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

[David Matlack <dmatlack@google.com>]

On 2026-09-16 07:07 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.
> > ...
> 
> > +static int pci_liveupdate_unpreserve_device(struct pci_flb_outgoing *outgoing,
> > +					    struct pci_dev *dev)
> >  {
> >  	struct pci_dev_ser *dev_ser = dev->liveupdate.outgoing;
> >  
> >  	if (!dev_ser) {
> >  		pci_warn(dev, "Cannot unpreserve device that is not preserved\n");
> > -		return;
> > +		return -EINVAL;
> > +	}
> > +
> > +	if (!dev_ser->refcount) {
> > +		pci_WARN(dev, 1, "Preserved device has a 0 refcount!\n");
> > +		return -EINVAL;
> >  	}
> >  
> > +	if (--dev_ser->refcount)
> > +		return 0;
> 
> I guess this is a nit: I was looking for a dev_ser->refcount++ in this
> patch to correspond with this decrement.  I *guess* the
> "dev_ser->refcount = 1" in pci_liveupdate_preserve_device() starts
> that, and the "dev->liveupdate.outgoing->refcount++" there is an
> increment of the same refcount?  The connections there don't seem
> quite obvious to me.
> 
> But I guess "dev_ser->refcount = 1" was added by a different patch, so
> probably *not* the same?

As mentioned in the other reply I can combine refcount=1 and refcount++
into a single refcount++ to make this simpler.

The refcount=1 is for when a device is first preserved. refcount++ is
for when a device is preserved a subsequent time. Bridges are the only
devices that can have a refcount > 1 so that's why this patch introduces
the refcount++.

> 
> >  	pci_info(dev, "Device will no longer be preserved across next Live Update\n");
> >  	outgoing->ser->nr_devices--;
> >  	memset(dev_ser, 0, sizeof(*dev_ser));
> >  	dev->liveupdate.outgoing = NULL;
> > +	return 0;
> > +}
> 
> > @@ -385,13 +418,23 @@ static int pci_liveupdate_preserve_device(struct pci_flb_outgoing *outgoing,
> >  	}
> >  
> >  	if (dev->liveupdate.outgoing) {
> > -		pci_warn(dev, "Device is already preserved\n");
> > -		return -EBUSY;
> > -	}
> > +		if (!dev->liveupdate.outgoing->refcount) {
> > +			pci_WARN(dev, 1, "Preserved device with 0 refcount!\n");
> > +			return -EINVAL;
> > +		}
> >  
> > -	if (!pci_is_root_bus(dev->bus)) {
> > -		pci_warn(dev, "Cannot preserve devices behind bridges\n");
> > -		return -EINVAL;
> > +		/*
> > +		 * Endpoint devices should not be preserved more than once.
> > +		 * Bridges are preserved once for every downstream device that
> > +		 * is preserved.
> > +		 */
> > +		if (!dev->subordinate) {
> > +			pci_warn(dev, "Device is already preserved\n");
> > +			return -EBUSY;
> > +		}
> > +
> > +		dev->liveupdate.outgoing->refcount++;
> > +		return 0;
> >  	}
> >  
> >  	dev_ser = pci_flb_alloc_dev_ser(outgoing);