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

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).

> From: Michael S. Tsirkin <mst at redhat.com>
> Sent: Tuesday, August 9, 2022 5:38 PM
[..]
> > > 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 */
> 
It is the field from struct virtio_net_config.mtu. right?
This is RO field for driver.
> there is no guarantee device will not get a bigger packet.
Right. That is what I also hinted.
Hence, allocating buffers worth upto mtu is safer.
When user overrides it, driver can be further optimized to honor such new value
on rx buffer posting.
> And there is no guarantee such a packet will be dropped as opposed to
> wedging the device if userspace insists on adding smaller buffers.
>
If user space insists on small buffers, so be it. It only works when user
exactly know what user is doing in the whole network.
When user prefers to override the device RO field, device is in the dark and
things work on best effort basis.
This must be a reasonably advance user who has good knowledge of its network
topology etc.

For such case, may be yes, driver should be further optimized.

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 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.
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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