linux-kernel - Re: [PATCH] x86/build: Move '-mindirect-branch-cs-prefix' out of GCC-only block

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Date:   Thu, 18 Aug 2022 09:45:39 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] x86/build: Move '-mindirect-branch-cs-prefix' out of
 GCC-only block

On Wed, Aug 17, 2022 at 03:08:01PM -0700, Nick Desaulniers wrote:
> On Wed, Aug 17, 2022 at 11:56 AM Nathan Chancellor <nathan@...nel.org> wrote:
> >
> > LLVM 16 will have support for this flag so move it out of the GCC-only
> > block to allow LLVM builds to take advantage of it.
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/1665
> > Link: https://github.com/llvm/llvm-project/commit/6f867f9102838ebe314c1f3661fdf95700386e5a
> > Signed-off-by: Nathan Chancellor <nathan@...nel.org>
> 
> Thanks for the patch, I also boot tested this (in virtual guests for
> both 64b and 32b).
> 
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> 
> Peter, is there anything else special about these prefixes needed to
> make use of them?

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)


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 ;-)