Re: [PATCH v6 01/12] PCI: liveupdate: Set up FLB handler for the PCI core

[Pranjal Shrivastava <praan@google.com>]

On Mon, Jun 08, 2026 at 08:51:00PM +0000, David Matlack wrote:
> On 2026-06-05 05:41 AM, Pranjal Shrivastava wrote:
> > On Fri, May 22, 2026 at 08:23:59PM +0000, David Matlack wrote:
> > > Set up a File-Lifecycle-Bound (FLB) handler for the PCI core to enable
> > > it to participate in the preservation of PCI devices across Live Update.
> > > Essentially, this commit enables the PCI core to allocate a struct
> > > (struct pci_ser) and preserve it across a Live Update whenever at least
> > > one device is preserved.
> > > 
> > > Preserving PCI devices across Live Update is built on top of the Live
> > > Update Orchestrator's (LUO) support for file preservation. Drivers are
> > > expected to expose a file to userspace to represent a single PCI device
> > > and support preservation of that file. This is intended primarily to
> > > support preservation of PCI devices bound to VFIO drivers.
> > > 

[...]

> > > + * struct pci_ser - PCI Subsystem Live Update State
> > > + *
> > > + * This struct tracks state about all devices that are being preserved across
> > > + * a Live Update for the next kernel.
> > > + *
> > > + * @max_nr_devices: The length of the devices[] flexible array.
> > > + * @nr_devices: The number of devices that were preserved.
> > > + * @devices: Flexible array of pci_dev_ser structs for each device.
> > > + */
> > > +struct pci_ser {
> > > +	u32 max_nr_devices;
> > > +	u32 nr_devices;
> > > +	struct pci_dev_ser devices[];
> > > +} __packed;
> > > +
> > > +/* Ensure all elements of devices[] are naturally aligned. */
> > > +static_assert(offsetof(struct pci_ser, devices) % sizeof(unsigned long) == 0);
> > > +static_assert(sizeof(struct pci_dev_ser) % sizeof(unsigned long) == 0);
> > 
> > Minor Nit: Shall we consider using specific bitwidth types here?
> > I'm wondering if down the line another u32 field is added to 
> > struct pci_dev_ser.. in that case on a 32-bit machine 12 % 4 == 0 but on
> > a 64-bit machine 12 % 8 != 0..
> 
> I think natural alignment is what matters for efficient access of the
> array elements. So failing the assert only on 64-bit architectures seems
> like the correct behavior.
> 

Ack. I guess what I was trying to say was we'd anyway need to keep both
architectures in mind, i.e. adding members should be 64-bit aligned
(which implicitly also handles 32-bit) as with the current assert
any new addition would have to be both 32 & 64-bit aligned. I guess we
can keep this as is.

Thanks,
Praan