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

[David Matlack <dmatlack@google.com>]

On 2026-09-10 06:51 PM, Bjorn Helgaas wrote:
> 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.

To protect against pci_has_incoming_preserved_devices() flipping from
true to false while the PCI core is in the middle of a scan. It is not
likely to ever happen given most host bridge scanning should happen
during early boot, but theoretically possible with the way the PCI core
code is structured. I did not see way to structurally ensure these 2
things cannot race. A lot of the host bridge scanning happens without
taking the rescan lock, for example.