Re: [PATCH v2] pci: fix device presence detection for VFs

Bjorn Helgaas
2022-11-08T15:02:35Z

On Tue, Nov 08, 2022 at 08:53:00AM -0600, Bjorn Helgaas wrote:
> On Wed, Oct 26, 2022 at 02:11:21AM -0400, Michael S. Tsirkin wrote:
> > virtio uses the same driver for VFs and PFs. Accordingly,
> > pci_device_is_present is used to detect device presence. This function
> > isn't currently working properly for VFs since it attempts reading
> > device and vendor ID.
>
> > As VFs are present if and only if PF is present,
> > just return the value for that device.
>
> VFs are only present when the PF is present *and* the PF has VF Enable
> set. Do you care about the possibility that VF Enable has been
> cleared?

Can you also include a hint about how the problem manifests, and a URL
to the report if available?

It's beyond the scope of this patch, but I've never liked the
semantics of pci_device_is_present() because it's racy by design. All
it tells us is that some time in the *past*, the device was present.
It's telling that almost all calls test for !pci_device_is_present(),
which does make a little more sense.

> > Reported-by: Wei Gong <gongwei833x@gmail.com>
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >
> > Wei Gong, thanks for your testing of the RFC!
> > As I made a small change, would appreciate re-testing.
> >
> > Thanks!
> >
> > changes from RFC:
> > use pci_physfn() wrapper to make the code build without PCI_IOV
> >
> >
> > drivers/pci/pci.c | 5 +++++
> > 1 file changed, 5 insertions(+)
> >
> > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > index 2127aba3550b..899b3f52e84e 100644
> > --- a/drivers/pci/pci.c
> > +++ b/drivers/pci/pci.c
> > @@ -6445,8 +6445,13 @@ bool pci_devs_are_dma_aliases(struct pci_dev *dev1, struct pci_dev *dev2)
> >
> > bool pci_device_is_present(struct pci_dev *pdev)
> > {
> > + struct pci_dev *physfn = pci_physfn(pdev);
> > u32 v;
> >
> > + /* Not a PF? Switch to the PF. */
> > + if (physfn != pdev)
> > + return pci_device_is_present(physfn);
> > +
> > if (pci_dev_is_disconnected(pdev))
> > return false;
> > return pci_bus_read_dev_vendor_id(pdev->bus, pdev->devfn, &v, 0);
> > --
> > MST
> >