Re: [PATCH v6 06/12] PCI: liveupdate: Auto-preserve upstream bridges across Live Update

[Pranjal Shrivastava <praan@google.com>]

On Fri, May 22, 2026 at 08:24:04PM +0000, David Matlack wrote:
> When a PCI device is preserved across a Live Update, all of its upstream
> bridges up to the root port must also be preserved. This enables the PCI
> core and any drivers bound to the bridges to manage bridges correctly
> across a Live Update.
> 
> Notably, this will be used in subsequent commits to ensure that
> preserved devices can continue performing memory transactions without a
> disruption or change in routing.
> 
> To preserve bridges, the PCI core tracks the number of downstream
> devices preserved under each bridge using a reference count in struct
> pci_dev_ser. This allows a bridge to remain preserved until all its
> downstream preserved devices are unpreserved or finish their
> participation in the Live Update.
> 
> Signed-off-by: David Matlack <dmatlack@google.com>
> ---
>  drivers/pci/liveupdate.c    | 136 +++++++++++++++++++++++++++++++-----
>  include/linux/kho/abi/pci.h |   5 +-
>  2 files changed, 122 insertions(+), 19 deletions(-)
> 

[...]

> +
> +#define for_each_pci_dev_in_path(_d, _start, _end) \
> +	for ((_d) = (_start); (_d) != (_end); (_d) = (_d)->bus->self)
> +
> +static void __pci_liveupdate_unpreserve_path(struct pci_ser *ser,
> +					     struct pci_dev *start,
> +					     struct pci_dev *end)
> +{
> +	struct pci_dev *dev;
> +
> +	for_each_pci_dev_in_path(dev, start, end) {
> +		if (pci_liveupdate_unpreserve_device(ser, dev))

I might be reading this wrong but are we leaking some upstream devs if 
an intermediate node fails?

			  EP0
			/
Assume we have: RC -> B1 -> B2 
				\
				 EP1

and EP0 & EP1 were preserved successfully.

And then we try unpreserving EP1, we follow:

unpreserve EP1 -> unpreserve B2 failed due to a corruption.

This aborts the loop, skipping B1 and RC completely?
Their refcounts remain elevated, effectively leaking them as preserved 
state permanently? (i.e. if we unpreserve EP0 after this, B1 & RC will
still get preserved).

> +			return;
> +	}
> +}
> +
> +static void pci_liveupdate_unpreserve_path(struct pci_ser *ser,
> +					   struct pci_dev *start)
> +{
> +	__pci_liveupdate_unpreserve_path(ser, start, /*end=*/NULL);
> +}
> +
> +static int pci_liveupdate_preserve_path(struct pci_ser *ser,
> +					struct pci_dev *start)
> +{
> +	struct pci_dev *dev;
> +	int ret;
> +
> +	for_each_pci_dev_in_path(dev, start, NULL) {
> +		ret = pci_liveupdate_preserve_device(ser, dev);
> +		if (ret) {
> +			__pci_liveupdate_unpreserve_path(ser, start, dev);
> +			return ret;
> +		}
> +	}
> +
> +	return 0;
> +}
> +
>  /**
>   * pci_liveupdate_preserve() - Preserve a PCI device across Live Update
>   * @dev: The PCI device to preserve.
> @@ -321,6 +403,9 @@ static int pci_liveupdate_preserve_device(struct pci_ser *ser, struct pci_dev *d
>   * pci_liveupdate_preserve() from their struct liveupdate_file_handler
>   * preserve() callback to ensure the outgoing struct pci_ser is already set up.
>   *
> + * pci_liveupdate_preserve() automatically preserves all bridges upstream of
> + * @dev.
> + *
>   * Returns: 0 on success, <0 on failure.
>   */
>  int pci_liveupdate_preserve(struct pci_dev *dev)
> @@ -336,7 +421,7 @@ int pci_liveupdate_preserve(struct pci_dev *dev)
>  	if (IS_ERR(ser))
>  		return PTR_ERR(ser);
>  
> -	return pci_liveupdate_preserve_device(ser, dev);
> +	return pci_liveupdate_preserve_path(ser, dev);

Minor nit: I might be too nitpicky here (and it's NOT a strong opinion)
but naming it pci_liveupdate_preserve_path_for_dev() reads better to me.

>  }
>  EXPORT_SYMBOL_GPL(pci_liveupdate_preserve);
>  

[...]

Thanks,
Praan