Re: Linux 2.6.32-rc1

[Arjan van de Ven <arjan@infradead.org>]

On Tue, 29 Sep 2009 14:56:28 -0700 (PDT)
Linus Torvalds <torvalds@linux-foundation.org> wrote:

> 
> 
> 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..

so I debugged this guy (had a few bugs ;-)

patch, including a new cmpxchg8b_emu below:

>From 5a76986c5dd272ea16a9b8abb7349ff3d6791c2b Mon Sep 17 00:00:00 2001
From: Arjan van de Ven <arjan@linux.intel.com>
Date: Wed, 30 Sep 2009 17:04:35 +0200
Subject: [PATCH] x86: Provide an alternative() based cmpxchg64()

Based on Linus' patch, this patch provides cmpxchg64() using
the alternative() infrastructure.

Note: the fallback is NOT smp safe, just like the current fallback
is not SMP safe.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
---
 arch/x86/include/asm/cmpxchg_32.h |   29 ++++++++++--------
 arch/x86/kernel/i386_ksyms_32.c   |    3 ++
 arch/x86/lib/Makefile             |    2 +-
 arch/x86/lib/cmpxchg8b_emu.S      |   61 +++++++++++++++++++++++++++++++++++++
 4 files changed, 81 insertions(+), 14 deletions(-)
 create mode 100644 arch/x86/lib/cmpxchg8b_emu.S

diff --git a/arch/x86/include/asm/cmpxchg_32.h b/arch/x86/include/asm/cmpxchg_32.h
index 82ceb78..3b21afa 100644
--- a/arch/x86/include/asm/cmpxchg_32.h
+++ b/arch/x86/include/asm/cmpxchg_32.h
@@ -312,19 +312,22 @@ 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("call cmpxchg8b_emu",			\
+			"lock; cmpxchg8b (%%esi)" ,		\
+		       X86_FEATURE_CX8,				\
+		       "=A" (__ret),				\
+		       "S" ((ptr)), "0" (__old),		\
+		       "b" ((unsigned int)__new),		\
+		       "c" ((unsigned int)(__new>>32)));	\
+	__ret; })
+
+
+
 #define cmpxchg64_local(ptr, o, n)					\
 ({									\
 	__typeof__(*(ptr)) __ret;					\
diff --git a/arch/x86/kernel/i386_ksyms_32.c b/arch/x86/kernel/i386_ksyms_32.c
index 43cec6b..f17930e 100644
--- a/arch/x86/kernel/i386_ksyms_32.c
+++ b/arch/x86/kernel/i386_ksyms_32.c
@@ -10,6 +10,9 @@
 EXPORT_SYMBOL(mcount);
 #endif
 
+extern void cmpxchg8b_emu(void); /* dummy proto */
+EXPORT_SYMBOL(cmpxchg8b_emu);
+
 /* Networking helper routines. */
 EXPORT_SYMBOL(csum_partial_copy_generic);
 
diff --git a/arch/x86/lib/Makefile b/arch/x86/lib/Makefile
index 9e60920..3e549b8 100644
--- a/arch/x86/lib/Makefile
+++ b/arch/x86/lib/Makefile
@@ -15,7 +15,7 @@ ifeq ($(CONFIG_X86_32),y)
         obj-y += atomic64_32.o
         lib-y += checksum_32.o
         lib-y += strstr_32.o
-        lib-y += semaphore_32.o string_32.o
+        lib-y += semaphore_32.o string_32.o cmpxchg8b_emu.o
 
         lib-$(CONFIG_X86_USE_3DNOW) += mmx_32.o
 else
diff --git a/arch/x86/lib/cmpxchg8b_emu.S b/arch/x86/lib/cmpxchg8b_emu.S
new file mode 100644
index 0000000..b8af4c7
--- /dev/null
+++ b/arch/x86/lib/cmpxchg8b_emu.S
@@ -0,0 +1,61 @@
+/*
+ *	This program is free software; you can redistribute it and/or
+ *	modify it under the terms of the GNU General Public License
+ *	as published by the Free Software Foundation; version 2
+ *	of the License.
+ *
+ */
+
+#include <linux/linkage.h>
+#include <asm/alternative-asm.h>
+#include <asm/frame.h>
+#include <asm/dwarf2.h>
+
+
+.text
+
+/*
+ * Inputs:
+ * %esi : memory location to compare
+ * %eax : low 32 bits of old value
+ * %edx : high 32 bits of old value
+ * %ebx : low 32 bits of new value
+ * %ecx : high 32 bits of new value
+ */
+ENTRY(cmpxchg8b_emu)
+	CFI_STARTPROC
+
+	push %edi
+	push %ebx
+	push %ecx
+	/* disable interrupts */
+	pushf
+	pop %edi
+	cli
+
+	cmpl %edx, 4(%esi)
+	jne 1f
+	cmpl %eax, (%esi)
+	jne 1f
+
+	xchg (%esi), %ebx
+	xchg 4(%esi), %ecx
+	mov %ebx, %eax
+	mov %ecx, %edx
+
+2:
+	/* restore interrupts */
+	push %edi
+	popf
+
+	pop %ecx
+	pop %ebx
+	pop %edi
+	ret
+1:
+	mov (%esi), %eax
+	mov 4(%esi), %edx
+	jmp 2b
+	CFI_ENDPROC
+ENDPROC(cmpxchg8b_emu)
+
-- 
1.6.2.5