netdev - Re: [PATCH net-next 2/4] veth: allow enabling NAPI even without XDP

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Date:   Fri, 09 Apr 2021 17:20:12 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Toshiaki Makita <toshiaki.makita1@...il.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>
Subject: Re: [PATCH net-next 2/4] veth: allow enabling NAPI even without XDP

On Fri, 2021-04-09 at 16:58 +0200, Toke Høiland-Jørgensen wrote:
> Paolo Abeni <pabeni@...hat.com> writes:
> 
> > Currently the veth device has the GRO feature bit set, even if
> > no GRO aggregation is possible with the default configuration,
> > as the veth device does not hook into the GRO engine.
> > 
> > Flipping the GRO feature bit from user-space is a no-op, unless
> > XDP is enabled. In such scenario GRO could actually take place, but
> > TSO is forced to off on the peer device.
> > 
> > This change allow user-space to really control the GRO feature, with
> > no need for an XDP program.
> > 
> > The GRO feature bit is now cleared by default - so that there are no
> > user-visible behavior changes with the default configuration.
> > 
> > When the GRO bit is set, the per-queue NAPI instances are initialized
> > and registered. On xmit, when napi instances are available, we try
> > to use them.
> 
> Am I mistaken in thinking that this also makes XDP redirect into a veth
> work without having to load an XDP program on the peer device? That's
> been a long-outstanding thing we've been meaning to fix, so that would
> be awesome! :)

I have not experimented that, and I admit gross ignorance WRT this
argument, but AFAICS the needed bits to get XDP redirect working on
veth are the ptr_ring initialization and the napi instance available.

With this patch both are in place when GRO is enabled, so I guess XPD
redirect should work, too (modulo bugs for untested scenario).

Thanks!

Paolo