Re: [PATCH v8 05/12] PCI: liveupdate: Preserve bus numbers during Live Update

[Bjorn Helgaas <helgaas@kernel.org>]

On Tue, Jul 28, 2026 at 10:09:59PM +0000, David Matlack wrote:
> During a Live Update, preserved devices must be allowed to continue
> performing memory transactions so the kernel cannot change the fabric
> topology, including bus numbers, since that would require disabling and
> flushing any memory transactions first.
> 
> To keep bus numbers constant, always preserve the secondary and
> subordinate bus numbers assigned to bridges during scanning, instead of
> assigning new ones, if any PCI devices were preserved. Note that the
> kernel preserves bus numbers even on bridges without any downstream
> endpoints that were preserved. This avoids accidentally assigning a
> bridge a new window that overlaps with a preserved device that is
> downstream of a different bridge.

> +bool pci_liveupdate_preserve_bus_numbers(struct pci_bus *bus, struct pci_dev *dev)
> +{
> +	struct pci_dev *parent = bus->self;
> +
> +	if (dev->liveupdate.preserve_bus_numbers)
> +		return true;
> +
> +	if (parent && parent->liveupdate.preserve_bus_numbers) {
> +		/*
> +		 * Preserve bus numbers if the parent bridge is required to
> +		 * preserve bus numbers. Otherwise the PCI core could expand
> +		 * this bridge's reservation beyond its parent (which cannot
> +		 * expand).
> +		 */
> +		dev->liveupdate.preserve_bus_numbers = true;
> +	} else {
> +		/*
> +		 * Otherwise preserve bus numbers if there are any incoming
> +		 * preserved devices. This ensures that the PCI core does not
> +		 * allocate a bus number to a non-preserved device that
> +		 * conflicts with the bus number already assigned to a preserved
> +		 * device.
> +		 *
> +		 * This is slightly more restrictive than it needs to be. For
> +		 * example, each host bridges have their own range of bus
> +		 * numbers that won't conflict with other host bridges. But the
> +		 * previous kernel should have assigned a sane bus topology and
> +		 * it is simpler to just adopt that entire topology.

s/each ... bridges have their/each ... bridge has its/

It's true there should be no bus number conflicts between host
bridges, but it does require the domain as well.

> +		 */
> +		dev->liveupdate.preserve_bus_numbers =
> +			pci_has_incoming_preserved_devices();
> +	}
> +
> +	return dev->liveupdate.preserve_bus_numbers;

I'm not sure why you don't just return
pci_has_incoming_preserved_devices() in all cases, which is what the
commit log suggests this patch does.  What's gained by all the logic
here?  It's not like devices will be hot-added during the kexec.