Re: [Qemu-devel] [PATCH v4 0/3] virtio-net: Add support to MTU feature

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

Hi Daniel,

On 12/12/2016 11:02 AM, Daniel P. Berrange wrote:
> On Sat, Dec 10, 2016 at 04:30:35PM +0100, Maxime Coquelin wrote:
>> Thanks for the reviews,
>>
>> This series implements Virtio spec update from Aaron Conole which
>> defines a way for the host to expose its max MTU to the guest.
>>
>> "host_mtu" parameter is added to provide QEMU with the MTU value,
>> and the backend, if supported, gets notified of the MTU value when the
>> MTU feature neogotiation succeeds.
>>
>> Only user backend currently supports MTU notification. A new protocol
>> feature has been implemented for sending MTU value to the backend.
>>
>> For kernel backend, it is expected the management tool also configures
>> the tap/macvtap interface with same MTU value.
>> Daniel, I would be interrested about your feedback on this implementation
>> from management tool point of view.
>
> I can't give real feedback yet, as I'm not seeing clear information on
> what problem this series is designed to solve....

Right, I agree it is missing a bit of context here, I'll add more about
the background in next revision.

The goal of this series is to address two things:
1. Providing a way for the guests to use the same MTU as the host,
    in order to have a consistent MTU value across the infrastructure.

2. Improve Rx performance for small packets, as if all parties agrees
    on a MTU value for which packets fit into a single descriptor, then
    Rx mergeable buffers feature can be disabled (saving one
    cache-miss on guest side by not accessing the virtio header, when
    offloading has not been negotiated).

Thanks,
Maxime