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

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Message-ID: <CA+CK2bDm5bpEgtw5U_ENkAsh28QJ+hQC+YF95zKKV0+ugTVMOQ@mail.gmail.com>
Date: Sat, 29 Nov 2025 15:10:32 -0500
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Lukas Wunner <lukas@...ner.de>
Cc: 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>, 
	Jason Gunthorpe <jgg@...dia.com>, Jason Gunthorpe <jgg@...pe.ca>, 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 5:34 AM Lukas Wunner <lukas@...ner.de> 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.
>
> The most portable way to identify PCI devices across kernels is to
> store their path from the root down the hierarchy.  Because the bus
> number might change but the device/function number on each bus stays
> the same.
>
> This is what EFI does with device paths:
> https://uefi.org/specs/UEFI/2.10/10_Protocols_Device_Path_Protocol.html
>
> Example:
> Acpi(PNP0A03,0)/Pci(1E|0)/Pci(0|0)
>
> Source:
> https://raw.githubusercontent.com/tianocore-docs/edk2-UefiDriverWritersGuide/main/3_foundation/39_uefi_device_paths/README.9.md
>
> We've got a device path *parser* in drivers/firmware/efi/dev-path-parser.c,
> but we don't have a *generator* for device paths in the kernel yet.

Hi Lukas,

Thanks for the input.

You are right that bus numbers can change in standard boot scenarios.
However, for Live Update, we skip firmware, and we would likely list
pci=assign-busses as an unsupported parameter. So, BDF should be
sufficient.

That said, if there is a better method using a stable hierarchical
path, and more importantly, if that method can be extended to other
bus types, we are open to considering it. The main hurdle is that we
would need a way to generate this stable path in the kernel and also
parse it during early boot.

Thanks,
Pasha