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Date:   Wed, 29 Apr 2020 22:53:15 -0500
From:   Josh Poimboeuf <jpoimboe@redhat.com>
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
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        daniel@iogearbox.net
Subject: Re: BPF vs objtool again
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On Wed, Apr 29, 2020 at 07:10:52PM -0700, Alexei Starovoitov wrote:
> > For example:
> > 
> > #define GOTO    ({ goto *jumptable[insn->code]; })
> > 
> > and then replace all 'goto select_insn' with 'GOTO;'
> > 
> > The problem is that with RETPOLINE=y, the function text size grows from
> > 5k to 7k, because for each of the 160+ retpoline JMPs, GCC (stupidly)
> > reloads the jump table register into a scratch register.
> 
> that would be a tiny change, right?
> I'd rather go with that and gate it with 'ifdef CONFIG_FRAME_POINTER'
> Like:
> #ifndef CONFIG_FRAME_POINTER
> #define CONT     ({ insn++; goto select_insn; })
> #define CONT_JMP ({ insn++; goto select_insn; })
> #else
> #define CONT     ({ insn++; goto *jumptable[insn->code]; })
> #define CONT_JMP ({ insn++; goto *jumptable[insn->code]; })
> #endif
> 
> The reason this CONT and CONT_JMP macros are there because a combination
> of different gcc versions together with different cpus make branch predictor
> behave 'unpredictably'.
> 
> I've played with CONT and CONT_JMP either both doing direct goto or
> indirect goto and observed quite different performance characteristics
> from the interpreter.
> What you see right now was the best tune I could get from a set of cpus
> I had to play with and compilers. If I did the same tuning today the outcome
> could have been different.
> So I think it's totally fine to use above code. I think some cpus may actually
> see performance gains with certain versions of gcc.
> The retpoline text increase is unfortunate but when retpoline is on
> other security knobs should be on too. In particular CONFIG_BPF_JIT_ALWAYS_ON
> should be on as well. Which will remove interpreter from .text completely.

This would actually be contingent on RETPOLINE, not FRAME_POINTER.

(FRAME_POINTER was the other issue with the "optimize" attribute, which
we're reverting so it'll no longer be a problem.)

So if you're not concerned about the retpoline text growth, it could be
as simple as:

#define CONT     ({ insn++; goto *jumptable[insn->code]; })
#define CONT_JMP ({ insn++; goto *jumptable[insn->code]; })


Or, if you wanted to avoid the text growth, it could be:

#ifdef CONFIG_RETPOLINE
/*
 * Avoid a 40% increase in function text size by getting GCC to generate a
 * single retpoline jump instead of 160+.
 */
#define CONT	 ({ insn++; goto select_insn; })
#define CONT_JMP ({ insn++; goto select_insn; })

select_insn:
	goto *jumptable[insn->code];

#else /* !CONFIG_RETPOLINE */
#define CONT	 ({ insn++; goto *jumptable[insn->code]; })
#define CONT_JMP ({ insn++; goto *jumptable[insn->code]; })
#endif /* CONFIG_RETPOLINE */


But since this is legacy path, I think the first one is much nicer.


Also, JMP_TAIL_CALL has a "goto select_insn", is it ok to convert that
to CONT?

-- 
Josh