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

[Si-Wei Liu <si-wei.liu@oracle.com>]

On 8/9/2022 12:18 PM, Parav Pandit wrote:
> > 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.
For big_packet path, yes, we need improvement; for mergeable, it's 
adaptable to any incoming packet size so 1500 is what it is today.
> 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".
Right, that's what it happens today on device side (i.e. vhost-net, btw 
mlx5 vdpa device seems to have a bug not pro-actively dropping packets 
that exceed the MTU size, causing guest panic in small packet path).
> 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).

Usually, the use case I'm aware of would set the peer's MTU to 1500 
(e.g. on a virtual network appliance), or it would rely on path mtu 
discovery to avoid packet drop across links.

-Siwei