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x86/alternatives: Macrofy lock prefixes to work around GCC inlining bugs 

As described in:

  77b0bf55bc67: ("kbuild/Makefile: Prepare for using macros in inline assembly code to work around asm() related GCC inlining bugs")

GCC's inlining heuristics are broken with common asm() patterns used in
kernel code, resulting in the effective disabling of inlining.

The workaround is to set an assembly macro and call it from the inline
assembly block - i.e. to macrify the affected block.

As a result GCC considers the inline assembly block as a single instruction.

This patch handles the LOCK prefix, allowing more aggresive inlining:

      text     data     bss      dec     hex  filename
  18140140 10225284 2957312 31322736 1ddf270  ./vmlinux before
  18146889 10225380 2957312 31329581 1de0d2d  ./vmlinux after (+6845)

This is the reduction in non-inlined functions:

  Before: 40286
  After:  40218 (-68)

Tested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Nadav Amit <namit@vmware.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20181003213100.189959-6-namit@vmware.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Nadav Amit 2018-10-03 14:30:55 -07:00 committed by Ingo Molnar
parent 9e1725b410
commit 77f48ec28e
3 changed files with 17 additions and 15 deletions
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20
arch/x86/include/asm/alternative-asm.h
View File

@ -7,16 +7,24 @@
#include <asm/asm.h>
#ifdef CONFIG_SMP
.macro LOCK_PREFIX
672: lock
.macro LOCK_PREFIX_HERE
.pushsection .smp_locks,"a"
.balign 4
.long 672b - .
.long 671f - . # offset
.popsection
.endm
671:
.endm
.macro LOCK_PREFIX insn:vararg
LOCK_PREFIX_HERE
lock \insn
.endm
#else
.macro LOCK_PREFIX
.endm
.macro LOCK_PREFIX_HERE
.endm
.macro LOCK_PREFIX insn:vararg
.endm
#endif
/*

11
arch/x86/include/asm/alternative.h
View File

@ -31,15 +31,8 @@
*/
#ifdef CONFIG_SMP
#define LOCK_PREFIX_HERE \
".pushsection .smp_locks,\"a\"\n" \
".balign 4\n" \
".long 671f - .\n" /* offset */ \
".popsection\n" \
"671:"
#define LOCK_PREFIX LOCK_PREFIX_HERE "\n\tlock; "
#define LOCK_PREFIX_HERE "LOCK_PREFIX_HERE\n\t"
#define LOCK_PREFIX "LOCK_PREFIX "
#else /* ! CONFIG_SMP */
#define LOCK_PREFIX_HERE ""
#define LOCK_PREFIX ""

1
arch/x86/kernel/macros.S
View File

@ -8,3 +8,4 @@
#include <linux/compiler.h>
#include <asm/refcount.h>
#include <asm/alternative-asm.h>