On Tue, Aug 09, 2022 at 09:13:42PM +0000, Parav Pandit wrote: > > From: Si-Wei Liu <si-wei.liu@xxxxxxxxxx> > > 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@xxxxxxxxxx> > > >> Sent: Tuesday, August 9, 2022 3:09 PM > > >>>> From: Si-Wei Liu <si-wei.liu@xxxxxxxxxx> > > >>>> 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 _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
