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 09:13:42PM +0000, Parav Pandit wrote:
> > 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 doesn’t 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 doesn’t 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.

That's generally what one has to do with mtu, yes - it has to be set
consistently across the LAN. While e.g. pMTU might help work around some
misconfigured LANs with a mix of different MTUs it was never designed
for that.

-- 
MST


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