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

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Message-ID: <aXj6/yhMoveAHv2w@Asurada-Nvidia>
Date: Tue, 27 Jan 2026 09:50:55 -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 v2 10/10] iommu/arm-smmu-v3: Allow sharing domain across
 SMMUs

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.

Thanks
Nicolin