Re: [PATCH v4 02/11] PCI: liveupdate: Track outgoing preserved PCI devices

[Jacob Pan <jacob.pan@linux.microsoft.com>]

Hi David,

On Thu, 23 Apr 2026 21:23:06 +0000
David Matlack <dmatlack@google.com> wrote:

> Add APIs to allow drivers to notify the PCI core of which devices are
> being preserved across a Live Update for the next kernel, i.e.
> "outgoing" devices.
> 
> Drivers must notify the PCI core when devices are preserved so that
> the PCI core can update its FLB data (struct pci_ser) and track the
> list of outgoing devices. pci_liveupdate_preserve() notifies the PCI
> core that a device must be preserved across Live Update.
> pci_liveupdate_unpreserve() reverses this (cancels the preservation
> of the device).
> 
> This tracking ensures the PCI core is fully aware of which devices may
> need special handling during shutdown and kexec, and so that it can be
> handed off to the next kernel.
> 
> Signed-off-by: David Matlack <dmatlack@google.com>
> ---
>  drivers/pci/liveupdate.c    | 101
> ++++++++++++++++++++++++++++++++++++ include/linux/kho/abi/pci.h |
> 7 +-- include/linux/pci.h         |  26 ++++++++++
>  3 files changed, 131 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/pci/liveupdate.c b/drivers/pci/liveupdate.c
> index d4fa61625d56..2dd8daa2f17c 100644
> --- a/drivers/pci/liveupdate.c
> +++ b/drivers/pci/liveupdate.c
> @@ -43,6 +43,26 @@
>   *
>   *  * ``pci_liveupdate_register_flb(driver_file_handler)``
>   *  * ``pci_liveupdate_unregister_flb(driver_file_handler)``
> + *
> + * Device Tracking
> + * ===============
> + *
> + * Drivers must notify the PCI core when specific devices are
> preserved or
> + * unpreserved with the following APIs:
> + *
> + *  * ``pci_liveupdate_preserve(pci_dev)``
> + *  * ``pci_liveupdate_unpreserve(pci_dev)``
> + *
> + * This allows the PCI core to keep it's FLB data (struct pci_ser)
> up to date
> + * with the list of **outgoing** preserved devices for the next
> kernel.
> + *
> + * Restrictions
> + * ============
> + *
> + * The PCI core enforces the following restrictions on which devices
> can be
> + * preserved. These may be relaxed in the future:
> + *
> + *  * The device cannot be a Virtual Function (VF).
>   */
>  
>  #define pr_fmt(fmt) "PCI: liveupdate: " fmt
> @@ -57,6 +77,8 @@
>  #include <linux/pci.h>
>  #include <linux/sort.h>
>  
> +static DEFINE_MUTEX(pci_flb_outgoing_lock);
> +
>  static int pci_flb_preserve(struct liveupdate_flb_op_args *args)
>  {
>  	struct pci_dev *dev = NULL;
> @@ -124,6 +146,85 @@ static struct liveupdate_flb pci_liveupdate_flb
> = { .compatible = PCI_LUO_FLB_COMPATIBLE,
>  };
>  
> +int pci_liveupdate_preserve(struct pci_dev *dev)
> +{
> +	struct pci_ser *ser;
> +	int i, ret;
> +
> +	guard(mutex)(&pci_flb_outgoing_lock);
> +
> +	ret = liveupdate_flb_get_outgoing(&pci_liveupdate_flb, (void
> **)&ser);
> +	if (ret)
> +		return ret;
> +
> +	if (!ser)
> +		return -ENOENT;
> +
> +	if (dev->is_virtfn)
> +		return -EINVAL;
This can be left outside the mutex?

> +
> +	if (dev->liveupdate_outgoing)
> +		return -EBUSY;
> +
> +	if (ser->nr_devices == ser->max_nr_devices)
> +		return -ENOSPC;
> +
> +	for (i = 0; i < ser->max_nr_devices; i++) {
> +		/*
> +		 * Start searching at index ser->nr_devices. This
> should result
> +		 * in a constant time search under expected
> conditions (devices
> +		 * are not getting unpreserved).
> +		 */
> +		int index = (ser->nr_devices + i) %
> ser->max_nr_devices;
> +		struct pci_dev_ser *dev_ser = &ser->devices[index];
> +
> +		if (dev_ser->refcount)
> +			continue;
> +
> +		pci_info(dev, "Device will be preserved across next
> Live Update\n");
> +		ser->nr_devices++;
> +
> +		dev_ser->domain = pci_domain_nr(dev->bus);
> +		dev_ser->bdf = pci_dev_id(dev);
> +		dev_ser->refcount = 1;
> +
> +		dev->liveupdate_outgoing = dev_ser;
> +		return 0;
> +	}
> +
> +	return -ENOSPC;
> +}
> +EXPORT_SYMBOL_GPL(pci_liveupdate_preserve);
> +
> +void pci_liveupdate_unpreserve(struct pci_dev *dev)
> +{
> +	struct pci_dev_ser *dev_ser;
> +	struct pci_ser *ser = NULL;
> +	int ret;
> +
> +	guard(mutex)(&pci_flb_outgoing_lock);
> +
> +	ret = liveupdate_flb_get_outgoing(&pci_liveupdate_flb, (void
> **)&ser); +
> +	if (ret || !ser) {
> +		pci_warn(dev, "Cannot unpreserve device without
> outgoing Live Update state\n");
> +		return;
> +
> +	}
> +
> +	dev_ser = dev->liveupdate_outgoing;
> +	if (!dev_ser) {
> +		pci_warn(dev, "Cannot unpreserve device that is not
> preserved\n");
> +		return;
> +	}
> +
> +	pci_info(dev, "Device will no longer be preserved across
> next Live Update\n");
> +	ser->nr_devices--;
> +	memset(dev_ser, 0, sizeof(*dev_ser));
> +	dev->liveupdate_outgoing = NULL;
> +}
> +EXPORT_SYMBOL_GPL(pci_liveupdate_unpreserve);
> +
>  int pci_liveupdate_register_flb(struct liveupdate_file_handler *fh)
>  {
>  	pr_debug("Registering file handler \"%s\"\n",
> fh->compatible); diff --git a/include/linux/kho/abi/pci.h
> b/include/linux/kho/abi/pci.h index 5c0e92588c00..5b4c8d9e462c 100644
> --- a/include/linux/kho/abi/pci.h
> +++ b/include/linux/kho/abi/pci.h
> @@ -23,19 +23,20 @@
>   * incrementing the version number in the PCI_LUO_FLB_COMPATIBLE
> string. */
>  
> -#define PCI_LUO_FLB_COMPATIBLE "pci-v1"
> +#define PCI_LUO_FLB_COMPATIBLE "pci-v2"
>  
>  /**
>   * 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.
> - * @reserved: Reserved (to naturally align struct pci_dev_ser).
> + * @refcount: Reference count used by the PCI core to keep track of
> whether it
> + *            is done using a device's struct pci_dev_ser.
>   */
>  struct pci_dev_ser {
>  	u32 domain;
>  	u16 bdf;
> -	u16 reserved;
> +	u16 refcount;
>  } __packed;
>  
>  /**
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index d70080babd52..eb94cbd8ab9d 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -41,6 +41,7 @@
>  #include <linux/msi_api.h>
>  #include <uapi/linux/pci.h>
>  #include <linux/liveupdate.h>
> +#include <linux/kho/abi/pci.h>
>  
>  #include <linux/pci_ids.h>
>  
> @@ -594,6 +595,9 @@ struct pci_dev {
>  	u8		tph_mode;	/* TPH mode */
>  	u8		tph_req_type;	/* TPH requester type
> */ #endif
> +#ifdef CONFIG_PCI_LIVEUPDATE
> +	struct pci_dev_ser *liveupdate_outgoing; /* State preserved
> for next kernel */ +#endif
>  };
>  
>  static inline struct pci_dev *pci_physfn(struct pci_dev *dev)
> @@ -2880,6 +2884,14 @@ void pci_uevent_ers(struct pci_dev *pdev, enum
>  pci_ers_result err_type); #ifdef CONFIG_PCI_LIVEUPDATE
>  int pci_liveupdate_register_flb(struct liveupdate_file_handler *fh);
>  void pci_liveupdate_unregister_flb(struct liveupdate_file_handler
> *fh); +
> +int pci_liveupdate_preserve(struct pci_dev *dev);
> +void pci_liveupdate_unpreserve(struct pci_dev *dev);
> +
> +static inline struct pci_dev_ser *pci_liveupdate_outgoing(struct
> pci_dev *dev) +{
> +	return dev->liveupdate_outgoing;
> +}
>  #else
>  static inline int pci_liveupdate_register_flb(struct
> liveupdate_file_handler *fh) {
> @@ -2889,6 +2901,20 @@ static inline int
> pci_liveupdate_register_flb(struct liveupdate_file_handler *fh static
> inline void pci_liveupdate_unregister_flb(struct
> liveupdate_file_handler *fh) { }
> +
> +static inline int pci_liveupdate_preserve(struct pci_dev *dev)
> +{
> +	return -EOPNOTSUPP;
> +}
> +
> +static inline void pci_liveupdate_unpreserve(struct pci_dev *dev)
> +{
> +}
> +
> +static inline struct pci_dev_ser *pci_liveupdate_outgoing(struct
> pci_dev *dev) +{
> +	return NULL;
> +}
>  #endif
>  
>  #endif /* LINUX_PCI_H */