Re: [PATCH v6 05/12] PCI: liveupdate: Keep bus numbers constant during Live Update

[Pranjal Shrivastava <praan@google.com>]

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 <dmatlack@google.com>
> ---
>  .../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 <praan@google.com>

Thanks,
Praan