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

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Message-ID: <CA+CK2bB0V9jdmrcNjgsmWHmSFQpSpxdVahf1pb3Bz2WA3rKcng@mail.gmail.com>
Date: Sat, 29 Nov 2025 20:20:34 -0500
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Jason Gunthorpe <jgg@...dia.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 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?

Pasha

>
> Jason