Re: Linux 2.6.32-rc1

Arjan van de Ven
2009-09-30T15:07:40Z

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