Re: [PATCH v1 2/9] iommu/arm-smmu-v3: Add alloc_id/free_id functions to arm_smmu_invs

On Fri, Jan 02, 2026 at 11:57:15AM -0400, Jason Gunthorpe wrote:

On Tue, Dec 30, 2025 at 10:52:53AM -0800, Nicolin Chen wrote:

quoted

Hi Jason,

On Fri, Dec 19, 2025 at 01:05:51PM -0400, Jason Gunthorpe wrote:

quoted

+static int arm_smmu_get_tag(struct arm_smmu_domain *smmu_domain,
+			    struct arm_smmu_master *master,
+			    struct arm_vsmmu *vsmmu,
+			    struct arm_smmu_iotlb_tag *tag, bool no_alloc)

[...]

quoted

+	case ARM_SMMU_DOMAIN_S2:
+		if (smmu_domain->nest_parent) {
+			/* FIXME we can support attaching a nest_parent without
+			 * a vsmmu, but to do that we need to fix
+			 * arm_smmu_get_id_from_invs() to never return the vmid
+			 * of a vsmmu. Probably by making a
+			 * INV_TYPE_S2_VMID_VSMMU */
+			id = vsmmu->vmid;
+			return 0;
+		}

Would you mind elaborating why arm_smmu_get_id_from_invs() can't
return vsmmu->vmid to share with a naked S2 STE?

A "naked" S2 domain doesn't have a pointer to the vsmmu, so it is
impossible to get vsmmu->vmid.

An S2 parent domain should be per VM. And a vSMMU on top of an S2
should be per SMMU. So, it could have stored a list of vSMMUs and
device attaching to a naked S2 could match its master->smmu with
vSMMU->smmu in the list?

But, it's probably not very efficient, so I took your advise and
added an INV_TYPE_S2_VMID_VSMMU. Overall, this looks clean to me.

The only domains which have it are nested domains using a bypass vSTE.

So, if userspace attaches a bypass vSTE then it should use vsmmu->vmid

However if they attach a raw S2 HWPT without a vSTE then there is no
vsmmu and it should work like any other S2 attach and allocate a VMID
for this domain, ignoring any vSMMU that may exist.

In sort, the only case where we use the vsmmu->vmid is for vSTEs.

Yea, I think I got these right. I have all the code reworked and
will run some tests and post v2.

Thanks
Nicolin