Re: [PATCH v2 14/14] bpf,x86: Respect X86_FEATURE_RETPOLINE*

[Alexei Starovoitov]

On Wed, Oct 20, 2021 at 12:44:56PM +0200, Peter Zijlstra wrote:
> +
> +	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

One more question.
What's a deal with AMD? I thought the retpoline is effective on it as well.
lfence is an optimization or retpoline turned out to be not enough
in some cases?