On 27/10/2022 17:08, Rahul Singh wrote:
Hi Julien,
Hi Rahul,
On 26 Oct 2022, at 8:48 pm, Julien Grall <jul...@xen.org> wrote:
On 26/10/2022 15:33, Rahul Singh wrote:
Hi Julien,
Hi Rahul,
On 26 Oct 2022, at 2:36 pm, Julien Grall <jul...@xen.org> wrote:
On 26/10/2022 14:17, Rahul Singh wrote:
Hi All,
Hi Rahul,
At Arm, we started to implement the POC to support 2 levels of page
tables/nested translation in SMMUv3.
To support nested translation for guest OS Xen needs to expose the virtual
IOMMU. If we passthrough the
device to the guest that is behind an IOMMU and virtual IOMMU is enabled for
the guest there is a need to
add IOMMU binding for the device in the passthrough node as per [1]. This email
is to get an agreement on
how to add the IOMMU binding for guest OS.
Before I will explain how to add the IOMMU binding let me give a brief overview
of how we will add support for virtual
IOMMU on Arm. In order to implement virtual IOMMU Xen need SMMUv3 Nested
translation support. SMMUv3 hardware
supports two stages of translation. Each stage of translation can be
independently enabled. An incoming address is logically
translated from VA to IPA in stage 1, then the IPA is input to stage 2 which
translates the IPA to the output PA. Stage 1 is
intended to be used by a software entity( Guest OS) to provide isolation or
translation to buffers within the entity, for example,
DMA isolation within an OS. Stage 2 is intended to be available in systems
supporting the Virtualization Extensions and is
intended to virtualize device DMA to guest VM address spaces. When both stage 1
and stage 2 are enabled, the translation
configuration is called nesting.
Stage 1 translation support is required to provide isolation between different
devices within the guest OS. XEN already supports
Stage 2 translation but there is no support for Stage 1 translation for guests.
We will add support for guests to configure
the Stage 1 transition via virtual IOMMU. XEN will emulate the SMMU hardware
and exposes the virtual SMMU to the guest.
Guest can use the native SMMU driver to configure the stage 1 translation. When
the guest configures the SMMU for Stage 1,
XEN will trap the access and configure the hardware accordingly.
Now back to the question of how we can add the IOMMU binding between the
virtual IOMMU and the master devices so that
guests can configure the IOMMU correctly. The solution that I am suggesting is
as below:
For dom0, while handling the DT node(handle_node()) Xen will replace the phandle in the
"iommus" property with the virtual
IOMMU node phandle.
Below, you said that each IOMMUs may have a different ID space. So shouldn't we
expose one vIOMMU per pIOMMU? If not, how do you expect the user to specify the
mapping?
Yes you are right we need to create one vIOMMU per pIOMMU for dom0. This also
helps in the ACPI case
where we don’t need to modify the tables to delete the pIOMMU entries and
create one vIOMMU.
In this case, no need to replace the phandle as Xen create the vIOMMU with the
same pIOMMU
phandle and same base address.
For domU guests one vIOMMU per guest will be created.
IIRC, the SMMUv3 is using a ring like the GICv3 ITS. I think we need to be open
here because this may end up to be tricky to security support it (we have N
guest ring that can write to M host ring).
If xl want to creates the one vIOMMU per pIOMMU for domU then xl needs to know
the below information:
- Find the number of holes in guest memory same as the number of vIOMMU that
needs the creation to create the vIOMMU DT nodes. (Think about a big system
that has 50+ IOMMUs)
Yes, we will create vIOMMU for only those devices that are assigned to
guests but still we need to find the hole in guest memory.
I agree this is a problem with the one vIOMMU per pIOMMU.
- Find the pIOMMU attached to the assigned device and create mapping b/w vIOMMU
-> pIOMMU to register the MMIO handler.
Either we need to modify the current hyerpcall or need to implement a new
hypercall to find this information.
Adding hypercalls are is not a big problem.
Because of the above reason I thought of creating one vIOMMU for domU. Yes you
are right this may end up to be tricky to security support
but as per my understanding one vIOMMU per domU guest is easy to implement and
simple to handle as compared to one vIOMMU per pIOMMU
I am not sure about this. My gut feeling is the code in Xen will end up
to be tricky (there more that Xen doesn't support preemption). So I
think we will trade-off complexity in Xen over simplicity in libxl.
That said, I haven't looked deeper in the code. So I may be wrong. I
will need to see the code to confirm.
For domU guests, when passthrough the device to the guest as per [2], add the
below property in the partial device tree
node that is required to describe the generic device tree binding for IOMMUs
and their master(s)
"iommus = < &magic_phandle 0xvMasterID>
• magic_phandle will be the phandle ( vIOMMU phandle in xl) that will
be documented so that the user can set that in partial DT node (0xfdea).
Does this mean only one IOMMU will be supported in the guest?
Yes.
• vMasterID will be the virtual master ID that the user will provide.
The partial device tree will look like this:
/dts-v1/;
/ {
/* #*cells are here to keep DTC happy */
#address-cells = <2>;
#size-cells = <2>;
aliases {
net = &mac0;
};
passthrough {
compatible = "simple-bus";
ranges;
#address-cells = <2>;
#size-cells = <2>;
mac0: ethernet@10000000 {
compatible = "calxeda,hb-xgmac";
reg = <0 0x10000000 0 0x1000>;
interrupts = <0 80 4 0 81 4 0 82 4>;
iommus = <0xfdea 0x01>;
};
};
};
In xl.cfg we need to define a new option to inform Xen about vMasterId to
pMasterId mapping and to which IOMMU device this
the master device is connected so that Xen can configure the right IOMMU. This
is required if the system has devices that have
the same master ID but behind a different IOMMU.
In xl.cfg, we already pass the device-tree node path to passthrough. So Xen
should already have all the information about the IOMMU and Master-ID. So it
doesn't seem necessary for Device-Tree.
For ACPI, I would have expected the information to be found in the IOREQ.
So can you add more context why this is necessary for everyone?
We have information for IOMMU and Master-ID but we don’t have information for
linking vMaster-ID to pMaster-ID.
I am confused. Below, you are making the virtual master ID optional. So
shouldn't this be mandatory if you really need the mapping with the virtual ID?
vMasterID is optional if user knows pMasterID is unique on the system. But if
pMasterId is not unique then user needs to provide the vMasterID.
So the expectation is the user will be able to know that the pMasterID
is uniq. This may be easy with a couple of SMMUs, but if you have 50+
(as suggested above). This will become a pain on larger system.
IHMO, it would be much better if we can detect that in libxl (see below).
The device tree node will be used to assign the device to the guest and
configure the Stage-2 translation. Guest will use the
vMaster-ID to configure the vIOMMU during boot. Xen needs information to link
vMaster-ID to pMaster-ID to configure
the corresponding pIOMMU. As I mention we need vMaster-ID in case a system
could have 2 identical Master-ID but
each one connected to a different SMMU and assigned to the guest.
I am afraid I still don't understand why this is a requirement. Libxl could
have enough knowledge (which will be necessarry for the PCI case) to know the
IOMMU and pMasterID associated with a device.
So libxl could allocate the vMasterID, tell Xen the corresponding mapping and
update the device-tree.
IOW, it doesn't seem to be necessary to involve the user in the process here.
Yes, libxl could allocate the vMasterID but there is no way we can find the
link b/w vMasterID created to pMasterID from dtdev.
What I understand from the code is that there is no link between the
passthrough node and dtdev config option. The passthrough
node is directly copied to guest DT without any modification. Dtdev is used to
add and assign the device to IOMMU.
Let's take an example if the user wants to assign two devices to the guest via
passthrough node.
/dts-v1/;
/ {
/* #*cells are here to keep DTC happy */
#address-cells = <2>;
#size-cells = <2>;
aliases {
net = &mac0;
};
passthrough {
compatible = "simple-bus";
ranges;
#address-cells = <2>;
#size-cells = <2>;
mac0: ethernet@10000000 {
compatible = "calxeda,hb-xgmac";
reg = <0 0x10000000 0 0x1000>;
interrupts = <0 80 4 0 81 4 0 82 4>;
};
mac1: ethernet@20000000 {
compatible = “r8169";
reg = <0 0x10000000 0 0x1000>;
interrupts = <0 80 4 0 81 4 0 82 4>;
};
};
};
dtdev = [ "/soc/ethernet@10000000”, “/soc/ethernet@f2000000” ]
There is no link which dtdev entry belongs to which node. Therefor there is no
way to link the vMasterID created to pMasterID.
I agree there is no link today. But we could add a property in the
partial device-tree to mention which physical device is associated.
With that, I think all, the complexity is moved to libxl and it will be
easier for the user to use vIOMMU.
[...]
iommu_devid_map = [ “PMASTER_ID[@VMASTER_ID],IOMMU_BASE_ADDRESS” ,
“PMASTER_ID[@VMASTER_ID],IOMMU_BASE_ADDRESS”]
• PMASTER_ID is the physical master ID of the device from the physical
DT.
• VMASTER_ID is the virtual master Id that the user will configure in
the partial device tree.
• IOMMU_BASE_ADDRESS is the base address of the physical IOMMU device
to which this device is connected.
Below you give an example for Platform device. How would that fit in the
context of PCI passthrough?
In PCI passthrough case, xl will create the "iommu-map" property in vpci host
bridge node with phandle to vIOMMU node.
vSMMUv3 node will be created in xl.
This means that libxl will need to know the associated pMasterID to a PCI
device. So, I don't understand why you can't do the same for platform devices.
For the PCI passthrough case, we don’t need to provide the MasterID to create
"iommu-map” property as for
PCI device MasterID is RID ( BDF ). For non-PCI devices, MasterID is required
to create “iommus” property.
Are you talking about the physical MasterID or virtual one? If physical
MasterID then I don't think this is always the RID (see [1]). But for
the virtual Master ID we could make this association.
This still means that in some way the toolstack need to let Xen know (or
the other way around) the mapping between the pMasterID and vMasterID.
[1] Documentation/devicetree/bindings/pci/pci-iommu.txt.
Cheers,
--
Julien Grall
