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

[Suthikulpanit, Suravee]

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@xxxxxxx>
> > > > 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