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

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Date:   Wed, 13 Oct 2021 14:06:05 -0700
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 02:22:26PM +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).

-- 
Josh