Virtualization - Aug 2022 - [virtio-dev] [PATCH] virtio-net: use mtu size as buffer length for big packets

> From: Si-Wei Liu <si-wei.liu at oracle.com>
> Sent: Tuesday, August 9, 2022 3:09 PM
> >> From: Si-Wei Liu <si-wei.liu at 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.

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".

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).

On 8/9/2022 12:18 PM, Parav Pandit wrote:
>> From: Si-Wei Liu <si-wei.liu at oracle.com>
>> Sent: Tuesday, August 9, 2022 3:09 PM
>>>> From: Si-Wei Liu <si-wei.liu at 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.

-Siwei

On Tue, Aug 09, 2022 at 07:18:30PM +0000, Parav Pandit
wrote:
> > From: Si-Wei Liu <si-wei.liu at oracle.com>
> > Sent: Tuesday, August 9, 2022 3:09 PM
> 
> > >> From: Si-Wei Liu <si-wei.liu at 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.
> 
> 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.

> 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).