Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Wed, 20 Oct 2021 17:07:53 -0700
From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     x86@kernel.org, jpoimboe@redhat.com, andrew.cooper3@citrix.com,
        linux-kernel@vger.kernel.org, ndesaulniers@google.com,
        bpf@vger.kernel.org, daniel@iogearbox.net
Subject: Re: [PATCH v2 14/14] bpf,x86: Respect X86_FEATURE_RETPOLINE*
Message-ID: <20211021000753.kdxtjl3nzre2zshb@ast-mbp.dhcp.thefacebook.com>
References: <20211020104442.021802560@infradead.org>
 <20211020105843.345016338@infradead.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20211020105843.345016338@infradead.org>
Precedence: bulk

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?