RE: [PATCH v2] PCI: hv: Fix a crash in hv_pci_restore_msi_msg() during hibernation

[Dexuan Cui]

> From: Kuppuswamy Sathyanarayanan
> <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
> Sent: Wednesday, August 16, 2023 11:12 AM
> [...]
> On 8/16/2023 10:59 AM, Dexuan Cui wrote:
> > When a Linux VM with an assigned PCI device runs on Hyper-V, if the PCI
> > device driver is not loaded yet (i.e. MSI-X/MSI is not enabled on the
> > device yet), doing a VM hibernation triggers a panic in
> > hv_pci_restore_msi_msg() -> msi_lock_descs(&pdev->dev), because
> > pdev->dev.msi.data is still NULL.
> >
> > Avoid the panic by checking if MSI-X/MSI is enabled.
> >
> > Fixes: dc2b453290c4 ("PCI: hv: Rework MSI handling")
> > Signed-off-by: Dexuan Cui <decui@xxxxxxxxxxxxx>
> > ---
> >
> > Changes in v2:
> >      Replaced the test "if (!pdev->dev.msi.data)" with
> > 		      "if (!pdev->msi_enabled && !pdev->msix_enabled)".
> >        Thanks Michael!
> >      Updated the changelog accordingly.
> >
> >   drivers/pci/controller/pci-hyperv.c | 3 +++
> >   1 file changed, 3 insertions(+)
> >
> > diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-
> hyperv.c
> > index 2d93d0c4f10d..bed3cefdaf19 100644
> > --- a/drivers/pci/controller/pci-hyperv.c
> > +++ b/drivers/pci/controller/pci-hyperv.c
> > @@ -3983,6 +3983,9 @@ static int hv_pci_restore_msi_msg(struct pci_dev
> *pdev, void *arg)
> >   	struct msi_desc *entry;
> >   	int ret = 0;
> >
> > +	if (!pdev->msi_enabled && !pdev->msix_enabled)
> > +		return 0;
> Isn't this is a error condition? Don't you want to return error here?

This is not an error.  If a PCI device driver is not loaded or not installed, 
MSI-X/MSI is not enabled on the device, so pdev->msi_enabled is 0
and pdev->msix_enabled is 0. In this case, it's still legit for a user to request
the system (i.e. here it's a Linux VM running on Hyper-V) to hibernate -- in
this case, we should not try to save/restore the MSI/MSI-X state, and we
should not let the hibernation fail; here we should just ignore the device
by returning a success ("return 0;").

> > +
> >   	msi_lock_descs(&pdev->dev);
> >   	msi_for_each_desc(entry, &pdev->dev, MSI_DESC_ASSOCIATED) {
> >   		irq_data = irq_get_irq_data(entry->irq);