Re: [PATCH v2 1/2] virtio-net: rephrase devconf fields description

[Victor Kaplansky <victork@redhat.com>]

On Mon, Aug 17, 2015 at 10:43:46AM +0800, Jason Wang wrote:
> 
> 
> On 08/16/2015 09:42 PM, Victor Kaplansky wrote:
> > Clarify general description of the mac, status and
> > max_virtqueue_pairs fields. Specifically, the old description is
> > vague about configuration layout and fields offsets when some of
> > the fields are non valid.
> >
> > Also clarify that validity of two status bits depends on two
> > different feature flags.
> >
> > Signed-off-by: Victor Kaplansky <victork@redhat.com>
> > ---
> > +
> > +\item [\field{max_virtqueue_pairs}] tells the driver the maximum
> > +    number of each of virtqueues (receiveq1\ldots receiveqN and
> > +    transmitq1\ldots transmitqN respectively) that can be configured
> > +    on the device once VIRTIO_NET_F_MQ is negotiated.
> > +    \field{max_virtqueue_pairs} is valid only if VIRTIO_NET_F_MQ is
> > +    set and can be read by the driver.
> > +
> 
> 
> I don't get the point that adding "can be read by the driver". Looks
> like it's hard for hypervisor to detect this?

AFAIU, if the device sets VIRTIO_NET_F_MQ, the device also sets
the value of 'max_virtqueue_pairs' even before driver negotiated
VIRTIO_NET_F_MQ. If so, the driver can read the value of
'max_virtqueue_pairs' during negotiation and potentially this
value can even affect negotiation decision of the driver.  

If above is correct, I'll change the description to make this
point more clear.

Thanks,
-- Victor