Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Thu, 18 Aug 2022 10:00:59 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Nathan Chancellor <nathan@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, llvm@lists.linux.dev
Subject: Re: [PATCH] x86/build: Move '-mindirect-branch-cs-prefix' out of
 GCC-only block
Message-ID: <Yv3xu7f3NDC1is6o@worktop.programming.kicks-ass.net>
References: <20220817185410.1174782-1-nathan@kernel.org>
 <CAKwvOd=0R76r6YqNiOhGNt_XrA_t_ZbuBbNNdNXV4CWpDy0+Bg@mail.gmail.com>
 <Yv3uI/MoJVctmBCh@worktop.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <Yv3uI/MoJVctmBCh@worktop.programming.kicks-ass.net>
Precedence: bulk

On Thu, Aug 18, 2022 at 09:45:39AM +0200, Peter Zijlstra wrote:

> The thing to do is boot with (warning, *very* verbose):
> 
> 	"spectre_v2=retpoline,lfence debug-alternative"
> 
> and observe that the retpoline sites all replace:
> 
> 	"cs call __x86_indirect_thunk_r11" (6 bytes)
> 
> with:
> 
> 	"lfence; jmp *%r11" (6 bytes)

		^^ call, ofc, unless the original was jmp then jmp...

/me goes get more morning juice in an attempt to wake up.

> 
> 
> This being clang, you'll ofcourse still have a few weird:
> 
> 	"Jcc.d32 __x86_indirect_thunk_r11"
> 
> sites that will not be able to be replaced, but there's nothing I can do
> about that other than to continue to encourage y'all to stop emitting
> them ;-)