linux-kernel - [PATCH] x86: Fix detection of GCC -mpreferred-stack-boundary support

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Message-Id: <e5297c192969adfa0d28b84cf8a22d59573db26d.1436126872.git.luto@kernel.org>
Date:	Sun,  5 Jul 2015 13:08:16 -0700
From:	Andy Lutomirski <luto@...nel.org>
To:	linux-kernel@...r.kernel.org, x86@...nel.org
Cc:	Andy Lutomirski <luto@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>, Ingo Molnar <mingo@...nel.org>,
	Borislav Petkov <bp@...en8.de>
Subject: [PATCH] x86: Fix detection of GCC -mpreferred-stack-boundary support

As per https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53383, GCC only
allows -mpreferred-stack-boundary=3 on x86_64 if -mno-sse is set.
That means that cc-option will not detect
-mpreferred-stack-boundary=3 support, because we test for it before
setting -mno-sse.

Fix it by reordering the Makefile bits.

Compile-tested only.  This should help avoid code generation issues
such as the one that was worked around in b96fecbfa8c8 ("x86/fpu:
Fix boot crash in the early FPU code").

I'm a bit concerned that we could still have problems on older GCC
versions given that our asm code does not respect GCC's idea of the
ABI-required stack alignment.

Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Jan Kara <jack@...e.cz>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Borislav Petkov <bp@...en8.de>
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/Makefile | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 118e6debc483..344dd2110b2a 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -39,6 +39,12 @@ ifdef CONFIG_X86_NEED_RELOCS
         LDFLAGS_vmlinux := --emit-relocs
 endif
 
+# prevent gcc from generating any FP code by mistake
+# This must be before we try -mpreferred-stack-boundary -- see
+# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53383
+KBUILD_CFLAGS += -mno-sse -mno-mmx -mno-sse2 -mno-3dnow
+KBUILD_CFLAGS += $(call cc-option,-mno-avx,)
+
 ifeq ($(CONFIG_X86_32),y)
         BITS := 32
         UTS_MACHINE := i386
@@ -167,9 +173,6 @@ KBUILD_CFLAGS += -pipe
 KBUILD_CFLAGS += -Wno-sign-compare
 #
 KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
-# prevent gcc from generating any FP code by mistake
-KBUILD_CFLAGS += -mno-sse -mno-mmx -mno-sse2 -mno-3dnow
-KBUILD_CFLAGS += $(call cc-option,-mno-avx,)
 
 KBUILD_CFLAGS += $(mflags-y)
 KBUILD_AFLAGS += $(mflags-y)
-- 
2.4.3

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