Re: [PATCH v2] docs/system/arm/virt: Document user-creatable SMMUv3

[Eric Auger <eric.auger@redhat.com>]

On 10/27/25 12:27 PM, Peter Maydell wrote:
> On Fri, 24 Oct 2025 at 09:46, Shameer Kolothum <skolothumtho@nvidia.com> wrote:
>> The virt machine now supports creating multiple SMMUv3 instances, each
>> associated with a separate PCIe root complex.
>>
>> Update the documentation with an example.
>>
>> Signed-off-by: Shameer Kolothum <skolothumtho@nvidia.com>
> Thanks. I have some minor word-smithing tweaks, but I'll just
> apply them to this patch in target-arm.next if you're OK with
> that:
>
>
> --- a/docs/system/arm/virt.rst
> +++ b/docs/system/arm/virt.rst
> @@ -224,23 +224,24 @@ SMMU configuration
>  """"""""""""""""""
>
>  Machine-wide SMMUv3 IOMMU
> -  See the machine-specific ``iommu`` option above. This allows specifying
> -  a single, machine-wide SMMUv3 instance that applies to all devices in
> -  the PCIe topology.
> +  Setting the machine-specific option ``iommu=smmuv3`` causes QEMU to
> +  create a single, machine-wide SMMUv3 instance that applies to all
> +  devices in the PCIe topology.
>
>    For information about selectively bypassing devices, refer to
>    ``docs/bypass-iommu.txt``.
>
>  User-creatable SMMUv3 devices
> -  Allows creating multiple user-defined SMMUv3 devices, each associated
> -  with a separate PCIe root complex. This is only permitted if the
> -  machine-wide SMMUv3 (``iommu=smmuv3``) option is not used.
> +  You can use the ``-device arm-smmuv3`` option to create multiple
> +  user-defined SMMUv3 devices, each associated with a separate PCIe
> +  root complex. This is only permitted if the machine-wide SMMUv3
> +  (``iommu=smmuv3``) option is not used. Each ``arm-smmuv3`` device
> +  uses the ``primary-bus`` sub-option to specify which PCIe root
> +  complex it is associated with.
>
>    This model is useful when you want to mirror a host configuration where
>    each NUMA node typically has its own SMMU, allowing the VM topology to
> -  align more closely with the host’s hardware layout. Supporting multiple
> -  SMMUv3 instances is also a prerequisite for future accelerated SMMUv3
> -  support.
> +  align more closely with the host’s hardware layout.
>
>    Example::
>
>
>
>
> (I deleted the sentence about SMMU acceleration because that
> isn't upstream yet. When it lands then we can update the
> docs if the version of it that lands can't support acceleration
> of a single machine-wide accelerator.)
>
> thanks
> -- PMM
>
Looks good to me as well

Fell free to add my R-b

Reviewed-by: Eric Auger <eric.auger@redhat.com>

Thanks!

Eric