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

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Message-ID: <CA+CK2bBJ_RoRuCxiHuraDH4Gya-ZON3S6PE9PgPfsxObvBRY4w@mail.gmail.com>
Date: Thu, 2 Oct 2025 17:30:53 -0400
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: Samiullah Khawaja <skhawaja@...gle.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

> Maybe finish is too broad grained? What if each session had a finish?
> All the objects in the session are cleaned up, invoke the session
> finish and the memfd's in the session unfreeze?

All sessions have their own finish:
https://lore.kernel.org/all/20250929010321.3462457-15-pasha.tatashin@soleen.com
LIVEUPDATE_SESSION_SET_EVENT

Each session can go into a "finished" state independently. However, I
am still thinking about whether a dependency graph is needed. I feel
that if we require FDs to be added to a session in a specific order
(i.e., dependencies must be added first), and every subsequent FD
checks that all prerequisites are already in the session via the
existing can_preserve() callback, we should be okay, as long as we
finish() them in reverse order.

There are two issues:
1. What do we do with LIVEUPDATE_SESSION_UNPRESERVE_FD ?
We can simply remove this IOCTL all together. Stuff can be unpreserved
by simply closing session FD.

2. Remembering this order on the way back, and since we are using the
token as an iterator, that is not going to work, unless the graph is
also preserved. However, now that we have sessions and the token
values are independent for each session, I am thinking we can go back
to the model where the kernel issues tokens when FDs are preserved, as
each session will always start from token=0. This way FD preservation
order and token order will always match.

Pasha