Re: [PATCH 2/2] iommu/amd: Force identity mode for selected GPUs only

On Wed, Aug 05, 2026 at 06:12:05PM +0000, Deucher, Alexander wrote:

> > On Wed, Aug 05, 2026 at 05:22:22PM +0000, Deucher, Alexander wrote:
> >
> > > IIRC, they supported SME, but it wasn't usable due to other
> > > limitations (e.g., the requirement for identity mappings due to the
> > > display hw bug and the latency impact of encryption for displays in
> > > system memory).  Do you could effectively pick either SME or displays.
> >
> > IMHO the GPU driver should fail if it cannot work because it can't reach the
> > IOVA space requried for SME ...
> >
> > I don't think this should be a iommu problem to deal with
> 
> That means no display for the user which is a not a good user
> experience. Most users of systems with integrated graphics would
> probably prefer displays over SME.

You said IOVA translation doesn't work at all on these devices due to
a bug, so what is the iommu even supposed to do? It can't select
identity because the device doesn't work that way and it can't select
paging because it doesn't work that way either.

Or are you saying paging sort of works enough to do something
half-busted?

Why did AMD even offer SME in the BIOS on these systems if it doesn't
even work and turn it into Linux's problem?

Jason