RE: [virtio-dev] [PATCH] virtio-net: use mtu size as buffer length for big packets

[Parav Pandit <parav@nvidia.com>]

> From: Si-Wei Liu <si-wei.liu@oracle.com>
> Sent: Tuesday, August 9, 2022 3:09 PM

> >> From: Si-Wei Liu <si-wei.liu@oracle.com>
> >> Sent: Tuesday, August 9, 2022 2:39 PM Currently it is not. Not a
> >> single patch nor this patch, but the context for the eventual goal is
> >> to allow XDP on a MTU=9000 link when guest users intentionally lower
> >> down MTU to 1500.
> > Which application benefit by having asymmetry by lowering mtu to 1500
> to send packets but want to receive 9K packets?

Below details doesnât answer the question of asymmetry. :)

> I think virtio-net driver doesn't differentiate MTU and MRU, in which case
> the receive buffer will be reduced to fit the 1500B payload size when mtu is
> lowered down to 1500 from 9000. 
How? Driver reduced the mXu to 1500, say it is improved to post buffers of 1500 bytes.

Device doesn't know about it because mtu in config space is RO field.
Device keep dropping 9K packets because buffers posted are 1500 bytes.
This is because device follows the spec " The device MUST NOT pass received packets that exceed mtu".

So, I am lost what virtio net device user application is trying to achieve by sending smaller packets and dropping all receive packets.
(it doesnât have any relation to mergeable or otherwise).