Re: [PATCH v20 4/4] PCI/MSI: Enable memory decoding before restoring MSI-X messages

[Niklas Schnelle]

On Mon, 2026-06-22 at 10:18 -0700, Farhan Ali wrote:
> The current MSI-X restoration path assumes the Command register Memory bit
> is enabled when writing MSI-X messages. But its possible the last saved and

SpellingL "it's"

> restored state of device may not have the Memory bit enabled, even if a

Missing "a" in "of a device"


> 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.

The official spec term is "Memory Space Enable" and I think "can lead
to" is actually underselling it, as the PCIe spec says that accesses
are "caused to be handled as Unsupported Requests". Also as I
understand you did see this in practice. Maybe we even want a Fixes
tag?

> 
> Signed-off-by: Farhan Ali <alifm@xxxxxxxxxxxxx>
> ---
>  drivers/pci/msi/msi.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/pci/msi/msi.c b/drivers/pci/msi/msi.c
> index 81d24a270a79..46a0d9f68a57 100644
> --- a/drivers/pci/msi/msi.c
> +++ b/drivers/pci/msi/msi.c
> @@ -874,6 +874,7 @@ void __pci_restore_msix_state(struct pci_dev *dev)
>  {
>  	struct msi_desc *entry;
>  	bool write_msg;
> +	u16 cmd;
>  
>  	if (!dev->msix_enabled)
>  		return;
> @@ -882,6 +883,8 @@ void __pci_restore_msix_state(struct pci_dev *dev)
>  	pci_intx_for_msi(dev, 0);
>  	pci_msix_clear_and_set_ctrl(dev, 0,
>  				PCI_MSIX_FLAGS_ENABLE | PCI_MSIX_FLAGS_MASKALL);
> +	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);
>  
> @@ -893,6 +896,7 @@ 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);
>  }
>  

Code wise this all looks good to me so feel free to add.

Reviewed-by: Niklas Schnelle <schnelle@xxxxxxxxxxxxx>

Thanks,
Niklas