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.
-Siwei
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