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.
