x86/asm/tsc: Save an instruction in DECLARE_ARGS users

Before, the code to do RDTSC looked like:

  rdtsc
  shl $0x20, %rdx
  mov %eax, %eax
  or  %rdx, %rax

The "mov %eax, %eax" is required to clear the high 32 bits of RAX.

By declaring low and high as 64-bit variables, the code is
simplified to:

  rdtsc
  shl $0x20,%rdx
  or  %rdx,%rax

Yes, it's a 2-byte instruction that's not on a critical path,
but there are principles to be upheld.

Every user of EAX_EDX_RET has been checked. I tried to check
users of EAX_EDX_ARGS, but there weren't any, so I deleted it to
be safe.

( There's no benefit to making "high" 64 bits, but it was the
  simplest way to proceed. )

Signed-off-by: George Spelvin <linux@horizon.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
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: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: jacob.jun.pan@linux.intel.com
Link: http://lkml.kernel.org/r/20150618075906.4615.qmail@ns.horizon.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
index 02bdd6c65017006e4dbdedd0ac57859f70651263..131eec2ca137a3ca127cde5374e4a91889e5b962 100644 (file)
--- a/arch/x86/include/asm/msr.h
+++ b/arch/x86/include/asm/msr.h
@@ -47,14 +47,13 @@ static inline unsigned long long native_read_tscp(unsigned int *aux)
  * it means rax *or* rdx.
  */
 #ifdef CONFIG_X86_64
-#define DECLARE_ARGS(val, low, high)   unsigned low, high
-#define EAX_EDX_VAL(val, low, high)    ((low) | ((u64)(high) << 32))
-#define EAX_EDX_ARGS(val, low, high)   "a" (low), "d" (high)
+/* Using 64-bit values saves one instruction clearing the high half of low */
+#define DECLARE_ARGS(val, low, high)   unsigned long low, high
+#define EAX_EDX_VAL(val, low, high)    ((low) | (high) << 32)
 #define EAX_EDX_RET(val, low, high)    "=a" (low), "=d" (high)
 #else
 #define DECLARE_ARGS(val, low, high)   unsigned long long val
 #define EAX_EDX_VAL(val, low, high)    (val)
-#define EAX_EDX_ARGS(val, low, high)   "A" (val)
 #define EAX_EDX_RET(val, low, high)    "=A" (val)
 #endif