Re: [PATCH v2 1/2] iommu/amd: Add PerfOpt IOMMU performance optimization support

[Jason Gunthorpe <jgg@ziepe.ca>]

On Fri, Sep 25, 2026 at 07:33:08PM -0500, Mario Limonciello wrote:
> 
> 
> On 9/25/26 5:39 PM, Jason Gunthorpe wrote:
> > On Fri, Sep 25, 2026 at 08:11:41AM -0500, Mario Limonciello wrote:
> > > 
> > > 
> > > On 9/25/26 07:28, Jason Gunthorpe wrote:
> > > > On Fri, Sep 25, 2026 at 09:20:02AM +0200, Joerg Roedel wrote:
> > > > > On Thu, Sep 24, 2026 at 07:50:18PM -0300, Jason Gunthorpe wrote:
> > > > > > Why does the GPU driver have to request this? Why can't the iommu
> > > > > > driver know that this is a special device that can use this magic fast
> > > > > > path and then auto set it when identity is asked for?
> > > > > 
> > > > > This is a layering violation either way. Either the IOMMU driver needs to poke
> > > > > into device details and their relations or the device driver needs to poke into
> > > > > IOMMU details.
> > > > 
> > > > There are no device details, this is just a module option in the gpu
> > > > driver.
> > > 
> > > Well the GPU driver has knowledge whether it's a GPU inside of an APU to
> > > decide if it's valid.  That information is part of the discovery table
> > > setup.
> > 
> > Where was that in patch 2?
> 
> amdgpu_device_use_perfopt()

Ah, sneaky, that looks like it just decodes from a a giant list of PCI IDs
 
> > The right way for this to work is to quirk it through the iommu driver
> > so it can choose the fast mode, and not involve the GPU driver at
> > all. Like we've done for every other GPU weirdness. And AMD should be
> > making this work better by providing ACPI support so the iommu driver
> > can understand how it should work without inspecting PCI IDs.
> 
> There is a heuristic that Vasant added recently that we detect APU from the
> IOMMU driver.  We might be able to use that for now.
> 
> I'll take a look at what an incremental patch looks like on top of
> iommu/next that I think reworks all these paths the way you want.  I should
> be able to post something next week.

ACPI is the right answer to these kinds of problems.

Jason