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

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

On 7/9/24 18:59, Nicolin Chen wrote:
> 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/
OK I thought this was still envisionned althought not really meaningful.
By the way, please add Yi and Zhenzhong in cc since thre problematics
are connected I think.
>
>> 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.
OK at least it deserves some explanation about the "why"

Thanks

Eric
>
> Thank you
> Nicolin
>