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

[Nicolin Chen]

Hi Jason,

On Fri, Dec 19, 2025 at 01:05:51PM -0400, Jason Gunthorpe wrote:
> +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)
[...]
> +	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?

I'm having a bit trouble justifying this INV_TYPE_S2_VMID_VSMMU.

Since it's the same S2 domain/iopt, if anything that is attached
(whether nested or naked) changes the S2 iopt, we should always
flush the nested S1 domain too, right?

If so, a naked S2 STE should refer to the same VMID in order to
allow its following INV_TYPE_S2_VMID_S1_CLEAR to work properly?

Thanks
Nicolin