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

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Message-ID: <aVQfhaqfvfLFNZqe@Asurada-Nvidia>
Date: Tue, 30 Dec 2025 10:52:53 -0800
From: Nicolin Chen <nicolinc@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: <will@...nel.org>, <robin.murphy@....com>, <joro@...tes.org>,
	<jpb@...nel.org>, <praan@...gle.com>, <miko.lenczewski@....com>,
	<linux-arm-kernel@...ts.infradead.org>, <iommu@...ts.linux.dev>,
	<linux-kernel@...r.kernel.org>, <patches@...ts.linux.dev>
Subject: Re: [PATCH v1 2/9] iommu/arm-smmu-v3: Add alloc_id/free_id functions
 to arm_smmu_invs

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