Re: [PATCH v2] add mtu set in virtio

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Wed, Sep 07, 2016 at 11:16:47AM +0200, Maxime Coquelin wrote:
> 
> 
> 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.

Thanks for the explanation!

I see. So, the point is to avoid using mergeable buffers while it is
enabled.

> Does that sound reasonnable?

Yeah, maybe. Just don't know how well it may work in real life. Have
you got any rought data so far?

	--yliu

> Do I miss something?
> 
> Thanks,
> Maxime