netdev - Re: [patch net-next v2 8/9] switchdev: introduce Netlink API

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Message-ID: <CA+mtBx-ZvkZ2ALGLsLEN7Cgn9gN_rY36ZhuqYC3mZ9WVxjFDaQ@mail.gmail.com>
Date:	Mon, 22 Sep 2014 08:10:08 -0700
From:	Tom Herbert <therbert@...gle.com>
To:	Thomas Graf <tgraf@...g.ch>
Cc:	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Jiri Pirko <jiri@...nulli.us>,
	John Fastabend <john.r.fastabend@...el.com>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Neil Horman <nhorman@...driver.com>,
	Andy Gospodarek <andy@...yhouse.net>,
	Daniel Borkmann <dborkman@...hat.com>,
	Or Gerlitz <ogerlitz@...lanox.com>,
	Jesse Gross <jesse@...ira.com>,
	Pravin Shelar <pshelar@...ira.com>,
	Andy Zhou <azhou@...ira.com>,
	Ben Hutchings <ben@...adent.org.uk>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	Vladislav Yasevich <vyasevic@...hat.com>,
	Cong Wang <xiyou.wangcong@...il.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Scott Feldman <sfeldma@...ulusnetworks.com>,
	Florian Fainelli <f.fainelli@...il.com>,
	Roopa Prabhu <roopa@...ulusnetworks.com>,
	John Linville <linville@...driver.com>,
	"dev@...nvswitch.org" <dev@...nvswitch.org>,
	Jason Wang <jasowang@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Nicolas Dichtel <nicolas.dichtel@...nd.com>,
	ryazanov.s.a@...il.com, Lennert Buytenhek <buytenh@...tstofly.org>,
	aviadr@...lanox.com, Felix Fietkau <nbd@...nwrt.org>,
	Neil Jerram <Neil.Jerram@...aswitch.com>, ronye@...lanox.com,
	simon.horman@...ronome.com,
	Alexander Duyck <alexander.h.duyck@...el.com>
Subject: Re: [patch net-next v2 8/9] switchdev: introduce Netlink API

On Mon, Sep 22, 2014 at 1:13 AM, Thomas Graf <tgraf@...g.ch> wrote:
> On 09/20/14 at 03:50pm, Alexei Starovoitov wrote:
>> I think HW should not be limited by SW abstractions whether
>> these abstractions are called flows, n-tuples, bridge or else.
>> Really looking forward to see "device reporting the headers as
>> header fields (len, offset) and the associated parse graph"
>> as the first step.
>>
>> Another topic that this discussion didn't cover yet is how this
>> all connects to tunnels and what is 'tunnel offloading'.
>> imo flow offloading by itself serves only academic interest.
>
> We haven't touched encryption yet either ;-)
>
> Certainly true for the host case. The Linux on TOR case is less
> dependant on this and L2/L3 offload w/o encap already has value.
>
Thomas, can you (or someone else) quantify what the host case is. I
suppose there may be merit in using a switch on NIC for kernel bypass
scenarios, but I'm still having a hard time understanding how this
could be integrated into the host stack with benefits that outweigh
complexity. The history of stateful offloads in NICs is not great, and
encapsulation (stuffing a few bytes of header into a packet) is in
itself not nearly an expensive enough operation to warrant offloading
to the NIC. Personally, I wish if NIC vendors are going to focus on
stateful offload I rather see it be for encryption which I believe
currently does warrant offload at 40G and higher speeds.

Tom

> I'm with you though, all of this has little value on the host in
> the DC if stateful encap offload is not incorporated. I expect the
> HW to provide filters on the outer header plus metadata in the
> encap. Actually, this was a follow-up question I had for John as
> this is not easily describable with offset/len filters. How would
> we represent such capabilities?
>
> The TX side of this was one of the reasons why I initially thought
> it would be beneficial to implement a cache like offload as we could
> serve an initial encap in SW, do the FIB lookup and offload it
> transparently to avoid replicating the FIB in user space.
>
> What seems most feasisble to me right now is to separate the offload
> of the encap action from the IP -> dev mapping decision. The eSwitch
> would send the first encap for an unknown dest IP to the CPU due
> to a miss in the IP mapping table, the CPU would do the FIB lookup,
> update the table and send it back.
>
> What do you have in mind?
> --
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