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 4:33 PM
> 
> 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.
Ok. Somehow the application knows the mtu to set on (all) peer(s) and hope for the best.
Understood.