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

[David Matlack <dmatlack@google.com>]

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.
> > 
> > This commit enables drivers to register their liveupdate_file_handler
> > with the PCI core so that the PCI core can do its own tracking and
> > enforcement of which devices are preserved.
> > 
> >   pci_liveupdate_register_flb(driver_file_handler);
> >   pci_liveupdate_unregister_flb(driver_file_handler);
> > 
> > When the first file (with a handler registered with the PCI core) is
> > preserved, the PCI core will be notified to allocate its tracking struct
> > (pci_ser). When the last file is unpreserved (i.e. preservation
> > cancelled) the PCI core will be notified to free struct pci_ser.
> > 
> > This struct is preserved across a Live Update using KHO and can be
> > fetched by the PCI core during early boot (e.g. during device
> > enumeration) so that it knows which devices were preserved.
> > 
> > Note: This commit only allocates struct pci_ser and preserves it across
> > Live Update. A subsequent commit will add an API for drivers to tell the
> > PCI core exactly which devices are being preserved.
> > 
> > Note: There is no reason to check for kho_is_enabled() since it can be
> > assumed to return true. If KHO was not enabled then Live Update would
> > not be enabled and these routines would never run.
> > 
> 
> [...]
> 
> > +/**
> > + * struct pci_dev_ser - Serialized state about a single PCI device.
> > + *
> > + * @domain: The device's PCI domain number (segment).
> > + * @bdf: The device's PCI bus, device, and function number.
> > + * @padding: Padding to naturally align struct pci_dev_ser.
> > + */
> > +struct pci_dev_ser {
> > +	u32 domain;
> > +	u16 bdf;
> > +	u16 padding;
> > +} __packed;
> > +
> > +/**
> > + * 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.

> 
> [...]
> 
> With the nit:
> 
> Reviewed-by: Pranjal Shrivastava <praan@google.com>
> 
> Thanks,
> Praan