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

[Farhan Ali]

On 6/22/2026 1:29 PM, Thomas Gleixner wrote:
> On Mon, Jun 22 2026 at 10:18, 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
> > restored state of 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.
> >
> > 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);
> Can we please have a comment there which explains this? Three month down
> the road this will results in head scratching otherwise.
>
> I agree with Niklas that this wants a Fixes and a Cc:stable tag.
>
> Other than that:
>
> Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxx>
>
> Thanks,
>
>          tglx
I can add a comment, how about something like below?

"The restored device state may not have Memory decoding enabled in 
Command register. Since the MSI-X was enabled for the device, enable 
Memory decoding before restoring MSI-X"

For the Fixes tag, do you have a suggestion for a commit? This behavior 
has been present since 41017f0cac92 ("[PATCH] PCI: MSI(X) save/restore 
for suspend/resume" which introduced these restore functions. So should 
be Fixes against 41017f0cac92?

Thanks

Farhan