netdev - Re: [patch net-next 0/9] net: sched: introduce chain templates support with offloading to mlxsw

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Date:   Mon, 25 Jun 2018 21:58:50 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Jiri Pirko <jiri@...nulli.us>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, jhs@...atatu.com,
        xiyou.wangcong@...il.com, simon.horman@...ronome.com,
        john.hurley@...ronome.com, dsahern@...il.com, mlxsw@...lanox.com
Subject: Re: [patch net-next 0/9] net: sched: introduce chain templates
 support with offloading to mlxsw

On Mon, 25 Jun 2018 23:01:39 +0200, Jiri Pirko wrote:
> From: Jiri Pirko <jiri@...lanox.com>
> 
> For the TC clsact offload these days, some of HW drivers need
> to hold a magic ball. The reason is, with the first inserted rule inside
> HW they need to guess what fields will be used for the matching. If
> later on this guess proves to be wrong and user adds a filter with a
> different field to match, there's a problem. Mlxsw resolves it now with
> couple of patterns. Those try to cover as many match fields as possible.
> This aproach is far from optimal, both performance-wise and scale-wise.
> Also, there is a combination of filters that in certain order won't
> succeed.
> 
> Most of the time, when user inserts filters in chain, he knows right away
> how the filters are going to look like - what type and option will they
> have. For example, he knows that he will only insert filters of type
> flower matching destination IP address. He can specify a template that
> would cover all the filters in the chain.

Perhaps it's lack of sleep, but this paragraph threw me a little off
the track.  IIUC the goal of this set is to provide a way to inform the
HW about expected matches before any rule is programmed into the HW.
Not before any rule is added to a particular chain.  One can just use
the first rule in the chain to make a guess about the chain, but thanks
to this set user can configure *all* chains before any rules are added.

And that's needed because once any rule is added the tcam config can no
longer be easily modified?