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

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Date:   Tue, 8 Nov 2022 09:02:28 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Wei Gong <gongwei833x@...il.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org
Subject: Re: [PATCH v2] pci: fix device presence detection for VFs

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@...il.com>
> > Signed-off-by: Michael S. Tsirkin <mst@...hat.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
> >