netdev - Re: [PATCH bpf] bpf, x64: implement retpoline for tail call

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Date:   Wed, 21 Feb 2018 19:43:26 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>, ast@...nel.org,
        torvalds@...ux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH bpf] bpf, x64: implement retpoline for tail call

On Wed, Feb 21, 2018 at 07:04:02PM -0800, Eric Dumazet wrote:
> On Thu, 2018-02-22 at 01:05 +0100, Daniel Borkmann wrote:
> 
> ...
> 
> > +/* Instead of plain jmp %rax, we emit a retpoline to control
> > + * speculative execution for the indirect branch.
> > + */
> > +static void emit_retpoline_rax_trampoline(u8 **pprog)
> > +{
> > +	u8 *prog = *pprog;
> > +	int cnt = 0;
> > +
> > +	EMIT1_off32(0xE8, 7);	 /* callq <set_up_target> */
> > +	/* capture_spec: */
> > +	EMIT2(0xF3, 0x90);	 /* pause */
> > +	EMIT3(0x0F, 0xAE, 0xE8); /* lfence */
> > +	EMIT2(0xEB, 0xF9);	 /* jmp <capture_spec> */
> > +	/* set_up_target: */
> > +	EMIT4(0x48, 0x89, 0x04, 0x24); /* mov %rax,(%rsp) */
> > +	EMIT1(0xC3);		 /* retq */
> > +
> > +	BUILD_BUG_ON(cnt != RETPOLINE_SIZE);
> > +	*pprog = prog;
> 
> You might define the actual code sequence (and length) in 
> arch/x86/include/asm/nospec-branch.h
> 
> If we need to adjust code sequences for RETPOLINE, then we wont
> forget/miss that arch/x86/net/bpf_jit_comp.c had it hard-coded.

like adding a comment to asm/nospec-branch.h that says
"dont forget to adjust bpf_jit_comp.c" ?
but clang/gcc generate slightly different sequences for
retpoline anyway, so even if '.macro RETPOLINE_JMP' in
nospec-branch.h changes it doesn't mean that x64 jit has to change.
So what kinda comment there would make sense?