linux-kernel - Re: [PATCH 02/21] PCI: Add API to track PCI devices preserved across Live Update

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Message-ID: <20251201132934.GA1075897@nvidia.com>
Date: Mon, 1 Dec 2025 09:29:34 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Pasha Tatashin <pasha.tatashin@...een.com>
Cc: Lukas Wunner <lukas@...ner.de>, David Matlack <dmatlack@...gle.com>,
	Alex Williamson <alex@...zbot.org>,
	Adithya Jayachandran <ajayachandra@...dia.com>,
	Alex Mastro <amastro@...com>, Alistair Popple <apopple@...dia.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>, Chris Li <chrisl@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Jacob Pan <jacob.pan@...ux.microsoft.com>,
	Josh Hilke <jrhilke@...gle.com>, Kevin Tian <kevin.tian@...el.com>,
	kvm@...r.kernel.org, Leon Romanovsky <leonro@...dia.com>,
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
	linux-pci@...r.kernel.org, Mike Rapoport <rppt@...nel.org>,
	Parav Pandit <parav@...dia.com>,
	Philipp Stanner <pstanner@...hat.com>,
	Pratyush Yadav <pratyush@...nel.org>,
	Saeed Mahameed <saeedm@...dia.com>,
	Samiullah Khawaja <skhawaja@...gle.com>,
	Shuah Khan <shuah@...nel.org>, Tomita Moeko <tomitamoeko@...il.com>,
	Vipin Sharma <vipinsh@...gle.com>, William Tu <witu@...dia.com>,
	Yi Liu <yi.l.liu@...el.com>, Yunxiang Li <Yunxiang.Li@....com>,
	Zhu Yanjun <yanjun.zhu@...ux.dev>
Subject: Re: [PATCH 02/21] PCI: Add API to track PCI devices preserved across
 Live Update

On Sat, Nov 29, 2025 at 08:20:34PM -0500, Pasha Tatashin wrote:
> On Sat, Nov 29, 2025 at 7:51 PM Jason Gunthorpe <jgg@...dia.com> wrote:
> >
> > On Sat, Nov 29, 2025 at 11:34:49AM +0100, Lukas Wunner wrote:
> > > On Wed, Nov 26, 2025 at 07:35:49PM +0000, David Matlack wrote:
> > > > Add an API to enable the PCI subsystem to track all devices that are
> > > > preserved across a Live Update, including both incoming devices (passed
> > > > from the previous kernel) and outgoing devices (passed to the next
> > > > kernel).
> > > >
> > > > Use PCI segment number and BDF to keep track of devices across Live
> > > > Update. This means the kernel must keep both identifiers constant across
> > > > a Live Update for any preserved device.
> > >
> > > While bus numbers will *usually* stay the same across next and previous
> > > kernel, there are exceptions.  E.g. if "pci=assign-busses" is specified
> > > on the command line, the kernel will re-assign bus numbers on every boot.
> >
> > Stuff like this has to be disabled for this live update stuff, if the
> > bus numbers are changed it will break the active use of the iommu
> > across the kexec.
> >
> > So while what you say is all technically true, I'm not sure this is
> > necessary.
> 
> I agree. However, Lukas's comment made me wonder about the future: if
> we eventually need to preserve non-PCI devices (like a TPM), should we
> be designing a common identification mechanism for all buses now? Or
> should we settle on BDF for PCI and invent stable identifiers for
> other bus types as they become necessary?

Well, at least PCI subsystem should use BDF..

You are probably right that the matching of preserved data to a struct
device should be more general though.

Jason