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

On Thu, Nov 18, 2021 at 10:54 AM Peter Zijlstra [off-list ref] 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

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?

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

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

-- 
Thanks,
~Nick Desaulniers

On Thu, Nov 18, 2021 at 02:08:04PM -0800, Nick Desaulniers wrote:

On Thu, Nov 18, 2021 at 10:54 AM Peter Zijlstra [off-list ref] wrote:

quoted

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@google.com>

Thanks, let me repost as two patches.