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

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Message-ID: <CACePvbUnY_0HXmfBH5Y2fASZTw1aBLQhxCMXKCNOEKHTf4NnyQ@mail.gmail.com>
Date: Wed, 3 Dec 2025 01:20:44 +0400
From: Chris Li <chrisl@...nel.org>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Lukas Wunner <lukas@...ner.de>, David Matlack <dmatlack@...gle.com>, 
	Pasha Tatashin <pasha.tatashin@...een.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>, 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>, Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Subject: Re: [PATCH 02/21] PCI: Add API to track PCI devices preserved across
 Live Update

On Tue, Dec 2, 2025 at 10:19 PM Jason Gunthorpe <jgg@...dia.com> wrote:
>
> On Tue, Dec 02, 2025 at 08:36:53PM +0400, Chris Li wrote:
>
> > Jason, please correct me if I am wrong. My understanding is that not
> > only the device that is actively doing the DMA requires the bus number
> > to stay the same, I think all the parent bridge, all the way to the
> > root PCI host bridge, bus number must remain the same. After all, the
> > DMA will need to route through the parent bridges.
>
> The completions need to route back through the parent bridges, so yes
> you cannot do anything to disturb RID based routing in the active
> fabric either, with also means few changes to the subordinate bus
> range of any bridge are possible.

Thank you Jason for the confirmation.

Lukas, that means if we are using the path, we will need to save the
bus number along each path node. Different liveupdate devices might
share the parent bridges, we might want to de-duplicate that. Then you
end up with something very similar to the BDF design, where the path
part is just redundant if you have BDF.

That is what I mean previously, using the BDF has the same protections
as path design, just simpler.

Chris