Re: [PATCH 1/2] iommu: Add device ATS not supported capability

[Samiullah Khawaja]

On Tue, Mar 03, 2026 at 07:28:35PM -0400, Jason Gunthorpe wrote:
> On Tue, Mar 03, 2026 at 10:42:48PM +0000, Samiullah Khawaja wrote:
> > On Tue, Mar 03, 2026 at 03:03:47PM +0000, Shameer Kolothum wrote:
> > > PCIe ATS may be disabled by platform firmware, root complex limitations,
> > > or kernel policy even when a device advertises the ATS capability in its
> > > PCI configuration space.
> > >
> > > Add a new IOMMU_CAP_PCI_ATS_NOT_SUPPORTED capability to allow IOMMU
> > > drivers to report the effective ATS decision for a device.
> >
> > nit: Maybe have IOMMU_CAP_PCI_ATS_SUPPORTED instead of the negative
> > variant of the capability. As this will affect only new users who might
> > check this capability so this does not really have to be negative.
>
> It is deliberately negative for UAPI reasons.
>
> Currently all userspace should assume ATS is supported.
>
> The negative means new userspace that is sensitive will still work
> correctly on old and new kernels that both return 0 as meaning ATS is
> supported.

Ah yes, for old userspace missing cap means ATS is supported.
> Perhaps this should be touched on in the commit message.

+1
> Jason