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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Aug 09, 2022 at 07:18:30PM +0000, 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.
> 
> 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".


The "mtu" here is the device config field, which is

        /* Default maximum transmit unit advice */

there is no guarantee device will not get a bigger packet.
And there is no guarantee such a packet will be dropped
as opposed to wedging the device if userspace insists on
adding smaller buffers.


> 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).