Re: [PATCH] vfio/pci: Add extra test for extended capabilities

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 2014-06-12 at 16:12 -0400, Bandan Das wrote:
> Alex Williamson <alex.williamson@redhat.com> writes:
> 
> > If the PCI core set cfg_size isn't large enough for extended
> > capabilities, then they're not there.  Extended config space may be
> > inaccessible due to a PCI bridge, in which case cfg_size may be cut
> > short.
> 
> Just curious, is this part of the spec or just a convention ? I mean
> could there be devices with quirks ?

The PCIe bridge spec indicates that PCIe-to-PCI bridges should handle
extended config accesses as an unsupported request.  But as in 78916b00,
we've seen that's not always the case.  This just makes us trust
PCI-core before the capabilities we find on the device.  The current
quirk in the referenced commit is for a relatively uncommon topology,
but never rule out that we couldn't have a device quirked to avoid
touching extended config space too.  Thanks,

Alex 

> > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> > ---
> >  drivers/vfio/pci/vfio_pci_config.c |   13 +++++++++----
> >  1 file changed, 9 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/vfio/pci/vfio_pci_config.c b/drivers/vfio/pci/vfio_pci_config.c
> > index 83cd157..4a62a37 100644
> > --- a/drivers/vfio/pci/vfio_pci_config.c
> > +++ b/drivers/vfio/pci/vfio_pci_config.c
> > @@ -1027,8 +1027,11 @@ static int vfio_cap_len(struct vfio_pci_device *vdev, u8 cap, u8 pos)
> >  
> >  		if (PCI_X_CMD_VERSION(word)) {
> >  			/* Test for extended capabilities */
> > -			pci_read_config_dword(pdev, PCI_CFG_SPACE_SIZE, &dword);
> > -			vdev->extended_caps = (dword != 0);
> > +			if (pdev->cfg_size > PCI_CFG_SPACE_SIZE) {
> > +				pci_read_config_dword(pdev, PCI_CFG_SPACE_SIZE,
> > +						      &dword);
> > +				vdev->extended_caps = (dword != 0);
> > +			}
> >  			return PCI_CAP_PCIX_SIZEOF_V2;
> >  		} else
> >  			return PCI_CAP_PCIX_SIZEOF_V0;
> > @@ -1041,8 +1044,10 @@ static int vfio_cap_len(struct vfio_pci_device *vdev, u8 cap, u8 pos)
> >  		return byte;
> >  	case PCI_CAP_ID_EXP:
> >  		/* Test for extended capabilities */
> > -		pci_read_config_dword(pdev, PCI_CFG_SPACE_SIZE, &dword);
> > -		vdev->extended_caps = (dword != 0);
> > +		if (pdev->cfg_size > PCI_CFG_SPACE_SIZE) {
> > +			pci_read_config_dword(pdev, PCI_CFG_SPACE_SIZE, &dword);
> > +			vdev->extended_caps = (dword != 0);
> > +		}
> >  
> >  		/* length based on version */
> >  		if ((pcie_caps_reg(pdev) & PCI_EXP_FLAGS_VERS) == 1)
> >
> > --
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