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

On 2026-09-16 07:18 PM, Bjorn Helgaas wrote:
> On Fri, Sep 11, 2026 at 05:00:10PM +0000, David Matlack wrote:
> > 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.
> 
> Yes, I agree tracking it per-device is good.  I was looking for a
> traversal upstream to increment refcounts on bridges, and I guess that
> happens via for_each_pci_dev_in_path() in pci_liveupdate_preserve().
> 
> The actual refcount still confuses me a bit (see
> https://lore.kernel.org/all/20260917000723.GA992337@bhelgaas).  Maybe
> it would help if pci_liveupdate_preserve_device() alloc the dev_ser
> *first* (right after all the bail-out checks)?  I wonder if the
> refcount increment could then happen in exactly one place, separated
> from the one-time dev_ser housekeeping?  E.g., something like:
> 
>   if (!dev->liveupdate.outgoing) {
>     dev_ser = pci_flb_alloc_dev_ser(outgoing);
>     ...
>     dev->liveupdate.outgoing = dev_ser;
>   }
> 
>   dev->liveupdate.outgoing->refcount++;

Ack, will fix