Re: [PATCH v2 net-next] udp: allow forwarding of plain (non-fraglisted) UDP GRO packets

[Paolo Abeni]

On Wed, 2021-01-13 at 10:32 +0000, Alexander Lobakin wrote:
> Commit 9fd1ff5d2ac7 ("udp: Support UDP fraglist GRO/GSO.") actually
> not only added a support for fraglisted UDP GRO, but also tweaked
> some logics the way that non-fraglisted UDP GRO started to work for
> forwarding too.
> Commit 2e4ef10f5850 ("net: add GSO UDP L4 and GSO fraglists to the
> list of software-backed types") added GSO UDP L4 to the list of
> software GSO to allow virtual netdevs to forward them as is up to
> the real drivers.
> 
> Tests showed that currently forwarding and NATing of plain UDP GRO
> packets are performed fully correctly, regardless if the target
> netdevice has a support for hardware/driver GSO UDP L4 or not.
> Add the last element and allow to form plain UDP GRO packets if
> there is no socket -> we are on forwarding path.

If I read correctly, the above will make UDP GRO in the forwarding path
always enabled (admin can't disable that, if forwarding is enabled).

UDP GRO can introduce measurable latency for UDP packets staging in the
napi GRO hash (no push flag for UDP ;).

Currently the admin (for fraglist) or the application (for socket-based 
"plain" GRO) have to explicitly enable the feature, but this change
will impact every user.

I think we need at lest an explict switch for this.

Cheers,

Paolo