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Date:   Wed, 29 Apr 2020 21:24:00 -0700
From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     x86@kernel.org, tglx@linutronix.de, linux-kernel@vger.kernel.org,
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        peterz@infradead.org, rdunlap@infradead.org,
        Arnd Bergmann <arnd@arndb.de>, bpf@vger.kernel.org,
        daniel@iogearbox.net
Subject: Re: BPF vs objtool again
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On Wed, Apr 29, 2020 at 10:53:15PM -0500, Josh Poimboeuf wrote:
> 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

I'm a bit lost. So objtool is fine with the asm when retpoline is on?
Then pls do:
#if defined(CONFIG_RETPOLINE) || !defined(CONFIG_X86)

since there is no need to mess with other archs.

> /*
>  * 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?

yep