Re: [PATCH v3 3/3] iommu/arm-smmu-v3: Fix ATS state tracking via ats_prepared gate

[Pranjal Shrivastava <praan@google.com>]

On Tue, May 19, 2026 at 08:01:55PM +0000, Samiullah Khawaja wrote:
> On Tue, May 19, 2026 at 11:59:47AM -0300, Jason Gunthorpe wrote:
> > On Tue, May 19, 2026 at 02:55:54PM +0000, Pranjal Shrivastava wrote:
> > > On Tue, May 19, 2026 at 11:44:30AM -0300, Jason Gunthorpe wrote:
> > > > On Tue, May 19, 2026 at 01:53:22PM +0000, Pranjal Shrivastava wrote:
> > > > > @@ -4450,7 +4450,8 @@ static struct iommu_device *arm_smmu_probe_device(struct device *dev)
> > > > >  	if (dev_is_pci(dev)) {
> > > > >  		unsigned int stu = __ffs(smmu->pgsize_bitmap);
> > > > >
> > > > > -		pci_prepare_ats(to_pci_dev(dev), stu);
> > > > > +		if (!pci_prepare_ats(to_pci_dev(dev), stu))
> > > > > +			master->ats_prepared = true;
> > > > >  	}
> > > >
> > > > This should fail not keep going, it is a kernel bug if
> > > > pci_prepare_ats() fails.
> 
> I agree with this approach, as it captures the bug at right place and we
> can fix it. This maybe changes the simantics of prepare_ats(), but I
> agreed with this.
> > > 
> > > Are you suggesting it's a kernel bug if the STU doesn't match?
> > > IIUC, it can fail because of the following cases (including the PF STU):
> > > 
> > > 	if (!pci_ats_supported(dev))
> > > 		return -EINVAL;
> > 
> > It should not be called if ats is not supported, iommu driver bug.
> 
> So basically iommu drivers should call pci_ats_supported(dev) before
> calling prepare_ats() and probe fails if prepare_ats() fails.

Ack. I'll add those checks to Intel and AMD IOMMU too..
AFAICT, both of those too, don't fail an attach if pci_enable_ats fails
[1][2]. I'll add those in the next version as well.

Thanks,
Praan

[1] https://elixir.bootlin.com/linux/v7.1-rc3/source/drivers/iommu/amd/iommu.c#L2373
[2] https://elixir.bootlin.com/linux/v7.1-rc3/source/drivers/iommu/intel/iommu.c#L3340