Re: [PATCH v6 11/12] PCI: liveupdate: Do not disable bus mastering on preserved devices during kexec

[Pranjal Shrivastava <praan@google.com>]

On Fri, May 22, 2026 at 08:24:09PM +0000, David Matlack wrote:
> Do not disable bus mastering on outgoing preserved devices during
> pci_device_shutdown() for kexec.
> 
> Preserved devices must be allowed to perform memory transactions during
> a Live Update to ensure continuous operation.  Clearing the bus
> mastering bit would prevent these devices from issuing any memory
> requests while the new kernel boots.
> 
> Because bridges upstream of preserved endpoint devices are also
> automatically preserved, this change also avoids clearing bus mastering
> on them. This is critical because clearing bus mastering on an upstream
> bridge prevents the bridge from forwarding memory requests upstream (i.e.
> it would prevent the endpoint device from accessing system RAM and doing
> peer-to-peer transactions with devices not downstream of the bridge).
> 
> Signed-off-by: David Matlack <dmatlack@google.com>
> ---
>  drivers/pci/liveupdate.c | 11 +++++++++++
>  drivers/pci/liveupdate.h |  6 ++++++
>  drivers/pci/pci-driver.c |  7 +++++--
>  3 files changed, 22 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/liveupdate.c b/drivers/pci/liveupdate.c
> index d404e64a4e55..a6f2790bc1bf 100644
> --- a/drivers/pci/liveupdate.c
> +++ b/drivers/pci/liveupdate.c
> @@ -132,6 +132,10 @@
>   *  * 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.
> + *
> + *  * The PCI core does not disable bus mastering on outgoing preserved devices
> + *    during kexec. This allows preserved devices to issue memory transactions
> + *    throughout the Live Update.
>   */
>  
>  #define pr_fmt(fmt) "PCI: liveupdate: " fmt
> @@ -790,6 +794,13 @@ int pci_liveupdate_configure_ari(struct pci_dev *dev)
>  	return 0;
>  }
>  
> +bool pci_liveupdate_is_outgoing(struct pci_dev *dev)
> +{
> +	guard(rwsem_read)(&pci_liveupdate.rwsem);
> +	pci_WARN_ONCE(dev, !dev->liveupdate.frozen, "Preservation status is unstable!\n");
> +	return dev->liveupdate.outgoing;
> +}
> +

[...]

> diff --git a/drivers/pci/pci-driver.c b/drivers/pci/pci-driver.c
> index f7a5e65a7c75..0b1f8d01d7a5 100644
> --- a/drivers/pci/pci-driver.c
> +++ b/drivers/pci/pci-driver.c
> @@ -546,11 +546,14 @@ static void pci_device_shutdown(struct device *dev)
>  	/*
>  	 * If this is a kexec reboot, turn off Bus Master bit on the
>  	 * device to tell it to not continue to do DMA. Don't touch
> -	 * devices in D3cold or unknown states.
> +	 * devices being preserved for Live Update or in D3cold or
> +	 * unknown states.
> +	 *
>  	 * If it is not a kexec reboot, firmware will hit the PCI
>  	 * devices with big hammer and stop their DMA any way.
>  	 */
> -	if (kexec_in_progress && (pci_dev->current_state <= PCI_D3hot))
> +	if (kexec_in_progress && !pci_liveupdate_is_outgoing(pci_dev) &&
> +	    pci_dev->current_state <= PCI_D3hot)
>  		pci_clear_master(pci_dev);
>  }

Looks good. 

Reviewed-by: Pranjal Shrivastava <praan@google.com>

Thanks,
Praan