Re: [PATCH v8 06/12] PCI: liveupdate: Auto-preserve upstream bridges across Live Update

[David Matlack <dmatlack@google.com>]

On 2026-09-10 06:51 PM, Bjorn Helgaas wrote:
> On Tue, Jul 28, 2026 at 10:10:00PM +0000, David Matlack wrote:
> > When a PCI device is preserved across a Live Update, all of its upstream
> > bridges up to the root port must also be preserved. This enables the PCI
> > core and any drivers bound to the bridges to manage bridges correctly
> > across a Live Update.
> > 
> > Notably, this will be used in subsequent commits to ensure that
> > preserved devices can continue performing memory transactions without a
> > disruption or change in routing.
> > 
> > To preserve bridges, the PCI core tracks the number of downstream
> > devices preserved under each bridge using a reference count in struct
> > pci_dev_ser. This allows a bridge to remain preserved until all its
> > downstream preserved devices are unpreserved or finish their
> > participation in the Live Update.
> 
> This seems to hint that we're going to allow bridge reconfiguration in
> some cases, e.g., for hot-adds.  The simplest case is "leave config of
> all bridges the same", and I thought that was what the previous patch
> commit log said.
> 
> What's the benefit added by this patch?

It is used in the following patches:

 PCI: liveupdate: Adopt ACS controls in incoming preserved devices
 PCI: liveupdate: Adopt ARI Forwarding Enable on preserved bridges
 PCI: liveupdate: Do not disable bus mastering on preserved devices during kexec

to preserve certain configuration on bridges that have downstream
endpoints that are being preserved. To support P2PDMA we will also have
to preserve bridge memory windows (future series).

If we are ok with applying those policies to all bridges on the system
whenever one or more endpoints anywhere on the system are being
preserved, then I agree we don't need this patch. But I thought it would
be cleaner to track things per-device.