On Mon, 4 May 2020 17:03:54 -0600
Alex Williamson <alex.williamson@xxxxxxxxxx> wrote:
> On Mon, 4 May 2020 15:08:08 -0700
> Neo Jia <cjia@xxxxxxxxxx> wrote:
>
> > On Mon, May 04, 2020 at 12:52:53PM -0600, Alex Williamson wrote:
> > > External email: Use caution opening links or attachments
> > >
> > >
> > > On Mon, 4 May 2020 18:09:16 +0200
> > > Cornelia Huck <cohuck@xxxxxxxxxx> wrote:
> > >
> > > > On Fri, 01 May 2020 15:41:24 -0600
> > > > Alex Williamson <alex.williamson@xxxxxxxxxx> wrote:
> > > >
> > > > > There is no PCI spec defined capability with ID 0, therefore we don't
> > > > > expect to find it in a capability chain and we use this index in an
> > > > > internal array for tracking the sizes of various capabilities to handle
> > > > > standard config space. Therefore if a device does present us with a
> > > > > capability ID 0, we mark our capability map with nonsense that can
> > > > > trigger conflicts with other capabilities in the chain. Ignore ID 0
> > > > > when walking the capability chain, handling it as a hidden capability.
> > > > >
> > > > > Seen on an NVIDIA Tesla T4.
> > > > >
> > > > > Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
> > > > > ---
> > > > > drivers/vfio/pci/vfio_pci_config.c | 2 +-
> > > > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/drivers/vfio/pci/vfio_pci_config.c b/drivers/vfio/pci/vfio_pci_config.c
> > > > > index 87d0cc8c86ad..5935a804cb88 100644
> > > > > --- a/drivers/vfio/pci/vfio_pci_config.c
> > > > > +++ b/drivers/vfio/pci/vfio_pci_config.c
> > > > > @@ -1487,7 +1487,7 @@ static int vfio_cap_init(struct vfio_pci_device *vdev)
> > > > > if (ret)
> > > > > return ret;
> > > > >
> > > > > - if (cap <= PCI_CAP_ID_MAX) {
> > > >
> > > > Maybe add a comment:
> > > >
> > > > /* no PCI spec defined capability with ID 0: hide it */
> >
> > Hi Alex,
> >
> > I think this is NULL Capability defined in Codes and IDs spec, probably we
> > should just add a new enum to represent that?
>
> Yes, it looks like the 1.1 version of that specification from June 2015
> changed ID 0 from reserved to a NULL capability. So my description and
> this comment are wrong, but I wonder if we should did anything
> different with the handling of this capability. It's specified to
> contain only the ID and next pointer, so I'd expect it's primarily a
> mechanism for hardware vendors to blow fuses in config space to
> maintain a capability chain while maybe hiding a feature not supported
> by the product sku. Hiding the capability in vfio is trivial, exposing
> it implies some changes to our config space map that might be more
> subtle. I'm inclined to stick with this solution for now. Thanks,
>
> Alex
>From this description, I also think that we should simply hide these
NULL capabilities.
