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: <aVV+KWzpxPPfaNw4@Asurada-Nvidia>
Date: Wed, 31 Dec 2025 11:48:57 -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

On Tue, Dec 30, 2025 at 10:52:55AM -0800, Nicolin Chen wrote:
> 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?

I figured that having separate VMIDs on a nest_parent S2 isn't
a problem since invalidation will go through all the VMIDs. And
having an INV_TYPE_S2_VMID_VSMMU is for its unique lifecycle.

Nicolin