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

[Shameer Kolothum Thodi]

> -----Original Message-----
> From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Sent: 03 March 2026 23:29
> To: Samiullah Khawaja <skhawaja@xxxxxxxxxx>
> Cc: Shameer Kolothum Thodi <skolothumtho@xxxxxxxxxx>;
> iommu@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; joro@xxxxxxxxxx;
> will@xxxxxxxxxx; robin.murphy@xxxxxxx; baolu.lu@xxxxxxxxxxxxxxx;
> kevin.tian@xxxxxxxxx; Nicolin Chen <nicolinc@xxxxxxxxxx>; Nathan Chen
> <nathanc@xxxxxxxxxx>; Matt Ochs <mochs@xxxxxxxxxx>; Krishnakant Jaju
> <kjaju@xxxxxxxxxx>
> Subject: Re: [PATCH 1/2] iommu: Add device ATS not supported capability
> 
> 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.
> 
> Perhaps this should be touched on in the commit message.

Right, this is mentioned in the cover letter. I will update the commit
message as well.

Thanks,
Shameer