Re: [PATCH v3 1/5] iommu/amd: Remove iommu_v2 module

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Sep 21, 2023 at 03:15:42PM +0000, Deucher, Alexander wrote:
> > > Does this patch or the following patches make any functional changes
> > > for devices?  E.g., devices which supported ATS would have been put
> > > into an identity mapping mode previously Is that still retained?
> >
> > Huh? Why would we ever want to do that?
> >
> > Policy for the default domain belongs in the iommu subsystem except in
> > extreme cases, the AMD driver should not be doing random things like forcing
> > identity for ATS capable PCI devices.
> 
> I'm just concerned about regressions in random devices due to a
> change in policy in the IOMMU driver.  Previously the IOMMU driver
> would put ATS compatible devices into 1:1 mode.  Also some of the
> earlier integrated GPUs require 1:1 mapping for display from system
> memory due to hardware limitations.  There were also a lot of sbios
> bugs in the carrizo/raven timeframe because windows didn't enable
> the IOMMU so lots of OEMs had bogus IOMMU ACPI tables which didn't
> cause problems when 1:1 mode was used.

That is a fair concern.

But we need to root those out and fix them as narrow quirks. We don't
want a driver to have blanket ATS == identity configuration, it will
harm other legitimate applications.

eg if you want to narrowly quirk integrated GPUs and carrizo/raven
that would be great.

Jason