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

AMD General

> -----Original Message-----
> From: Jason Gunthorpe <[email protected]>
> Sent: Wednesday, August 12, 2026 7:27 PM
> To: Deucher, Alexander <[email protected]>
> Cc: Hegde, Vasant <[email protected]>; Limonciello, Mario
> <[email protected]>; Kuehling, Felix <[email protected]>;
> [email protected]; [email protected]; [email protected];
> [email protected]; [email protected]; Suthikulpanit, Suravee
> <[email protected]>; [email protected]; Longia,
> Amandeep Kaur <[email protected]>
> Subject: Re: [PATCH 2/2] iommu/amd: Force identity mode for selected GPUs
> only
>
> On Wed, Aug 12, 2026 at 09:03:58PM +0000, Deucher, Alexander wrote:
> > > Sure, but the end result is the GPU and SME are inherently
> > > incompatible, we can't really fix it.
> > >
> > > What's wrong with blowing up at boot with a clear message that Linux
> > > must have SME disabled on this HW printed to the UEFI console?
> >
> > It's not very user friendly, plus it would break working systems.
>
> Huh? You've explained that SME doesn't work with these GPUs? How are
> there any working systems with SME turned on already running Linux?

No one noticed it because the IOMMU driver used to default to 1:1 mapping for all AMD GPUs that supported ATS and PRI for compatibility with ROCm at the time back when the amd IOMMU was the only one that supported SVA.  Same reason we didn't notice the IOMMU problems with the display hardware.  Linux defaulted to 1:1 and windows didn't use the IOMMU.

>
> >  I'm not sure the average user would know what to do.
>
> Turn off SME in the bios? Doesn't it default to off?

I don't remember.  I suspect it defaulted to on.

>
> >  You aren't likely to want SME in the first place on one of these
> > laptops.  Why not flip the logic and print a clear message that if
> > the user wants SME they have to explicitly enable it via the kernel
> > command line.
>
> That seems like it would be API breaking for server users that do want SME

Only disable it by default for these specific platforms with the old iGPUs.

Alex