On 8/9/2022 2:37 PM, Michael S. Tsirkin wrote:
On Tue, Aug 09, 2022 at 07:18:30PM +0000, 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.
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.
It'd be nice to document this requirement or statement to the spec for
clarity purpose. Otherwise various device implementations are hard to
follow. The capture is that vhost-net drops bigger packets while the
driver only supplied smaller buffers. This is the status quo, and users
seemingly have relied on this behavior for some while.
-Siwei
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).
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