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: <aWnI3Ii3KCFmJV3/@Asurada-Nvidia>
Date: Thu, 15 Jan 2026 21:13:00 -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 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:
> > 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?
> 
> 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