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

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Date:   Thu, 14 Oct 2021 11:48:50 +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 Thu, Oct 14, 2021 at 11:46:11AM +0200, Peter Zijlstra wrote:
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -396,6 +396,37 @@ static int get_pop_bytes(bool *callee_re
>  	return bytes;
>  }
>  
> +#define EMIT_LFENCE()	EMIT3(0x0F, 0xAE, 0xE8)
> +
> +#ifdef CONFIG_RETPOLINE
> +#define INDIRECT_SIZE (5)

Bah, that should be:

#define INDIRECT_SIZE	(2 + 3*cpu_feature_enabled(X86_FEATURE_RETPOLINE))

> +#else
> +#define INDIRECT_SIZE (2)
> +#endif
> +
> +static void emit_indirect_jump(u8 **pprog, int reg, u8 *ip)
> +{
> +	static const void *reg_thunk[] = {
> +#undef GEN
> +#define GEN(reg) __x86_indirect_thunk_ ## reg,
> +#include <asm/GEN-for-each-reg.h>
> +	};
> +
> +	u8 *prog = *pprog;
> +
> +#ifdef CONFIG_RETPOLINE
> +	if (cpu_feature_enabled(X86_FEATURE_RETPOLINE_AMD)) {
> +		EMIT_LFENCE();
> +		EMIT2(0xFF, 0xE0 + reg);
> +	} else if (cpu_feature_enabled(X86_FEATURE_RETPOLINE)) {
> +		emit_jump(&prog, reg_thunk[reg], ip);
> +	} else
> +#endif
> +	EMIT2(0xFF, 0xE0 + reg);
> +
> +	*pprog = prog;
> +}