From: Andi Kleen <ak@suse.de>
Date: Wed, 2 May 2007 17:27:21 +0000 (+0200)
Subject: [PATCH] x86: Use RDTSCP for synchronous get_cycles if possible
X-Git-Tag: v2.6.22-rc1~1011^2~16
X-Git-Url: https://err.no/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=c5bcb5635a03da3158f121ae20ccbbf72b4fc62a;p=linux-2.6

[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>
---

diff --git a/include/asm-i386/tsc.h b/include/asm-i386/tsc.h
index 0181f9df75..3f3c1fa000 100644
--- a/include/asm-i386/tsc.h
+++ b/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: