Re: [PATCH v6 03/12] PCI: liveupdate: Track incoming preserved PCI devices

[Pranjal Shrivastava <praan@google.com>]

On Mon, Jun 08, 2026 at 08:57:45PM +0000, David Matlack wrote:
> On 2026-06-06 10:08 AM, Pranjal Shrivastava wrote:
> > On Fri, May 22, 2026 at 08:24:01PM +0000, David Matlack wrote:
> > > During PCI enumeration, the previous kernel might have passed state about
> > > devices that were preserved across kexec. The PCI core needs to fetch
> > > this state to identify which devices are "incoming" and require special
> > > handling.
> > > 
> > > Add pci_liveupdate_setup_device() which is called during device setup
> > > to fetch the serialized state (struct pci_ser) from the Live Update
> > > Orchestrator. The first time this happens, pci_flb_retrieve() will run
> > > and convert the array of pci_dev_ser structs into an xarray so that it
> > > can be looked up efficiently.
> > > 
> > > If a device is found in the xarray, the PCI core stores a pointer to its
> > > state in dev->liveupdate_incoming and holds a reference to the incoming
> > > FLB until pci_liveupdate_finish() is called by the driver.
> > > 
> > > This ensures proper lifecycle management for incoming preserved devices
> > > and allows the PCI core and drivers to apply specific Live Update
> > > logic to them in subsequent commits.
> > > 
> > > Drivers can check if a device is an incoming preserved device (e.g.
> > > during probe) by calling pci_liveupdate_is_incoming().
> > > 
> > > CONFIG_64BIT is now required to enable CONFIG_PCI_LIVEUPDATE so that the
> > > domain and bdf can be guaranteed to fit in an unsigned long and be used
> > > as the xarray key.
> > > 
> > > Signed-off-by: David Matlack <dmatlack@google.com>
> > > ---
> > >  MAINTAINERS                    |   1 +
> > >  drivers/pci/Kconfig            |   2 +-
> > >  drivers/pci/liveupdate.c       | 230 ++++++++++++++++++++++++++++++++-
> > >  drivers/pci/liveupdate.h       |   5 +
> > >  drivers/pci/probe.c            |   3 +
> > >  include/linux/pci_liveupdate.h |  13 ++
> > >  6 files changed, 251 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/MAINTAINERS b/MAINTAINERS
> > > index 6c618830cf61..0e262c0ceb43 100644
> > > --- a/MAINTAINERS
> > > +++ b/MAINTAINERS
> > > @@ -20537,6 +20537,7 @@ L:	linux-pci@vger.kernel.org
> > >  S:	Maintained
> > >  T:	git git://git.kernel.org/pub/scm/linux/kernel/git/liveupdate/linux.git
> > >  F:	drivers/pci/liveupdate.c
> > > +F:	drivers/pci/liveupdate.h
> > >  F:	include/linux/kho/abi/pci.h
> > >  F:	include/linux/pci_liveupdate.h
> > >  
> > > diff --git a/drivers/pci/Kconfig b/drivers/pci/Kconfig
> > > index 10c9b65aa242..e68ae5c172d4 100644
> > > --- a/drivers/pci/Kconfig
> > > +++ b/drivers/pci/Kconfig
> > > @@ -330,7 +330,7 @@ config VGA_ARB_MAX_GPUS
> > >  
> > >  config PCI_LIVEUPDATE
> > >  	bool "PCI Live Update Support"
> > > -	depends on PCI && LIVEUPDATE
> > > +	depends on PCI && LIVEUPDATE && 64BIT
> > 
> > I see that the static assertions in Patch 1 work because of the 64BIT
> > enforcement here. In that case, should we have the assertions check u64?
> 
> The static asserts have nothing to do with the 64BIT enforcement here.
> The static asserts just verify that the array elements in struct pci_ser
> are naturally aligned (unsigned long) so they can be accessed
> efficiently. The requirement here for CONFIG_64BIT is for the xarray
> key.
> 
> Theoretically if we got the xarray to work with 32-bit architectures
> then we could drop the CONFIG_64BIT requirement here.
> 

Ack. I see.

[...]
> > > +	kho_restore_free(ser);
> > 
> > I tend to partly agree with Sashiko[1] here.. it raises a policy-hole.
> > We may need a policy here, the options I have in mind are:
> > 
> > 1. Retrieve shall ONLY be tried once, if it fails (like -ENOMEM in the
> >    xArray alloc), it's a liveupdate failure. We can't retry liveupdate.
> > 
> > 2. Retrying retrieve is allowed.
> > 
> > The only downside with option 1 is, the user may want flexibility due to
> > certain subsystems OR may choose NOT to use the proposed LUOd and instead
> > have its own user-space component which might try funny things or have a
> > different use-case.
> > 
> > In such a situation, the system may have transiently run out of memory
> > during the kexec transition (for e.g. a subsystem uses GFP_ATOMIC to
> > allocate memory and temporarily runs out of the atomic pool). [Note we
> > removed it in IOMMU v1 [2] but subsystems may have a use-case for it]
> > 
> > If the kernel frees the KHO page on the first failure, it removes any
> > chance of recovery. :/
> > 
> > Thus, it might make sense to let the user decide if it wants to fail the
> > liveupdate or retry again based on the failure type / source?
> 
> The plan is to have LUO enforce that retrieve() is only called once:
> 
>   https://lore.kernel.org/kexec/20260528174140.1921129-3-dmatlack@google.com/
> 
> Supporting retry gets complicated since there's many different places
> where retrieve() could have failed.

Ack. Thanks for pointing me to the thread.
In that case, no problem.

Thanks,
Praan