Re: [PATCH v7 09/12] PCI: liveupdate: Inherit ARI Forwarding Enable on preserved bridges

[Bjorn Helgaas <helgaas@kernel.org>]

On Fri, Jul 10, 2026 at 09:26:12PM +0000, David Matlack wrote:
> Inherit the ARI Forwarding Enable on preserved bridges and update
> pci_dev->ari_enabled accordingly during a Live Update. This ensures that
> the preserved devices on the bridge's secondary bus can be identified
> with the same expanded 8-bit function number after a Live Update.
> 
> Signed-off-by: David Matlack <dmatlack@google.com>
> ---
>  drivers/pci/liveupdate.c | 18 ++++++++++++++++++
>  drivers/pci/liveupdate.h |  6 ++++++
>  drivers/pci/pci.c        |  8 +++++++-
>  3 files changed, 31 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/liveupdate.c b/drivers/pci/liveupdate.c
> index a95bfe5eff77..74a11e520f0d 100644
> --- a/drivers/pci/liveupdate.c
> +++ b/drivers/pci/liveupdate.c
> @@ -128,6 +128,10 @@
>   *    way after Live Update and ensures that IOMMU groups do not change. Note
>   *    that a device will use its inherited ACS flags for the lifetime of its
>   *    struct pci_dev (i.e. even after pci_liveupdate_finish()).
> + *
> + *  * The PCI core inherits ARI Forwarding Enable on all bridges with downstream
> + *    preserved devices to ensure that all preserved devices on the bridge's
> + *    secondary bus are addressable after the Live Update.
>   */
>  
>  #define pr_fmt(fmt) "PCI: " KBUILD_BASENAME ": " fmt
> @@ -816,6 +820,20 @@ int pci_liveupdate_enable_acs(struct pci_dev *dev)
>  	return 0;
>  }
>  
> +int pci_liveupdate_configure_ari(struct pci_dev *dev)
> +{
> +	u16 val;
> +
> +	guard(rwsem_read)(&pci_liveupdate.rwsem);
> +
> +	if (!dev->liveupdate.incoming)
> +		return -EINVAL;
> +
> +	pcie_capability_read_word(dev, PCI_EXP_DEVCTL2, &val);
> +	dev->ari_enabled = !!(val & PCI_EXP_DEVCTL2_ARI);
> +	return 0;
> +}

I'm guessing we're going to see a lot of this pattern, so it will
eventually become familiar, but it's not familiar yet ;)  This doesn't
"configure" anything (I do understand that avoiding configuration in
the new kernel is really the main point of liveupdate).

Maybe a comment is the solution.  Or maybe a rename to
"pci_liveupdate_ari_preserved" or something?  That would have to
reverse the sense of return values, but I think something like this in
the callers would read better:

  if (pci_liveupdate_ari_preserved(dev))
    return;

or maybe:

  if (pci_liveupdate_preserve_ari(dev))
    return;

>  /**
>   * pci_liveupdate_is_incoming() - Check if a device is incoming-preserved
>   * @dev: The PCI device to check
> diff --git a/drivers/pci/liveupdate.h b/drivers/pci/liveupdate.h
> index 4e8a01bcb4bb..6f21ec50927b 100644
> --- a/drivers/pci/liveupdate.h
> +++ b/drivers/pci/liveupdate.h
> @@ -18,6 +18,7 @@ bool pci_liveupdate_scan_bridge_begin(struct pci_bus *bus, struct pci_dev *dev,
>  void pci_liveupdate_scan_bridge_end(struct pci_dev *dev, int pass);
>  void pci_liveupdate_init_acs(struct pci_dev *dev);
>  int pci_liveupdate_enable_acs(struct pci_dev *dev);
> +int pci_liveupdate_configure_ari(struct pci_dev *dev);
>  #else
>  static inline void pci_liveupdate_setup_device(struct pci_dev *dev)
>  {
> @@ -46,6 +47,11 @@ static inline int pci_liveupdate_enable_acs(struct pci_dev *dev)
>  {
>  	return -EINVAL;
>  }
> +
> +static inline int pci_liveupdate_configure_ari(struct pci_dev *dev)
> +{
> +	return -EINVAL;
> +}
>  #endif
>  
>  #endif /* DRIVERS_PCI_LIVEUPDATE_H */
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 739ecaab2e76..e0c133b66a35 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -3528,7 +3528,7 @@ void pci_configure_ari(struct pci_dev *dev)
>  	u32 cap;
>  	struct pci_dev *bridge;
>  
> -	if (pcie_ari_disabled || !pci_is_pcie(dev) || dev->devfn)
> +	if (!pci_is_pcie(dev) || dev->devfn)
>  		return;
>  
>  	bridge = dev->bus->self;
> @@ -3539,6 +3539,12 @@ void pci_configure_ari(struct pci_dev *dev)
>  	if (!(cap & PCI_EXP_DEVCAP2_ARI))
>  		return;
>  
> +	if (!pci_liveupdate_configure_ari(bridge))
> +		return;
> +
> +	if (pcie_ari_disabled)
> +		return;
> +
>  	if (pci_find_ext_capability(dev, PCI_EXT_CAP_ID_ARI)) {
>  		pcie_capability_set_word(bridge, PCI_EXP_DEVCTL2,
>  					 PCI_EXP_DEVCTL2_ARI);
> -- 
> 2.55.0.795.g602f6c329a-goog
>