The patch below should resolve this - could you please test and Ack it? 
But this CPUID was present in v2.6.23 too, so why did it only show up in 
2.6.24-rc for you?

	Ingo

-------------->
Subject: x86: fix get_cycles_sync() overhead
From: Ingo Molnar <mingo@elte.hu>

get_cycles_sync() is causing massive overhead in KVM networking:

   http://lkml.org/lkml/2007/12/11/54

remove the explicit CPUID serialization - it causes VM exits and is
pointless: we care about GTOD coherency but that goes to user-space
via a syscall, and syscalls are serialization points anyway.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 include/asm-x86/tsc.h |   12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

Index: linux-x86.q/include/asm-x86/tsc.h
===================================================================
--- linux-x86.q.orig/include/asm-x86/tsc.h
+++ linux-x86.q/include/asm-x86/tsc.h
@@ -39,8 +39,8 @@ static __always_inline cycles_t get_cycl
 	unsigned eax, edx;
 
 	/*
-  	 * Use RDTSCP if possible; it is guaranteed to be synchronous
- 	 * and doesn't cause a VMEXIT on Hypervisors
+	 * 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,
 		       ASM_OUTPUT2("=a" (eax), "=d" (edx)),
@@ -50,11 +50,11 @@ static __always_inline cycles_t get_cycl
 		return ret;
 
 	/*
-	 * Don't do an additional sync on CPUs where we know
-	 * RDTSC is already synchronous:
+	 * Use RDTSC on other CPUs. This might not be fully synchronous,
+	 * but it's not a problem: the only coherency we care about is
+	 * the GTOD output to user-space, and syscalls are synchronization
+	 * points anyway:
 	 */
-	alternative_io("cpuid", ASM_NOP2, X86_FEATURE_SYNC_RDTSC,
-			  "=a" (eax), "0" (1) : "ebx","ecx","edx","memory");
 	rdtscll(ret);
 
 	return ret;
--

I don't think this is a good idea. I discussed exactly this item with
Andi Kleen a while ago and afair the serializing instruction was
necessary to fix a backwards walking gettimeofday() on some K8
revisions. Andi Kleen can tell more details, I added him to the CC list.

Joerg

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--

On Tue, 11 Dec 2007 15:27:17 +0100

isn't this probably wrong since this code is also used in the vsyscall code..
--

the TSC clocksource (and hence the vsyscall code) is turned off on 
systems that fail the TOD/CLOCK portion of this test:

  http://people.redhat.com/mingo/time-warp-test/time-warp-test.c

i.e. on the majority of systems in place.

	Ingo
--

Which is not on core2 which was the question about. And if it was
turned off it wouldn't use get_cycles_sync() at all.

-Andi
--

it is turned off on core2 too:

 # cat /sys/devices/system/clocksource/clocksource0/current_clocksource
 acpi_pm

	Ingo
--

but you are right that it's not turned off on all core2's, so my patch 
is wrong for them.

	Ingo
--

It works, actually I already commented it out.

Acked-by: Dor Laor <dor.laor@qumranet.com>

I tried to figure out but all the code movements for i386 go in the way.
In the previous email I reported to Andi that Fedora kernel 2.6.23-8 did 
not suffer from it.
Thanks for the ultra fast reply :)

--