Re: [PATCH] iommu: Allow ATS to work on VFs when the PF uses IDENTITY

On 2024/8/9 21:28, Jason Gunthorpe wrote:

On Fri, Aug 09, 2024 at 02:36:14AM +0000, Tian, Kevin wrote:

quoted

quoted

From: Jason Gunthorpe <jgg@nvidia.com>
Sent: Thursday, August 8, 2024 2:19 AM

PCI ATS has a global Smallest Translation Unit field that is located in
the PF but shared by all of the VFs.

The expectation is that the STU will be set to the root port's global STU
capability which is driven by the IO page table configuration of the iommu
HW. Today it becomes set when the iommu driver first enables ATS.

Thus, to enable ATS on the VF, the PF must have already had the correct
STU programmed, even if ATS is off on the PF.

Unfortunately the PF only programs the STU when the PF enables ATS. The
iommu drivers tend to leave ATS disabled when IDENTITY translation is
being used.

I think this is the common practice as it is not necessary to enable ATS
since iommu is passthrough mode. :)

quoted

Is there more context on this?

How do you mean?

quoted

Looking at intel-iommu driver ATS is disabled for IDENETITY when
the iommu is in legacy mode:

dmar_domain_attach_device()
{
	...
	if (sm_supported(info->iommu) || !domain_type_is_si(info->domain))
		iommu_enable_pci_caps(info);
	...
}

But this follows what VT-d spec says (section 9.3):

TT: Translate Type
10b: Untranslated requests are processed as pass-through. The SSPTPTR
field is ignored by hardware. Translated and Translation Requests are
blocked.

Yes, HW like this is exactly the problem, it ends up not enabling ATS
on the PF and then we don't have the STU programmed so the VF is
effectively disabled too.

Ideally iommus would continue to work with translated requests when
ATS is enabled. 

As Kevin's pasting, the Translated requests will be blocked. So it does
not work. :(

Not supporting this configuration creates a nasty
problem for devices that are using PASID.

The PASID may require ATS to be enabled (ie SVA), but the RID may be
IDENTITY for performance. The poor device has no idea it is not
allowed to use ATS on the RID side :(

If this is the only problematic case, the intel iommu driver in this
patch could check the scalable mode before enabling ATS in the
probe_device() op. In this way, the legacy mode iommu would keep the old
ATS enable policy.

Seems an alternative is to use paging domain for IDENTITY. This means
the TT would not be 10b, hence it would work with ATS enabled.

-- 
Regards,
Yi Liu