Re: [PATCH v2 02/22] PCI: Add API to track PCI devices preserved across Live Update

[Alex Williamson <alex@shazbot.org>]

On Thu, 26 Feb 2026 00:28:28 +0000
David Matlack <dmatlack@google.com> wrote:
> > > +static int pci_flb_preserve(struct liveupdate_flb_op_args *args)
> > > +{
> > > +	struct pci_dev *dev = NULL;
> > > +	int max_nr_devices = 0;
> > > +	struct pci_ser *ser;
> > > +	unsigned long size;
> > > +
> > > +	for_each_pci_dev(dev)
> > > +		max_nr_devices++;  
> > 
> > How is this protected against hotplug?  
> 
> Pranjal raised this as well. Here was my reply:
> 
> .  Yes, it's possible to run out space to preserve devices if devices are
> .  hot-plugged and then preserved. But I think it's better to defer
> .  handling such a use-case exists (unless you see an obvious simple
> .  solution). So far I am not seeing preserving hot-plugged devices
> .  across Live Update as a high priority use-case to support.
> 
> I am going to add a comment here in the next revision to clarify that.
> I will also add a comment clarifying why this code doesn't bother to
> account for VFs created after this call (preserving VFs are explicitly
> disallowed to be preserved in this patch since they require additional
> support).

TBH, without SR-IOV support and some examples of in-kernel PF
preservation in support of vfio-pci VFs, it seems like this only
supports a very niche use case.  I expect the majority of vfio-pci
devices are VFs and I don't think we want to present a solution where
the requirement is to move the PF driver to userspace.  It's not clear,
for example, how we can have vfio-pci variant drivers relying on
in-kernel channels to PF drivers to support migration in this model.
Thanks,

Alex