Linux Virtualization - Aug 2015 - [PATCH v2 1/2] virtio-net: rephrase devconf fields description

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 at 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

On 08/19/2015 07:54 PM, Victor Kaplansky wrote:
> 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 at 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.  
Looks not, it's legal to negotiate VIRTIO_NET_F_MQ with
max_virtqueue_pairs = 1.
>
> If above is correct, I'll change the description to make this
> point more clear.
>
> Thanks,
> -- Victor
Driver also sets 'mac' before driver negotiated VIRTIO_NET_F_MAC, so
I'm
not sure this is really needed.

Thanks