-	 * 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

So I suggest we'll wait for Andi Kleen's 24' patch using [l|m]fence.
Meanwhile one shouldn't use tsc clock source in guests or alternatively mascaraed the guest cpu as
old Intel version [one can do that with kvm, although it hurt performance].
Regards,
Dor