Re: [PATCH v4 22/27] hw/arm/smmuv3-accel: Add support for ATS

[Nicolin Chen via <qemu-devel@nongnu.org>]

Hi Eric,

On Mon, Oct 27, 2025 at 05:59:13PM +0100, Eric Auger wrote:
> On 9/29/25 3:36 PM, Shameer Kolothum wrote:
> > QEMU SMMUv3 does not enable ATS (Address Translation Services) by default.
> > When accelerated mode is enabled and the host SMMUv3 supports ATS, it can
> > be useful to report ATS capability to the guest so it can take advantage
> > of it if the device also supports ATS.
> >
> > Note: ATS support cannot be reliably detected from the host SMMUv3 IDR
> > registers alone, as firmware ACPI IORT tables may override them. The
> > user must therefore ensure the support before enabling it.

> This looks incomplete to me. ATS is a big topic in itself. I would
> prefer we do not advertise it until we do not have full support for it
> (including emulation). Comparing to
> c049bf5bb9dd ("intel_iommu: Add support for ATS") which was recently
> contributed we miss at least translation request implementations
> (PCIIOMMUOps ats_request_translation callback implementation).
> 
> See:
> https://lore.kernel.org/all/20250628180226.133285-11-clement.mathieu--drif@eviden.com/#r

In accelerated SMMUv3 case, ATS translation and invalidation are
done by the physical SMMU. Wondering why do we need this?

IIRC, intel is mixing their emulated translation and accelerated
one, and their pasid table is not integrated like SMMU's CD table
that we already passed entirely via the STE.

> Also in SMMU spec I see other stuff like STE.EATS, ATS skipping stage 1,
> ... Here I only see  SMMU_CMD_ATC_INV and this is only implemented for
> accel. I think I would rather stick to the minimum in this series, ie.
> reject in case of ATS mismatch (for testing purpose you will just need a
> small hack until we get ATS support), work on ATS enablement in parallel

That SMMU_CMD_ATC_INV seems to be the only thing we need for ATS..

Thanks
Nicolin