linux-kernel - Re: [PATCH v2 14/14] bpf,x86: Respect X86_FEATURE

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Message-ID: <20211021001808.3ng6qeggi5lfwx7k@treble>
Date:   Wed, 20 Oct 2021 17:18:08 -0700
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        andrew.cooper3@...rix.com, linux-kernel@...r.kernel.org,
        ndesaulniers@...gle.com, bpf@...r.kernel.org, daniel@...earbox.net
Subject: Re: [PATCH v2 14/14] bpf,x86: Respect X86_FEATURE_RETPOLINE*

On Wed, Oct 20, 2021 at 05:07:53PM -0700, Alexei Starovoitov wrote:
> 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?

Yes, it's basically an optimization.  AMD recommends it presumably
because it's quite a bit faster than a retpoline.

According to AMD it shrinks the speculative execution window enough so
that Spectre v2 isn't a threat.

-- 
Josh