netdev - Re: [RFC] netfilter: nf_tables: ignore errors on flowtable device hw offload setup

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Date:   Fri, 13 May 2022 10:15:21 +0200
From:   Pablo Neira Ayuso <pablo@...filter.org>
To:     Felix Fietkau <nbd@....name>
Cc:     netfilter-devel@...r.kernel.org, netdev@...r.kernel.org,
        Jo-Philipp Wich <jo@...n.io>
Subject: Re: [RFC] netfilter: nf_tables: ignore errors on flowtable device hw
 offload setup

On Fri, May 13, 2022 at 10:03:13AM +0200, Felix Fietkau wrote:
> 
> On 13.05.22 09:49, Pablo Neira Ayuso wrote:
> > Hi,
> > 
> > On Tue, May 10, 2022 at 10:27:39PM +0200, Felix Fietkau wrote:
> > > In many cases, it's not easily possible for user space to know, which
> > > devices properly support hardware offload.
> > 
> > Then, it is a matter of extending the netlink interface to expose this
> > feature? Probably add a FLOW_BLOCK_PROBE or similar which allow to
> > consult if this feature is available?
> > 
> > > Even if a device supports hardware flow offload, it is not
> > > guaranteed that it will actually be able to handle the flows for
> > > which hardware offload is requested.
> > 
> > When might this happen?
>
> I think there are many possible reasons: The flow might be using features
> not supported by the offload driver. Maybe it doesn't have any space left in
> the offload table. I'm sure there are many other possible reasons it could
> fail.

This fallback to software flowtable path for partial scenarios already
exists.

> > > Ignoring errors on the FLOW_BLOCK_BIND makes it a lot easier to set up
> > > configurations that use hardware offload where possible and gracefully
> > > fall back to software offload for everything else.
> > 
> > I understand this might be useful from userspace perspective, because
> > forcing the user to re-try is silly.
> > 
> > However, on the other hand, the user should have some way to know from
> > the control plane that the feature (hardware offload) that they
> > request is not available for their setup.
>
> In my opinion, most users of this API probably don't care and just want to
> have offload on a best effort basis.

OK, but if the setup does not support hardware offload at all, why
should the control plane accept this? I think user should know in
first place that no one single flow is going to be offloaded to
hardware.

> Assuming that is the case, wouldn't it be better if we simply have
> an API that indicates, which flowtable members hardware offload was
> actually enabled for?

What are you proposing?

I think it would be good to expose through netlink interface what the
device can actually do according to the existing supported flowtable
software datapath features.