netdev - Re: [PATCH RFC 0/3] Support fraglist GRO/GSO

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Message-ID: <20190128075154.GE3581@gauss3.secunet.de>
Date:   Mon, 28 Jan 2019 08:51:54 +0100
From:   Steffen Klassert <steffen.klassert@...unet.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
CC:     Network Development <netdev@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>
Subject: Re: [PATCH RFC 0/3] Support fraglist GRO/GSO

On Fri, Jan 25, 2019 at 08:57:00AM -0500, Willem de Bruijn wrote:
> On Fri, Jan 25, 2019 at 3:14 AM Steffen Klassert
> <steffen.klassert@...unet.com> wrote:
> >
> > On Mon, Jan 14, 2019 at 12:09:22PM -0500, Willem de Bruijn wrote:
> > > On Mon, Jan 14, 2019 at 7:50 AM Steffen Klassert
> > > <steffen.klassert@...unet.com> wrote:
> > > > On Sun, Dec 23, 2018 at 08:15:40PM -0500, Willem de Bruijn wrote:
> > >
> > > I don't think that the route lookup is needed. If listified is cheaper
> > > for local delivery, too, then we can make that the default unless a
> > > device is active with h/w offload and ip forwarding is enabled. If it
> > > isn't, then use it iff ip forwarding is enabled. I think it's fine to
> > > mispredict between the two in edge cases with netfilter mangling, as
> > > long as all paths can correctly handle both types of GRO packets.
> >
> > I'd need at least a route lookup for my usecase, because listified
> > GRO is always cheaper when a xfrm transformation is needed (even for
> > TCP). In this case is software GSO needed. So I'd need to either have
> > an early route lookup or maybe some early ingress hook where a route
> > lookup could be imlemented in.
> 
> Could you use a similar system wide approach as what we discussed
> previous wrt hardware offload? Use listified only if (forwarding is enabled
> and no device is registered that implements h/w segmentation offload) or
> any path requires xfrm transformation (?).

The xfrm transformation has to happen for the segments. So if we need to
do xfrm transformation in software, we need to do segmentation in
software too. I think that just forwarding is enabled and the presence
of a device that can do hardware segmentation offload is not a good
indicator. The more devices support hardware segmentation offload
the more likely is it that xfrm take a suboptimal path.

We have to do a route lookup anyway, why not just do it early
in case forwarding is enabled?