linux-kernel - Re: [PATCH v2 10/10] iommu/arm-smmu-v3: Allow sharing domain across SMMUs

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Message-ID: <20260127175936.GO1134360@nvidia.com>
Date: Tue, 27 Jan 2026 13:59:36 -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 10/10] iommu/arm-smmu-v3: Allow sharing domain across
 SMMUs

On Tue, Jan 27, 2026 at 09:50:55AM -0800, Nicolin Chen wrote:
> On Tue, Jan 27, 2026 at 11:41:11AM -0400, Jason Gunthorpe wrote:
> > On Wed, Jan 21, 2026 at 05:24:28PM -0800, Nicolin Chen wrote:
> > > VMM needs a domain holding the mappings between gPA to hPA. It can be an S1
> > > domain or an S2 nesting parent domain, depending on whether the VM is built
> > > with a vSMMU or not.
> > > 
> > > Given that the IOAS for this gPA mapping is the same across SMMU instances,
> > > this domain can be shared across devices even if they sit behind different
> > > SMMUs, so long as the underlying page table is compatible between the SMMU
> > > instances.
> > > 
> > > There is no direct information about the page table from the master device,
> > > but a comparison can be done between the physical SMMU that the domain was
> > > allocated for and the physical SMMU that the device is behind.
> > 
> > I would very much prefer this works by inspecting the cfg from the
> > iopgtable..
> > 
> > You can get it by doing 
> > 
> > 	struct io_pgtable_cfg *pgtbl_cfg =
> > 		&io_pgtable_ops_to_pgtable(domain->pgtbl_ops)->cfg;
> 
> OK. I will make it a detailed scan
> 
> > I think it is important that the domain->smmu pointer be removed as
> > well
> 
> I will try that too.
> 
> There are smmu_domain->smmu validations in two SVA functions,
> which presumably might be replaced with this can_share() too.

Those are because we replicate the mmu notifier for every
instance. That can now be revised too, I think. Let's leave that for
another series after this and leave the smmu pointer for now, but
don't use it outside that SVA stuff.

Jason