RE: [PATCH v1 4/4] iommu/arm-smmu-v3-iommufd: Allow a shared s2_parent to allocate vSMMU

[Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>]

> -----Original Message-----
> From: Nicolin Chen <nicolinc@nvidia.com>
> Sent: Wednesday, March 5, 2025 5:04 AM
> To: will@kernel.org; robin.murphy@arm.com; jgg@nvidia.com
> Cc: joro@8bytes.org; linux-arm-kernel@lists.infradead.org;
> iommu@lists.linux.dev; linux-kernel@vger.kernel.org; Shameerali Kolothum
> Thodi <shameerali.kolothum.thodi@huawei.com>
> Subject: [PATCH v1 4/4] iommu/arm-smmu-v3-iommufd: Allow a shared
> s2_parent to allocate vSMMU
> 
> Now, vmids are stored in vSMMU objects. So all vSMMUs assigned to the
> same
> VM can share a s2_parent domain. This means a vIOMMU allocation per
> device
> behind one SMMU can be given with a s2_parent domain that's allocated
> per
> another device behind another SMMU, i.e. s2_parent->smmu != master-
> >smmu.
> 
> Remove the validation line to allow this use case.
> 
> Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
> ---
>  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c
> b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c
> index 2c5a9d0abed5..9bfa5fa5bafa 100644
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c
> @@ -378,9 +378,6 @@ struct iommufd_viommu *arm_vsmmu_alloc(struct
> device *dev,
>  	if (!(smmu->features & ARM_SMMU_FEAT_NESTING))
>  		return ERR_PTR(-EOPNOTSUPP);
> 
> -	if (s2_parent->smmu != master->smmu)
> -		return ERR_PTR(-EINVAL);
> -

Not sure we can just relax this like this. What if the two physical SMMUs are different in
functionality/features? Do we need some kind of sanity check here?

Thanks,
Shameer