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SubjectRe: [PATCH v2] x86/retpoline: Add clang support
On Wed, Feb 07, 2018 at 10:28:38PM +0000, David Woodhouse wrote:
>
>
> 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
>
Nice.
>
> > 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).
>

Much better and more comprehensive than my patch. Let's go with yours.

> 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.

Let's see what Chandler's feedback is. Worst case we can handle that
in a separate patch; we won't be able to fix all clang problems in
a single patch anyway.

Thanks,
Guenter

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