netdev - Re: [PATCH v3 net-next 3/6] net: Add SW fallback infrastructure for offloaded sockets

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Message-ID: <20171219150546.GC6122@localhost.localdomain>
Date:   Tue, 19 Dec 2017 13:05:46 -0200
From:   Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
To:     Ilya Lesokhin <ilyal@...lanox.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "davejwatson@...com" <davejwatson@...com>,
        "tom@...bertland.com" <tom@...bertland.com>,
        "hannes@...essinduktion.org" <hannes@...essinduktion.org>,
        Boris Pismenny <borisp@...lanox.com>,
        Aviad Yehezkel <aviadye@...lanox.com>,
        Liran Liss <liranl@...lanox.com>,
        Steffen Klassert <steffen.klassert@...unet.com>
Subject: Re: [PATCH v3 net-next 3/6] net: Add SW fallback infrastructure for
 offloaded sockets

On Tue, Dec 19, 2017 at 07:51:02AM +0000, Ilya Lesokhin wrote:
> On Monday, December 18, 2017 9:18 PM, Marcelo Ricardo Leitner wrote:
> 
> > > +
> > > +	if (sk && sk_fullsock(sk) && sk->sk_offload_check)
> > 
> > Isn't this going to hurt the fast path, checking for sk fields here?
> > 
> 
> We do add code to the fast path but it seems unavoidable if you want to have SW fallback.
> The XFRM device offload also does that
> http://elixir.free-electrons.com/linux/v4.14.7/source/net/core/dev.c#L3058

Right, although a bit different. It's accessing skb->sp and not the
socket and depending on how compiler is doing things, the check
http://elixir.free-electrons.com/linux/v4.14.7/source/net/xfrm/xfrm_device.c#L32
will help in some cases.

But more importantly, all the above only exists if CONFIG_XFRM_OFFLOAD
is enabled.

> 
> The check can be optimized but I didn't want to do that before I saw that it's an issue.
> I'm also not sure what the correct solution is.
> I don't like that fact that each "stateful protocol" we offload requires its own check. 
> We need to think if we can find a generic way of doing it.
> 
> Perhaps we can hold the expected netdev somewhere in the SKB and only if we don't
> Go out of the expected netdev go to a slow path that does a check for each protocol.

This could be a good switch, yes.

Thanks,
Marcelo