Re: Linux 2.6.32-rc1

[Linus Torvalds <torvalds@linux-foundation.org>]

On Tue, 29 Sep 2009, Arjan van de Ven wrote:
> 
> can't we use alternatives() for this, to patch cmpxchg64 in ?
> I mean.. it'll be commonly supported nowadays.. the fallback to it not
> being supported could be a bit slower by now...

Yes, we could. It would limit us to some fixed address format, probably

	cmpxchg8b (%esi)

or something. Use something like this as a starting point, perhaps?

NOTE! Totally untested! And you'd actually need to implement the 
"cmpxchg8b_emu" function that takes it's arguments in %eax:%edx, %ebx:%ecx 
and %esi and doesn't trash any other registers..

		Linus

---
 arch/x86/include/asm/cmpxchg_32.h |   27 ++++++++++++++-------------
 1 files changed, 14 insertions(+), 13 deletions(-)

diff --git a/arch/x86/include/asm/cmpxchg_32.h b/arch/x86/include/asm/cmpxchg_32.h
index 82ceb78..d16a486 100644
--- a/arch/x86/include/asm/cmpxchg_32.h
+++ b/arch/x86/include/asm/cmpxchg_32.h
@@ -312,19 +312,20 @@ static inline unsigned long cmpxchg_386(volatile void *ptr, unsigned long old,
 
 extern unsigned long long cmpxchg_486_u64(volatile void *, u64, u64);
 
-#define cmpxchg64(ptr, o, n)						\
-({									\
-	__typeof__(*(ptr)) __ret;					\
-	if (likely(boot_cpu_data.x86 > 4))				\
-		__ret = (__typeof__(*(ptr)))__cmpxchg64((ptr),		\
-				(unsigned long long)(o),		\
-				(unsigned long long)(n));		\
-	else								\
-		__ret = (__typeof__(*(ptr)))cmpxchg_486_u64((ptr),	\
-				(unsigned long long)(o),		\
-				(unsigned long long)(n));		\
-	__ret;								\
-})
+#define cmpxchg64(ptr, o, n)					\
+({								\
+	__typeof__(*(ptr)) __ret;				\
+	__typeof__(*(ptr)) __old = (o);				\
+	__typeof__(*(ptr)) __new = (n);				\
+	alternative_io(LOCK_PREFIX "cmpxchg8b (%%esi)",		\
+		       "call cmpxchg8b_emu",			\
+		       X86_FEATURE_CMPXCHG8B,			\
+		       "=A" (__ret),				\
+		       "m" (*(ptr)), "0" (__old),		\
+		       "b" ((unsigned int)__new),		\
+		       "c" ((unsigned int)(__new>>32)) );	\
+	__ret; })
+
 #define cmpxchg64_local(ptr, o, n)					\
 ({									\
 	__typeof__(*(ptr)) __ret;					\