Re: [PATCH RFCv1 02/10] hw/arm/virt: Add iommufd link to virt-machine

[Nicolin Chen]

Hi Eric,

Thanks for the comments!

On Tue, Jul 09, 2024 at 11:11:56AM +0200, Eric Auger wrote:
> On 6/26/24 02:28, Nicolin Chen wrote:
> > A nested SMMU must use iommufd ioctls to communicate with the host-level
> > SMMU instance for 2-stage translation support. Add an iommufd link to the
> > ARM virt-machine, allowing QEMU command to pass in an iommufd object.

> If I am not wrong vfio devices are allowed to use different iommufd's
> (although there is no real benefice). So this command line wouldn't
> match with that option.

I think Jason's remarks highlighted that FD should be one per VM:
https://lore.kernel.org/qemu-devel/20240503141024.GE3341011@nvidia.com/

> Also while reading the commit msg it is not clear with the iommufd is
> needed in the machine whereas the vfio iommufd BE generally calls those
> ioctls.

I think I forgot to revisit it. Both intel_iommu and smmu-common
used to call iommufd_backend_connect() for counting, so there was
a need to pass in the same iommufd handler to the viommu driver.
For SMMU, since it is created in the virt code, we had to pass in
with this patch.

That being said, it looks like intel_iommu had removed that. So,
likely we don't need an extra user counting for SMMU too.

Thank you
Nicolin