linux-kernel - Re: [RFC PATCH 13/15] iommufd: Persist iommu domains for live update

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Message-ID: <20251003120936.GN3195829@ziepe.ca>
Date: Fri, 3 Oct 2025 09:09:36 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Samiullah Khawaja <skhawaja@...gle.com>
Cc: Pasha Tatashin <pasha.tatashin@...een.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Lu Baolu <baolu.lu@...ux.intel.com>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>, iommu@...ts.linux.dev,
	YiFei Zhu <zhuyifei@...gle.com>,
	Robin Murphy <robin.murphy@....com>,
	Pratyush Yadav <pratyush@...nel.org>,
	Kevin Tian <kevin.tian@...el.com>, linux-kernel@...r.kernel.org,
	Saeed Mahameed <saeedm@...dia.com>,
	Adithya Jayachandran <ajayachandra@...dia.com>,
	Parav Pandit <parav@...dia.com>,
	Leon Romanovsky <leonro@...dia.com>, William Tu <witu@...dia.com>,
	Vipin Sharma <vipinsh@...gle.com>, dmatlack@...gle.com,
	Chris Li <chrisl@...nel.org>, praan@...gle.com
Subject: Re: [RFC PATCH 13/15] iommufd: Persist iommu domains for live update

On Thu, Oct 02, 2025 at 04:56:57PM -0700, Samiullah Khawaja wrote:
> > So if you have a notion that finish is disallowed and when it is
> > actually finished maybe the order doesn't matter?
> 
> I think FINISH for FDs in a SESSION is not atomic. If a dependency
> memfd gets its FINISH call first, it might make itself mutable before
> the iommufd FINISH callback fails because old HWPT is not replaced
> yet. By then, it would be too late; the memfd has already become
> mutable. That is why order would be needed.

I'm thinking of having an counter in the session and the iommu_domain
holds it elevated until it is destroyed. Finish can't even start until
the counter is 0.

If the counter is 0 then it is fine to unfreeze all the remaning
objects in any order.

Jason