Re: [PATCH v23 5/5] PCI/MSI: Enable memory decoding before restoring MSI-X messages

[Bjorn Helgaas <helgaas@kernel.org>]

On Wed, Aug 05, 2026 at 09:55:18AM -0700, Farhan Ali wrote:
> The current MSI-X restoration path assumes the Command register Memory bit
> is enabled when writing MSI-X messages. But it's possible the last saved
> and restored state of a device may not have the Memory bit enabled, even if
> a device driver later enables Memory bit and MSI-X. Attempting to access
> Memory space without Memory bit enabled can lead to Unsupported Request
> (UR) from the device. Fix this by enabling Memory bit and restore it
> afterwards.
> 
> Fixes: 41017f0cac92 ("[PATCH] PCI: MSI(X) save/restore for suspend/resume")
> Cc: stable@vger.kernel.org
> Reviewed-by: Thomas Gleixner <tglx@kernel.org>
> Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>
> Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
> ---
>  drivers/pci/msi/msi.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/pci/msi/msi.c b/drivers/pci/msi/msi.c
> index 209373c92e9e..79c7e84d314b 100644
> --- a/drivers/pci/msi/msi.c
> +++ b/drivers/pci/msi/msi.c
> @@ -870,6 +870,7 @@ void __pci_restore_msix_state(struct pci_dev *dev)
>  {
>  	struct msi_desc *entry;
>  	bool write_msg;
> +	u16 cmd;
>  
>  	if (!dev->msix_enabled)
>  		return;
> @@ -879,6 +880,14 @@ void __pci_restore_msix_state(struct pci_dev *dev)
>  	pci_msix_clear_and_set_ctrl(dev, 0,
>  				PCI_MSIX_FLAGS_ENABLE | PCI_MSIX_FLAGS_MASKALL);
>  
> +	/*
> +	 * The restored device state may not have Memory decoding enabled
> +	 * in the Command register. Since the MSI-X was enabled for the
> +	 * device, enable Memory decoding before restoring MSI-X.

PCI_COMMAND_MEMORY must be set because the MSI-X Table and PBA are in
Memory space (in a BAR), right?  I think a more direct way of saying
this would be:

  * The restored device state may not have Memory Space enabled.
  * Since the MSI-X Table and PBA are in Memory Space, enable it
  * while restoring them.

> +	 */
> +	pci_read_config_word(dev, PCI_COMMAND, &cmd);
> +	pci_write_config_word(dev, PCI_COMMAND, cmd | PCI_COMMAND_MEMORY);
> +
>  	write_msg = arch_restore_msi_irqs(dev);
>  
>  	scoped_guard (msi_descs_lock, &dev->dev) {
> @@ -889,6 +898,7 @@ void __pci_restore_msix_state(struct pci_dev *dev)
>  		}
>  	}
>  
> +	pci_write_config_word(dev, PCI_COMMAND, cmd);
>  	pci_msix_clear_and_set_ctrl(dev, PCI_MSIX_FLAGS_MASKALL, 0);
>  }
>  
> -- 
> 2.43.0
>