[PATCH] x86: Use RDTSCP for synchronous get_cycles if possible

RDTSCP is already synchronous and doesn't need an explicit CPUID.
This is a little faster and more importantly avoids VMEXITs on Hypervisors.

Original patch from Joerg Roedel, but reworked by AK
Also includes miscompilation fix by Eric Biederman

Cc: "Joerg Roedel" <joerg.roedel@amd.com>

Signed-off-by: Andi Kleen <ak@suse.de>

1 files changed, 9 insertions, 0 deletions

diff --git a/include/asm-i386/tsc.h b/include/asm-i386/tsc.h
index 0181f9df7539..3f3c1fa000b4 100644
--- a/include/asm-i386/tsc.h
+++ b/include/asm-i386/tsc.h
@@ -38,6 +38,15 @@ static __always_inline cycles_t get_cycles_sync(void)
         unsigned eax;
 
         /*
+         * Use RDTSCP if possible; it is guaranteed to be synchronous
+         * and doesn't cause a VMEXIT on Hypervisors
+         */
+        alternative_io(ASM_NOP3, ".byte 0x0f,0x01,0xf9", X86_FEATURE_RDTSCP,
+                                 "=A" (ret), "0" (0ULL) : "ecx", "memory");
+        if (ret)
+                return ret;
+
+        /*
          * Don't do an additional sync on CPUs where we know
          * RDTSC is already synchronous:
          */