Re: [PATCH v9 01/13] PCI: liveupdate: Set up FLB handler for the PCI core

[Mike Rapoport <rppt@kernel.org>]

Hi David,

On Fri, Sep 18, 2026 at 08:06:27PM +0000, David Matlack wrote:
> Set up a File-Lifecycle-Bound (FLB) handler so that the PCI core can
> preserve its own state across a Live Update kexec.
> 
> Preserving a PCI device across kexec requires preserving two independent
> sets of state:
> 
>  - Driver state, e.g. everything vfio-pci needs so that userspace can
>    keep using the device in the new kernel. The driver preserves this
>    itself and the PCI core is not involved.
> 
>  - PCI core state, e.g. which devices are preserved, so that the new
>    kernel knows not to disturb them while they are still running and
>    doing DMA. That is what this commit adds, serialized into struct
>    pci_ser.
 
...

> diff --git a/drivers/pci/liveupdate.c b/drivers/pci/liveupdate.c
> new file mode 100644
> index 000000000000..66dbee0bd3cf
> --- /dev/null
> +++ b/drivers/pci/liveupdate.c
> @@ -0,0 +1,206 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +/*
> + * Copyright (c) 2026, Google LLC.
> + * David Matlack <dmatlack@google.com>
> + */
> +
> +/**
> + * DOC: PCI Live Update
> + *
> + * The PCI subsystem participates in the Live Update process to enable drivers
> + * to preserve their PCI devices across kexec.
> + *
> + * Preserving a device requires preserving two independent sets of state: the
> + * driver's own state, which the driver preserves with no involvement from the
> + * PCI core, and the PCI core's state about the device, which the next kernel
> + * needs so that enumeration does not disturb a device that is still running.
> + * This file implements the latter.

This does not look good in html docs :(

> + *
> + * :ref:`FLB <flb>` Data
> + * =====================
> + *
> + * Userspace decides which devices are preserved, using :ref:`LUO <luo>` file
> + * preservation: a driver exposes a file that represents a single PCI device,
> + * and userspace preserves the device with
> + * ``ioctl(LIVEUPDATE_SESSION_PRESERVE_FD)`` on that file. Binding preservation
> + * to a file gives it proper lifecycle management, e.g. the preservation is
> + * undone if userspace cancels it or goes away. How a driver exposes that file
> + * is up to the driver and invisible to the PCI core (vfio-pci variant drivers,
> + * the first intended use-case, use their per-device cdev).
> + *
> + * LUO only knows that a file was preserved; it does not know that the file
> + * represents a PCI device. Drivers therefore register their
> + * struct liveupdate_file_handler with the PCI core:
> + *
> + *  * ``pci_liveupdate_register_flb(driver_file_handler)``
> + *  * ``pci_liveupdate_unregister_flb(driver_file_handler)``
> + *
> + * LUO then refcounts the PCI core's FLB against the files preserved by that

Please spell out reference count

> + * handler, and that refcount drives the lifetime of struct pci_ser:

           and here too  ^

> + * pci_flb_preserve() allocates and preserves it when the first file is
> + * preserved, and pci_flb_unpreserve() frees it when the last file is
> + * unpreserved. In the next kernel, pci_flb_retrieve() hands the PCI core the
> + * struct pci_ser built by the previous kernel, whenever the PCI core asks for
> + * it (e.g. during enumeration), and pci_flb_finish() frees it once the PCI
> + * core is done with it.

...

> diff --git a/include/linux/kho/abi/pci.h b/include/linux/kho/abi/pci.h
> new file mode 100644
> index 000000000000..4096e3cd3324
> --- /dev/null
> +++ b/include/linux/kho/abi/pci.h
> @@ -0,0 +1,65 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +/*
> + * Copyright (c) 2026, Google LLC.
> + * David Matlack <dmatlack@google.com>
> + */
> +
> +#ifndef _LINUX_KHO_ABI_PCI_H
> +#define _LINUX_KHO_ABI_PCI_H
> +
> +#include <linux/bug.h>
> +#include <linux/compiler.h>
> +#include <linux/types.h>
> +
> +/**
> + * DOC: PCI File-Lifecycle Bound (FLB) Live Update ABI
> + *
> + * This header defines the ABI for preserving core PCI state across kexec using

This does not look nice in html docs either :(

> + * Live Update File-Lifecycle Bound (FLB) data.
> + *
> + * This interface is a contract. Any modification to any of the serialization
> + * structs defined here constitutes a breaking change. Such changes require
> + * incrementing the version number in the PCI_LUO_FLB_VERSION number.
> + */

-- 
Sincerely yours,
Mike.