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Date: Wed, 3 Dec 2025 06:44:34 +0100
From: Lukas Wunner <lukas@wunner.de>
To: Chris Li <chrisl@kernel.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>, David Matlack <dmatlack@google.com>,
	Pasha Tatashin <pasha.tatashin@soleen.com>,
	Alex Williamson <alex@shazbot.org>,
	Adithya Jayachandran <ajayachandra@nvidia.com>,
	Alex Mastro <amastro@fb.com>, Alistair Popple <apopple@nvidia.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Bjorn Helgaas <bhelgaas@google.com>,
	David Rientjes <rientjes@google.com>,
	Jacob Pan <jacob.pan@linux.microsoft.com>,
	Josh Hilke <jrhilke@google.com>, Kevin Tian <kevin.tian@intel.com>,
	kvm@vger.kernel.org, Leon Romanovsky <leonro@nvidia.com>,
	linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
	linux-pci@vger.kernel.org, Mike Rapoport <rppt@kernel.org>,
	Parav Pandit <parav@nvidia.com>,
	Philipp Stanner <pstanner@redhat.com>,
	Pratyush Yadav <pratyush@kernel.org>,
	Saeed Mahameed <saeedm@nvidia.com>,
	Samiullah Khawaja <skhawaja@google.com>,
	Shuah Khan <shuah@kernel.org>, Tomita Moeko <tomitamoeko@gmail.com>,
	Vipin Sharma <vipinsh@google.com>, William Tu <witu@nvidia.com>,
	Yi Liu <yi.l.liu@intel.com>, Yunxiang Li <Yunxiang.Li@amd.com>,
	Zhu Yanjun <yanjun.zhu@linux.dev>,
	Ilpo =?iso-8859-1?Q?J=E4rvinen?= <ilpo.jarvinen@linux.intel.com>
Subject: Re: [PATCH 02/21] PCI: Add API to track PCI devices preserved across
 Live Update
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On Tue, Dec 02, 2025 at 08:36:53PM +0400, Chris Li wrote:
> On Tue, Dec 2, 2025 at 6:59 PM Jason Gunthorpe <jgg@nvidia.com> wrote:
> > The device is active
> > during KHO, you CAN NOT do any resource reassignment, not bus numbers,
> > not mmio. It must be fully disabled.
> 
> I agree with Jason. The bus number is used in the low level hardware
> to do the DMA transfer. The bus number can not change for a device
> during livedupate with pending DMA transfer. The BDF MUST remain the
> same as the liveupdate with DMA transfer requirement.

Thank you both for the explanation.

> Another point is that, on the same machine it can have multiple PCI
> host bridges. Each PCI host bridge bus number is acquired from the
> ACPI table walk. I am not aware of any way to get the slot number of
> the PCI host bridge. Lukas, do you know how to get the PCI host bridge
> slot number to form a path?

Host bridges are identified by the segment number.  On ACPI-based systems,
it's retrieved by acpi_pci_root_add() through invocation of the _SEG method.

Thanks,

Lukas