linux-kernel - Re: [PATCH] x86: Use -mindirect-branch-cs-prefix for RETPOLINE builds

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Message-ID: <YZe+nfVmA5wrIcv6@hirez.programming.kicks-ass.net>
Date:   Fri, 19 Nov 2021 16:11:25 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        "H.J. Lu" <hjl.tools@...il.com>, llvm@...ts.linux.dev,
        linux-toolchains@...r.kernel.org,
        Masahiro Yamada <masahiroy@...nel.org>
Subject: Re: [PATCH] x86: Use -mindirect-branch-cs-prefix for RETPOLINE builds

On Thu, Nov 18, 2021 at 02:08:04PM -0800, Nick Desaulniers wrote:
> On Thu, Nov 18, 2021 at 10:54 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> >
> > In order to further enable commit:
> >
> >   bbe2df3f6b6d ("x86/alternative: Try inline spectre_v2=retpoline,amd")
> >
> > add the new GCC flag -mindirect-branch-cs-prefix:
> >
> >   https://gcc.gnu.org/g:2196a681d7810ad8b227bf983f38ba716620545e
> >
> > to RETPOLINE=y builds. This should allow fully inlining retpoline,amd
> > for GCC builds.
> 
> Do you mind adding the following link tags:
> 
> Link: https://gcc.gnu.org/g:2196a681d7810ad8b227bf983f38ba716620545e
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102952
> Link: https://bugs.llvm.org/show_bug.cgi?id=52323

My scripts will strip that, they only like to have a Link to to the
actual email as posted. I can put in the URLs, the first is already in
the Changelog.

> If the feature would be helpful to implement in Clang, would you mind
> keeping the cc-option, but moving the check outside of the `ifdef
> CONFIG_CC_IS_GCC` block? That way, we don't need to revisit the
> Makefile?

I was thinking that clang would want to name it differently to be
consistent with their own retpoline flags, something like:

  -mretpoline-cs-prefix

> (Tangential: I wonder why RETPOLINE_CFLAGS aren't defined in
> arch/x86/Makefile; they seem to be x86 specific at the moment).

Yeah, I think there was hoping that other archs would find it useful,
but so far x86 is the only one having grown them things. I can move them
I suppose. If ever another arch grows them we can reconsider.

> With the above link tags, and with or without moving this outside the
> gcc-specific guard:
> Acked-by: Nick Desaulniers <ndesaulniers@...gle.com>

Thanks, let me repost as two patches.