Re: [RFC PATCH 6/6] iommu/amd: Introduce nested translation support

Suthikulpanit, Suravee
2024-01-08T06:49:41Z

On 1/5/2024 9:37 PM, Jason Gunthorpe wrote:
> On Fri, Jan 05, 2024 at 08:39:52PM +0700, Suthikulpanit, Suravee wrote:
>> Hi Kevin,
>>
>> On 12/15/2023 2:45 PM, Tian, Kevin wrote:
>>>> From: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
>>>> Sent: Wednesday, December 13, 2023 12:02 AM
>>>>
>>>> To support nested translation on AMD IOMMU, the driver needs to
>>>> program DTE[GCR3 Table Root Pointer] with the address provided by
>>>> the guest via struct iommu_hwpt_amd_v2, which is passed as a parameter
>>>> of the struct iommu_ops.domain_alloc_user() with the flag
>>>> IOMMU_HWPT_ALLOC_NEST_PARENT.
>>>>
>>>> Note that current implementation only support GCR3TRPMode for
>>>> nested translation, which uses GPA to program GCR3 Table Root Pointer.
>>>>
>>>
>>> means there is a plan to support another mode in the future or
>>> actually the nested translation requires GCR3TRPMode as a
>>> functional requirement? imho the point of GPA is assumed
>>> in the nested configuration in concept...
>>
>> On (older) system, which does not support GCR3TRPMode, the IOMMU driver
>> needs to program the device's DTE[GCR3 Table Root Pointer] field w/ SPA.
>
> Meaning that on older systems the GCR3 Table Root Pointer is not
> translated by the parent v1 page table?

Correct.

>> When QEMU presents an AMD vIOMMU device to a guest, the guest programs the
>> guest DTE[GCR3 Table Root Pointer] with GPA. Then we need to :
>> 1. Traps the DTE write
>> 2. Translate the GPA->SPA
>> 3. Program DTE with SPA.
>>
>> With the GCR3TRPMode, we can skip step 2 above and directly program step 3
>> with GPA.
>
> Do you want to support this? It will be hard to do because it is not
> just those three steps (which are easy) but that you have to somehow
> maintain coherence with any changes to the parent page table, so you
> have to hook the iommu_domain unmap as well...

I'm debating this. Let me get back on this part.

Thanks,
Suravee