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

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Date:   Mon, 4 May 2020 18:09:16 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vfio-pci: Mask cap zero

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 */

?

> +		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?