On Fri, May 22, 2026 at 08:24:03PM +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 inherit the secondary and
> subordinate bus numbers assigned to bridges during scanning, instead of
> assigning new ones, if any PCI devices are being preserved. Note that
> the kernel inherits 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.
>
> If a bridge is scanned with a broken topology or has no bus numbers
> set during a Live Update, refuse to assign it new bus numbers and refuse
> to enumerate devices below it until the Live Update is finished. This is
> a safety measure to prevent topology conflicts.
>
> Require that CONFIG_CARDBUS is not enabled to enable
> CONFIG_PCI_LIVEUPDATE since inheriting bus numbers on PCI-to-CardBus
> bridges requires additional work but is not a priority at the moment.
>
> Signed-off-by: David Matlack <[email protected]>
> ---
> .../admin-guide/kernel-parameters.txt | 6 +-
> drivers/pci/Kconfig | 2 +-
> drivers/pci/liveupdate.c | 83 ++++++++++++++++++-
> drivers/pci/liveupdate.h | 14 ++++
> drivers/pci/probe.c | 17 +++-
> include/linux/pci_liveupdate.h | 4 +
> 6 files changed, 119 insertions(+), 7 deletions(-)
>
[...]
> + incoming = pci_liveupdate_flb_get_incoming();
> + if (!incoming) {
> + dev->liveupdate.inherit_buses = false;
> + goto out;
> + }
> +
> + /*
> + * It is safe to sample incoming->ser->nr_devices and then
> + * drop the rwsem since nr_devices will only decrease. Thus the
> + * only "race" is that the current scan will be overly
> + * conservative and force bus inheritance.
> + */
> + dev->liveupdate.inherit_buses = incoming->ser->nr_devices;
Nit: inherit_buses is a bool, while compiler will handle it correctly,
maybe we could:
dev->liveupdate.inherit_buses = !!incoming->ser->nr_devices
OR
dev->liveupdate.inherit_buses = (incoming->ser->nr_devices > 0)
for readability?
> + pci_liveupdate_flb_put_incoming();
> + }
> +
> +out:
> + return dev->liveupdate.inherit_buses;
> +}
> +
[...]
> /*
> @@ -1497,8 +1501,7 @@ static int pci_scan_bridge_extend(struct pci_bus *bus,
> struct pci_dev *dev,
> * do in the second pass.
> */
> if (!pass) {
> - if (pcibios_assign_all_busses() || broken)
> -
> + if (assign_new_buses || broken)
> /*
> * Temporarily disable forwarding of the
> * configuration cycles on all bridges in
> @@ -1512,6 +1515,11 @@ static int pci_scan_bridge_extend(struct pci_bus *bus,
> struct pci_dev *dev,
> goto out;
> }
>
> + if (liveupdate) {
> + pci_err(dev, "Cannot reconfigure bridge during Live
> Update, skipping\n");
> + goto out;
> + }
Quite helpful! Thanks :)
> +
> /* Clear errors */
> pci_write_config_word(dev, PCI_STATUS, 0xffff);
>
> @@ -1572,6 +1580,7 @@ static int pci_scan_bridge_extend(struct pci_bus *bus,
> struct pci_dev *dev,
> pci_write_config_word(dev, PCI_BRIDGE_CONTROL, bctl);
>
> pm_runtime_put(&dev->dev);
> + pci_liveupdate_scan_bridge_end(dev, pass);
>
> return max;
> }
With the minor nit above,
Reviewed-by: Pranjal Shrivastava <[email protected]>
Thanks,
Praan
