linux-kernel - Re: [PATCH v2 06/10] iommu/arm-smmu-v3: Allocate vmid in arm_vsmmu

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Message-ID: <20260127151138.GI1134360@nvidia.com>
Date: Tue, 27 Jan 2026 11:11:38 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...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 v2 06/10] iommu/arm-smmu-v3: Allocate vmid in
 arm_vsmmu_init

On Mon, Jan 26, 2026 at 07:06:24PM -0800, Nicolin Chen wrote:
> On Mon, Jan 26, 2026 at 05:16:36PM -0400, Jason Gunthorpe wrote:
> > On Wed, Jan 21, 2026 at 05:24:24PM -0800, Nicolin Chen wrote:
> > > VMID owned by a vSMMU should be allocated in the viommu_init callback, as
> > > HW like tegra241-cmdqv needs to setup VINTF with the VMID.
> > 
> > Even the architected SMMU needs this, the VMS (not implemented in
> > Linux) should all share the same VMID for the same VM.
> 
> But for standard SMMU, the allocation/sharing could happen at the
> device attachment to a nested (maybe bypass proxy) domain, right?
> 
> vmid is only needed for STE (attach) and invalidation.
> 
> Or do you see some other case where vmid must be allocated during
> viommu_init?

It has to be tied to the VSMMU, it doesn't matter if a single VMID is
allocated on-demand for the VSMMU or for the lifetime, it has to be a
single ID and shared across all STEs.

It is easy to understand that lifecycle by just allocating it for the
lifetime of the VSMMU..

Jason