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

[Nicolin Chen <nicolinc@nvidia.com>]

On Mon, Oct 27, 2025 at 06:38:08PM +0100, Eric Auger wrote:
> 
> 
> On 10/27/25 6:13 PM, Nicolin Chen wrote:
> > 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?
> 
> in 06/27 you still can have emulated EPs hotplugged in the loop, no?
> 
> I remember some discussions with Peter who was also reluctant in general
> to introduce some partial feature support. I think in general this is a
> good policy to have features emulated and then accelerated. That's also
> good for testing in case we can bring up some test env.

Hmm, that sounds a legit reason, though adding the ATS support to
the emulated SMMUv3 isn't seemingly a small effort...

Also, do we have any emulated EP to test out ATS translation and
invalidation from the emulated SMMUv3?

Thanks
Nicolin