netdev - Re: [RFC PATCH bpf-next v2 0/4] Implement bpf queue/stack maps

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Date:   Mon, 10 Sep 2018 18:04:42 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Mauricio Vasquez <mauricio.vasquez@...ito.it>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Network Development <netdev@...r.kernel.org>,
        Joe Stringer <joe@...d.net.nz>
Subject: Re: [RFC PATCH bpf-next v2 0/4] Implement bpf queue/stack maps

On Fri, Sep 7, 2018 at 1:40 PM, Mauricio Vasquez
<mauricio.vasquez@...ito.it> wrote:
>
> I read the Joe's proposal and using that for this problem looks like a nice
> solution.
>
> I think a good trade-off for now would be to go ahead with a queue/stack map
> without preallocating support (or maybe include it having always in mind
> that this issue has to be solved in the near future) and then, as a
> separated work, try to use Joe's proposal in the map helpers.
>
> What do you think?

the problem with such approach is that it would mean that
non-prealloc stack/queue api will be different from future one
after verifier will get smarter.
The alternative would be to support by-value api only.
Meaning let stack/queue support value_size = 1,2,4,8 byte only.
Then bpf_push|pop_elem() helper will be by-value
instead of returning a pointer.
No rcu callback issues and implementation on the kernel
side can be much simpler.
I think simple array of value_size elems with head/tail indices
will be enough.
Once we have Joe's verifier improvements
we can add alloc/free bpf object management facility
and use 8-byte stack/queue mapas a stack of pointers.
I think decoupling memory operations alloc/free from
stack push/pop would be additional benefit of such design.