass callbacks directly and >>>>>> use AioContext APIs to run in IOThreads. >>>>> >>>>> Yes. >>>>> >>>>>> >>>>>> Do you have an idea for using vhost code for built-in devices? Maybe >>>>>> it's fastest if you explain your idea and its advantages instead of me >>>>>> guessing. >>>>> >>>>> It's something like I'd proposed in [1]: >>>>> >>>>> 1) a vhost that is implemented via IOThreads >>>>> 2) memory translation is done via vhost memory table/IOTLB >>>>> >>>>> The advantages are: >>>>> >>>>> 1) No 3rd application like DPDK application >>>>> 2) Attack surface were reduced >>>>> 3) Better understanding/interactions with device model for things like >>>>> RSS and IOMMU >>>>> >>>>> There could be some dis-advantages but it's not obvious to me :) >>>> >>>> Why is QEMU's native device emulation API not the natural choice for >>>> writing built-in devices? I don't understand why the vhost interface >>>> is desirable for built-in devices. >>> >>> Unless the memory helpers (like address translations) were optimized >>> fully to satisfy this 10M+ PPS. >>> >>> Not sure if this is too hard, but last time I benchmark, perf told me >>> most of the time spent in the translation. >>> >>> Using a vhost is a workaround since its memory model is much more >>> simpler so it can skip lots of memory sections like I/O and ROM etc. >> >> So, we can have a thread running as part of QEMU process that implements >> vhost functionality for a virtio-net device. And this thread has an >> optimized way to access memory. What prevents current virtio-net emulation >> code accessing memory in the same optimized way? > > Current emulation using memory core accessors which needs to take care > of a lot of stuff like MMIO or even P2P. Such kind of stuff is not > considered since day0 of vhost. You can do some experiment on this e.g > just dropping packets after fetching it from the TX ring.
If I'm reading that right, virtio implementation is using address space caching by utilizing a memory listener and pre-translated addresses of interesting memory regions. Then it's performing address_space_read_cached, which is bypassing all the memory address translation logic on a cache hit. That sounds pretty similar to how memory table is prepared for vhost. > >> i.e. we likely don't >> actually need to implement the whole vhost-virtio communication protocol >> in order to have faster memory access from the device emulation code. >> I mean, if vhost can access device memory faster, why device itself can't? > > I'm not saying it can't but it would end up with something similar to > vhost. And that's why I'm saying using vhost is a shortcut (at least > for a POC). > > Thanks > >> >> With that we could probably split the "datapath" part of the virtio-net >> emulation into a separate thread driven by iothread loop. >> >> Then add batch API for communication with a network backend (af-xdp) to >> avoid per-packet calls. >> >> These are 3 more or less independent tasks that should allow the similar >> performance to a full fledged vhost control and dataplane implementation >> inside QEMU. >> >> Or am I missing something? (Probably) >> >>> >>> Thanks >>> >>>> >>>>> >>>>> It's something like linking SPDK/DPDK to Qemu. >>>> >>>> Sergio Lopez tried loading vhost-user devices as shared libraries that >>>> run in the QEMU process. It worked as an experiment but wasn't pursued >>>> further. >>>> >>>> I think that might make sense in specific cases where there is an >>>> existing vhost-user codebase that needs to run as part of QEMU. >>>> >>>> In this case the AF_XDP code is new, so it's not a case of moving >>>> existing code into QEMU. >>>> >>>>> >>>>>> >>>>>>>>>> Regarding pinning - I wonder if that's something that can be refined >>>>>>>>>> in the kernel by adding an AF_XDP flag that enables on-demand pinning >>>>>>>>>> of umem. That way only rx and tx buffers that are currently in use >>>>>>>>>> will be pinned. The disadvantage is the runtime overhead to pin/unpin >>>>>>>>>> pages. I'm not sure whether it's possible to implement this, I >>>>>>>>>> haven't >>>>>>>>>> checked the kernel code. >>>>>>>>> >>>>>>>>> It requires the device to do page faults which is not commonly >>>>>>>>> supported nowadays. >>>>>>>> >>>>>>>> I don't understand this comment. AF_XDP processes each rx/tx >>>>>>>> descriptor. At that point it can getuserpages() or similar in order to >>>>>>>> pin the page. When the memory is no longer needed, it can put those >>>>>>>> pages. No fault mechanism is needed. What am I missing? >>>>>>> >>>>>>> Ok, I think I kind of get you, you mean doing pinning while processing >>>>>>> rx/tx buffers? It's not easy since GUP itself is not very fast, it may >>>>>>> hit PPS for sure. >>>>>> >>>>>> Yes. It's not as fast as permanently pinning rx/tx buffers, but it >>>>>> supports unpinned guest RAM. >>>>> >>>>> Right, it's a balance between pin and PPS. PPS seems to be more >>>>> important in this case. >>>>> >>>>>> >>>>>> There are variations on this approach, like keeping a certain amount >>>>>> of pages pinned after they have been used so the cost of >>>>>> pinning/unpinning can be avoided when the same pages are reused in the >>>>>> future, but I don't know how effective that is in practice. >>>>>> >>>>>> Is there a more efficient approach without relying on hardware page >>>>>> fault support? >>>>> >>>>> I guess so, I see some slides that say device page fault is very slow. >>>>> >>>>>> >>>>>> My understanding is that hardware page fault support is not yet >>>>>> deployed. We'd be left with pinning guest RAM permanently or using a >>>>>> runtime pinning/unpinning approach like I've described. >>>>> >>>>> Probably. >>>>> >>>>> Thanks >>>>> >>>>>> >>>>>> Stefan >>>>>> >>>>> >>>> >>> >> >
