Re: [PATCH rc v2] iommu/arm-smmu-v3: Fix inconsistent ATS state tracking

[Jason Gunthorpe <jgg@ziepe.ca>]

On Mon, May 04, 2026 at 07:33:07PM +0000, Pranjal Shrivastava wrote:

> > > The issue was exposed under heavy load when running a VFIO-based DMA map
> > > stress test: iova_stress [1]
> > 
> > I wonder what's the real reason for pci_enable_ats() to fail:
> > 
> 
> Yes, It's the dev->is_virtfn case (the one you suspect below)

Oh.... This is a much larger problem then

The VF should not fail to enable ATS, that is a meaningful bug, I'm
not sure why are are just discovering this?

It looks like the PF unconditionally calls pci_prepare_ats() during
arm_smmu_probe_device(), so how does this happen:

> >         if (dev->is_virtfn) {
> >                 pdev = pci_physfn(dev);
> >                 if (pdev->ats_stu != ps)
> >                         return -EINVAL;		// maybe this one?

??

So, I think the right error handling for a case that shouldn't happen is
to fail the attachment not to try to continue it broken?

Jason