[dpdk-dev] [PATCH 0/7] virtio/vhost: Add MTU feature support

[Maxime Coquelin]

On 02/23/2017 08:10 AM, Yuanhan Liu wrote:
> On Mon, Feb 13, 2017 at 03:28:13PM +0100, Maxime Coquelin wrote:
>> This series adds support to new Virtio's MTU feature[1].
>
> Seems you missed a link here?
Oops, I meant this link:
https://www.spinics.net/lists/linux-virtualization/msg28908.html

Will fix in v2.

>
>> The MTU
>> value is set via QEMU parameters.
>>
>> If the feature is negotiated (i.e supported by both host andcguest,
>> and valid MTU value is set in QEMU via its host_mtu parameter), QEMU
>> shares the configured MTU value throught dedicated Vhost protocol
>> feature.
>>
>> On vhost side, the value is stored in the virtio_net structure, and
>> made available to the application thanks to new vhost lib's
>> rte_vhost_mtu_get() function.
>>
>> rte_vhost_mtu_set() functions is implemented, but only succeed if the
>> application sets the same value as the one set in QEMU. Idea is that
>> it would be used for the application to ensure configured MTU value
>> is consistent, but maybe the mtu_get() API is enough, and mtu_set()
>> could just be dropped.
>
> If the vhost MTU is designed to be read-only, then we may should drop
> the set_mtu function.

Ok, I will remove it in next revision.

>> Vhost PMD mtu_set callback is also implemented
>> in the same spirit.
>>
>> To be able to set eth_dev's MTU value at the right time, i.e. to call
>> rte_vhost_mtu_get() just after Virtio features have been negotiated
>> and before the device is really started, a new vhost flag has been
>> introduced (VIRTIO_DEV_READY), because the VIRTIO_DEV_RUNNING flag is
>> set too late (after .new_device() ops is called).
>
> Okay, and I think this kind of info should be in corresponding commit log.
Sure.
>
>> Regarding valid MTU values, the maximum MTU value accepted on vhost
>> side is 65535 bytes, as defined in Virtio Spec and supported in
>> Virtio-net Kernel driver. But in Virtio PMD, current maximum frame
>> size is 9728 bytes (~9700 bytes MTU). So maximum MTU size accepted in
>> Virtio PMD is the minimum between ~9700 bytes and host's MTU.
>>
>> Initially, we thought about disabling the rx-mergeable feature when
>> MTU value was low enough to ensure all received packets would fit in
>> receive buffers (when offloads are disabled). Doing this, we would
>> save one cache-miss in the receive path. Problem is that we don't
>> know the buffers size at Virtio feature neogotiation time.
>> It might be possible for the application to call the configure
>> callback again once the Rx queue is set up, but it seems a bit hacky.
>
> Worse, if multiple queue is involved, one queue could have it's own
> mempool, meaning the buffer size could be different, whereas the
> MTU feature is global.
Indeed, we should ensure that for every queues, the corresponding buf
size can handle a full packet to be able to disable the mergeable
feature.

Let's skip this for now and focus on improving the rx-mergeable path.

Thanks,
Maxime
>
> 	--yliu
>
>> So I decided to skip this optimization for now, even if feedback and
>> are of course appreciated.
>>
>> Finally, this series also adds MTU value printing  in testpmd's
>> "show port info" command when non-zero.
>>
>> This series target v17.05 release.
>>
>> Cheers,
>> Maxime
>>
>> Maxime Coquelin (7):
>>   vhost: Enable VIRTIO_NET_F_MTU feature
>>   vhost: vhost-user: Add MTU protocol feature support
>>   vhost: Add new ready status flag
>>   vhost: Add API to get/set MTU value
>>   net/vhost: Implement mtu_set callback
>>   net/virtio: Add MTU feature support
>>   app/testpmd: print MTU value in show port info
>>
>>  app/test-pmd/config.c               |  5 +++++
>>  doc/guides/nics/features/vhost.ini  |  1 +
>>  doc/guides/nics/features/virtio.ini |  1 +
>>  drivers/net/vhost/rte_eth_vhost.c   | 18 +++++++++++++++
>>  drivers/net/virtio/virtio_ethdev.c  | 22 +++++++++++++++++--
>>  drivers/net/virtio/virtio_ethdev.h  |  3 ++-
>>  drivers/net/virtio/virtio_pci.h     |  3 +++
>>  lib/librte_vhost/rte_virtio_net.h   | 31 ++++++++++++++++++++++++++
>>  lib/librte_vhost/vhost.c            | 42 ++++++++++++++++++++++++++++++++++-
>>  lib/librte_vhost/vhost.h            |  9 +++++++-
>>  lib/librte_vhost/vhost_user.c       | 44 +++++++++++++++++++++++++++++++------
>>  lib/librte_vhost/vhost_user.h       |  5 ++++-
>>  12 files changed, 171 insertions(+), 13 deletions(-)
>>
>> --
>> 2.9.3