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

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Date:   Mon, 4 May 2020 12:52:53 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Cornelia Huck <cohuck@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vfio-pci: Mask cap zero

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 */
> 

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