[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>

include/asm-i386/tsc.h

@@ -37,6 +37,15 @@ static __always_inline cycles_t get_cycles_sync(void)
 	unsigned long long ret;
 	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: