Re: Proposal for virtual IOMMU binding b/w vIOMMU and passthrough devices

[Rahul Singh <Rahul.Singh@xxxxxxx>]

Hi Julien,


> On 26 Oct 2022, at 8:48 pm, Julien Grall <julien@xxxxxxx> wrote:
> 
> 
> 
> On 26/10/2022 15:33, Rahul Singh wrote:
>> Hi Julien,
> 
> Hi Rahul,
> 
>>> On 26 Oct 2022, at 2:36 pm, Julien Grall <julien@xxxxxxx> 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.
 -  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. 

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


> 
>>> 
>>>> 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.

> 
>> 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.

> 
>>> 
>>>>  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.

Also, VPCI will create the virtual SBDF when we assigned PCI devices to the 
guest. Xen can easily find the
physical SBDF and pIOMMU from virtual SBDF as Xen has all the information for 
PCI devices.

> 
>>> 
>>>>  Example: Let's say the user wants to assign the below physical device in 
>>>> DT to the guest.
>>>>  iommu@4f000000 {
>>>>                 compatible = "arm,smmu-v3";
>>>>                    interrupts = <0x00 0xe4 0xf04>;
>>>>                 interrupt-parent = <0x01>;
>>>>                 #iommu-cells = <0x01>;
>>>>                 interrupt-names = "combined";
>>>>                 reg = <0x00 0x4f000000 0x00 0x40000>;
>>>>                 phandle = <0xfdeb>;
>>>>                 name = "iommu";
>>>> };
>>> 
>>> So I guess this node will be written by Xen. How will you the case where 
>>> there are extra property to added (e.g. dma-coherent)?
>> In this example this is physical IOMMU node. vIOMMU node wil be created by 
>> xl during guest creation.
> 
> Ok. I think it would be better to use very different phandle in your example 
> so it doesn't look like a mistake.
> 

Ack

Regards,
Rahul