[RFC PATCH v3 4/5] iommu/arm-smmu-v3-iommufd: Use KVM VMID for s2 stage

[Jason Gunthorpe]

On Wed, Mar 19, 2025 at 05:32:01PM +0000, Shameer Kolothum wrote:
> +static int arm_vsmmu_alloc_vmid(struct arm_smmu_device *smmu, struct kvm *kvm,
> +				bool *kvm_used)
> +{
> +#ifdef CONFIG_KVM
> +	/*
> +	 * There can only be one allocator for VMIDs active at once. If BTM is
> +	 * turned on then KVM's allocator always supplies the VMID, and the
> +	 * VMID is matched by CPU invalidation of the KVM S2. Right now there
> +	 * is no API to get an unused VMID from KVM so this also means BTM systems
> +	 * cannot support S2 without an associated KVM.
> +	 */
> +	if ((smmu->features & ARM_SMMU_FEAT_BTM)) {
> +		int vmid;
> +
> +		if (!kvm || !kvm_get_kvm_safe(kvm))
> +			return -EOPNOTSUPP;

Isn't kvm_get_kvm_safe() in modular KVM code most of the time? You can't
call it like this right?

Per my prior comments, I'm a little nervous to make drivers keep track
of this refcount instead of the core code but this does seem like it
could work..

I also think you should block using the S2 without a viommu & KVM in
BTM mode.. Maybe like this:

@@ -2938,8 +2938,16 @@ static int arm_smmu_attach_dev(struct iommu_domain *domain, struct device *dev)
                cdptr = arm_smmu_alloc_cd_ptr(master, IOMMU_NO_PASID);
                if (!cdptr)
                        return -ENOMEM;
-       } else if (arm_smmu_ssids_in_use(&master->cd_table))
-               return -EBUSY;
+       } else {
+               if (arm_smmu_ssids_in_use(&master->cd_table))
+                       return -EBUSY;
+               /*
+                * S2 cannot be used when BTM is turned on without a VIOMMU and
+                * VMID shared with KVM
+                */
+               if (smmu->features & ARM_SMMU_FEAT_BTM)
+                       return -EOPNOTSUPP;
+       }

That more closely matches how this will eventually work when the VMID
allocation is properly made to be local to the viommu.

Jason