linux-kernel - Re: [PATCH] vfio-pci: Mask cap zero

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Message-ID: <20200504220804.GA22939@nvidia.com>
Date:   Mon, 4 May 2020 15:08:08 -0700
From:   Neo Jia <cjia@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>
CC:     Cornelia Huck <cohuck@...hat.com>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] vfio-pci: Mask cap zero

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@...hat.com> wrote:
> 
> > On Fri, 01 May 2020 15:41:24 -0600
> > Alex Williamson <alex.williamson@...hat.com> 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@...hat.com>
> > > ---
> > >  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?

Thanks,
Neo

> >
> 
> Sure.
> 
> >
> > > +           if (cap && cap <= PCI_CAP_ID_MAX) {
> > >                     len = pci_cap_length[cap];
> > >                     if (len == 0xFF) { /* Variable length */
> > >                             len = vfio_cap_len(vdev, cap, pos);
> > >
> >
> > Is there a requirement for caps to be strictly ordered? If not, could
> > len hold a residual value from a previous iteration?
> 
> There is no ordering requirement for capabilities, but len is declared
> non-static with an initial value within the scope of the loop, it's
> reset every iteration.  Thanks,
> 
> Alex
>