Re: [PATCH v12 2/7] PCI: Avoid saving config space state if inaccessible

[Bjorn Helgaas]

On Mon, Mar 30, 2026 at 10:40:06AM -0700, Farhan Ali wrote:
> The current reset process saves the device's config space state before
> reset and restores it afterward. However errors may occur unexpectedly and
> it may then be impossible to save config space because the device may be
> inaccessible (e.g. DPC). This results in saving invalid values that get
> written back to the device during state restoration.
> 
> With a reset we want to recover/restore the device into a functional state.
> So avoid saving the state of the config space when the device config space
> is inaccessible.
> 
> Signed-off-by: Farhan Ali <alifm@xxxxxxxxxxxxx>
> Reviewed-by: Niklas Schnelle <schnelle@xxxxxxxxxxxxx>
> Reviewed-by: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
> ---
>  drivers/pci/pci.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 70162f15a72c..b36263834289 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -722,6 +722,27 @@ u16 pci_find_dvsec_capability(struct pci_dev *dev, u16 vendor, u16 dvsec)
>  }
>  EXPORT_SYMBOL_GPL(pci_find_dvsec_capability);
>  
> +static bool pci_dev_config_accessible(struct pci_dev *dev, char *msg)
> +{
> +	u32 val;
> +
> +	/*
> +	 * If device's config space is inaccessible it can return ~0 for
> +	 * any reads. Since VFs can also return ~0 for Device and Vendor ID
> +	 * check Command and Status registers. Note that this is racy
> +	 * because the device may become inaccessible partway through
> +	 * next access.
> +	 */
> +	pci_read_config_dword(dev, PCI_COMMAND, &val);
> +	if (PCI_POSSIBLE_ERROR(val)) {
> +		pci_warn(dev, "Device config space inaccessible; unable to %s\n",
> +				msg);
> +		return false;

I wonder if it's feasible to do the pci_save_state() into a temporary
buffer, check the buffer for PCI_POSSIBLE_ERROR(), and copy the temp
buffer into the real buffer if we think the save was successful.  I
know this is a lot more work, but I would like to avoid the raciness
if possible.

> +	}
> +
> +	return true;
> +}
> +
>  /**
>   * pci_find_parent_resource - return resource region of parent bus of given
>   *			      region
> @@ -5029,6 +5050,9 @@ static void pci_dev_save_and_disable(struct pci_dev *dev)
>  	 */
>  	pci_set_power_state(dev, PCI_D0);
>  
> +	if (!pci_dev_config_accessible(dev, "save state"))
> +		return;
> +
>  	pci_save_state(dev);
>  	/*
>  	 * Disable the device by clearing the Command register, except for
> -- 
> 2.43.0
>