linux-kernel - Re: [PATCH 9/9] bpf,x86: Respect X86_FEATURE

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Message-ID: <YWdVq8UWpvMwnzht@hirez.programming.kicks-ass.net>
Date:   Wed, 13 Oct 2021 23:54:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     x86@...nel.org, andrew.cooper3@...rix.com,
        linux-kernel@...r.kernel.org, alexei.starovoitov@...il.com,
        ndesaulniers@...gle.com
Subject: Re: [PATCH 9/9] bpf,x86: Respect X86_FEATURE_RETPOLINE*

On Wed, Oct 13, 2021 at 02:06:05PM -0700, Josh Poimboeuf wrote:
> On Wed, Oct 13, 2021 at 02:22:26PM +0200, Peter Zijlstra wrote:
> > Current BPF codegen doesn't respect X86_FEATURE_RETPOLINE* flags and
> > unconditionally emits a thunk call, this is sub-optimal and doesn't
> > match the regular, compiler generated, code.
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >  arch/x86/net/bpf_jit_comp.c |   18 +++++++++++-------
> >  1 file changed, 11 insertions(+), 7 deletions(-)
> > 
> > --- a/arch/x86/net/bpf_jit_comp.c
> > +++ b/arch/x86/net/bpf_jit_comp.c
> > @@ -2123,14 +2123,18 @@ static int emit_fallback_jump(u8 **pprog
> >  	int err = 0;
> >  
> >  #ifdef CONFIG_RETPOLINE
> > -	/* Note that this assumes the the compiler uses external
> > -	 * thunks for indirect calls. Both clang and GCC use the same
> > -	 * naming convention for external thunks.
> > -	 */
> > -	err = emit_jump(&prog, __x86_indirect_thunk_rdx, prog);
> > -#else
> > -	EMIT2(0xFF, 0xE2);	/* jmp rdx */
> > +	if (cpu_feature_enabled(X86_FEATURE_RETPOLINE)) {
> > +		if (cpu_feature_enabled(X86_FEATURE_RETPOLINE_AMD)) {
> > +			/* The AMD retpoline can be easily emitted inline. */
> > +			EMIT3(0x0F, 0xAE, 0xE8);	/* lfence */
> > +			EMIT2(0xFF, 0xE2);		/* jmp rdx */
> > +		} else {
> > +			/* Call the retpoline thunk */
> > +			err = emit_jump(&prog, __x86_indirect_thunk_rdx, prog);
> > +		}
> > +	} else
> >  #endif
> > +	EMIT2(0xFF, 0xE2);	/* jmp rdx */
> 
> But the rest of eBPF JIT just emits retpolines unconditionally
> regardless of feature, for example see RETPOLINE_RCX_BPF_JIT().  So I'm
> thinking this should probably be consistent with that (or that with
> this).

Argh, I grepped for __x86_indirect_thunk, and missed they're writing
retpolines themselves. Bah.

Yes, that needs cleaning up. I'll go prod at that tomorrow.