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

[Pasha Tatashin <pasha.tatashin@soleen.com>]

On 2026-06-06 11:10:29+00:00, Pranjal Shrivastava wrote:
> 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 

+1

> 
> 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