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

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Message-ID: <aSrMSRd8RJn2IKF4@wunner.de>
Date: Sat, 29 Nov 2025 11:34:49 +0100
From: Lukas Wunner <lukas@...ner.de>
To: David Matlack <dmatlack@...gle.com>
Cc: 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>,
	Pasha Tatashin <pasha.tatashin@...een.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 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.

Thanks,

Lukas