netdev - Re: [PATCH v3 net-next RFC] Generic XDP

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Date:   Mon, 17 Apr 2017 16:04:38 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     brouer@...hat.com, kubakici@...pl, netdev@...r.kernel.org,
        xdp-newbies@...r.kernel.org
Subject: Re: [PATCH v3 net-next RFC] Generic XDP

On Mon, Apr 17, 2017 at 03:49:55PM -0400, David Miller wrote:
> From: Jesper Dangaard Brouer <brouer@...hat.com>
> Date: Sun, 16 Apr 2017 22:26:01 +0200
> 
> > The bpf tail-call use-case is a very good example of why the
> > verifier cannot deduct the needed HEADROOM upfront.
> 
> This brings up a very interesting question for me.
> 
> I notice that tail calls are implemented by JITs largely by skipping
> over the prologue of that destination program.
> 
> However, many JITs preload cached SKB values into fixed registers in
> the prologue.  But they only do this if the program being JITed needs
> those values.
> 
> So how can it work properly if a program that does not need the SKB
> values tail calls into one that does?

For x86 JIT it's fine, since caching of skb values is not part of the prologue:
  emit_prologue(&prog);
  if (seen_ld_abs)
          emit_load_skb_data_hlen(&prog);
and tail_call jumps into the next program as:
  EMIT4(0x48, 0x83, 0xC0, PROLOGUE_SIZE);   /* add rax, prologue_size */
  EMIT2(0xFF, 0xE0);                        /* jmp rax */
whereas inside emit_prologue() we have:
B  UILD_BUG_ON(cnt != PROLOGUE_SIZE);

arm64 has similar proplogue skipping code and it's even
simpler than x86, since it doesn't try to optimize LD_ABS/IND in assembler
and instead calls into bpf_load_pointer() from generated code,
so no caching of skb values at all.

s390 jit has partial skipping of prologue, since bunch
of registers are save/restored during tail_call and it looks fine
to me as well.

It's very hard to extend test_bpf.ko with tail_calls, since maps need
to be allocated and populated with file descriptors which are
not feasible to do from .ko. Instead we need a user space based test for it.
We've started building one in tools/testing/selftests/bpf/test_progs.c
much more tests need to be added. Thorough testing of tail_calls
is on the todo list.