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

On 8/12/2026 3:07 PM, Bjorn Helgaas wrote:
> 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: [email protected]
>> Reviewed-by: Thomas Gleixner <[email protected]>
>> Reviewed-by: Niklas Schnelle <[email protected]>
>> Signed-off-by: Farhan Ali <[email protected]>
>> ---
>>   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?

Yes, since restoring MSI-X would need to access the BAR, we need to set 
PCI_COMMAND_MEMORY.


> 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.

Sure, we can improve this with what you suggested. Would you prefer me 
to re-spin with the updated comment?

Thanks

Farhan

>> +	 */
>> +	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
>>