Re: [dpdk-dev] [PATCH v2] add mtu set in virtio

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 09/07/2016 05:25 AM, Yuanhan Liu wrote:
> On Tue, Aug 30, 2016 at 09:57:39AM +0200, Maxime Coquelin wrote:
>> Hi Souvik,
>>
>> On 08/30/2016 01:02 AM, souvikdey33 wrote:
>>> Signed-off-by: Souvik Dey <sodey@sonusnet.com>
>>>
>>> Fixes: 1fb8e8896ca8 ("Signed-off-by: Souvik Dey <sodey@sonusnet.com>")
>>> Reviewed-by: Stephen Hemminger <stephen@networkplumber.org>
>>>
>>> Virtio interfaces should also support setting of mtu, as in case of cloud
>>> it is expected to have the consistent mtu across the infrastructure that
>>> the dhcp server sends and not hardcoded to 1500(default).
>>> ---
>>> drivers/net/virtio/virtio_ethdev.c | 12 ++++++++++++
>>> 1 file changed, 12 insertions(+)
>>
>> FYI, there are some on-going changes in the VIRTIO specification
>> so that the VHOST interface exposes its MTU to its VIRTIO peer.
>> It may also be used as an alternative of what you patch achieves.
>>
>> I am working on its implementation in Qemu/DPDK, our goal being to
>> reduce performance drops for small packets with Rx mergeable buffers
>> feature enabled.
>
> Mind to educate me a bit on how that works?

Of course.

Basically, this is a way to advise the MTU we want in the guest.
In the guest, if GRO is not enabled:
  - In case of Kernel virtio-net, it could be used to
size the SKBs at the expected MTU. If possible, we could disable Rx
mergeable buffers.
  - In case of virtio PMD, if the MTU advised by host is lower than the
pre-allocated mbuf size for the receive queue, then we should not need
mergeable buffers.

Does that sound reasonnable?
Do I miss something?

Thanks,
Maxime