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SubjectRe: [PATCH v2] x86/retpoline: Add clang support
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On Wed, 2018-02-07 at 13:52 -0800, Guenter Roeck wrote:
> clang has its own set of compiler options for retpoline support.
>
> Link: https://github.com/llvm-mirror/clang/commit/0d816739a82da29748caf88570affb9715e18b69
> Link: https://github.com/llvm-mirror/llvm/commit/fd5a8723ce9f2a6b250e85972ef859e4253ea95d
> Link: https://github.com/llvm-mirror/llvm/commit/59b64490fda69d29bb42cfdf7eec37bcc31ff833
> Cc: David Woodhouse <dwmw@amazon.co.uk>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: gnomes@lxorguk.ukuu.org.uk
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Andi Kleen <ak@linux.intel.com>
> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> Cc: thomas.lendacky@amd.com
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Jiri Kosina <jikos@kernel.org>
> Cc: Andy Lutomirski <luto@amacapital.net>
> Cc: Dave Hansen <dave.hansen@intel.com>
> Cc: Kees Cook <keescook@google.com>
> Cc: Tim Chen <tim.c.chen@linux.intel.com>
> Cc: Greg Kroah-Hartman <gregkh@linux-foundation.org>
> Cc: Paul Turner <pjt@google.com>
> Signed-off-by: Guenter Roeck <linux@roeck-us.net>
> ---
> v2: llvm has been updated to use the same thunk names as gcc.
>     Tested with:
>     clang version 7.0.0
> (https://git.llvm.org/git/clang.git/
> 848874aed95a913fb45f363120500cebfe54e2ef)
> (https://git.llvm.org/git/llvm.git/
> 3afd566557f3616881505db0d69f5d19bf55ae14)
>     cross-checked with gcc 7.3.0 (x86_64-linux-gcc.br_real (Buildroot
>     2018.02-rc1) 7.3.0).
>
> Tested with 64-bit builds only; 32-bit images fail to build with clang
> with various unrelated errors and are difficult to test.
>
> I had to change '+=' to '=' below since make otherwise sets
> RETPOLINE_CFLAGS to " ", and the subsequent ifneq would always match.
> This is also the reason for the "ifeq ($(RETPOLINE_CFLAGS),)".
> If there is another/different/better way to handle this, please let
> me know.

See
http://git.infradead.org/users/dwmw2/linux-retpoline.git/commitdiff/82a1f41600


> There are curently lots of warnings when building an image with clang.
>
> ./include/linux/init.h:134:6: warning:
> unknown attribute 'indirect_branch' ignored
>
> I was inclined to add "&& !defined(__clang__)" to the condition for the
> __noretpoline define to fix the problem, but concluded that this should
> be a separate patch unless it can be addressed in clang.

... where I fixed that too as well as providing the
__x86_indirect_thunk for 32-bit builds (which are broken for other
reasons anyway).

We do still need the %V asm constraint modifier to work in clang
though, because otherwise it still doesn't build if you have any
hypervisor guest support.
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