On 04/22/2010 03:11 AM, Glauber Costa wrote:
On Tue, Apr 20, 2010 at 12:42:17PM -0700, Jeremy Fitzhardinge wrote:
On 04/20/2010 11:54 AM, Avi Kivity wrote:
On 04/20/2010 09:23 PM, Jeremy Fitzhardinge wrote:
On 04/20/2010 02:31 AM, Avi Kivity wrote:
btw, do you want this code in pvclock.c, or shall we keep it kvmclock
specific?
I think its a pvclock-level fix. I'd been hoping to avoid having
something like this, but I think its ultimately necessary.
Did you observe drift on Xen, or is this "ultimately" pointing at the
future?
People are reporting weirdnesses that "clocksource=jiffies" apparently
resolves. Xen and KVM are faced with the same hardware constraints, and
it wouldn't surprise me if there were small measurable
non-monotonicities in the PV clock under Xen. May as well be safe.
Of course, it kills any possibility of being able to usefully export
this interface down to usermode.
My main concern about this kind of simple fix is that if there's a long
term systematic drift between different CPU's tscs, then this will
somewhat mask the problem while giving really awful time measurement on
the "slow" CPU(s). In that case it really needs to adjust the scaling
factor to correct for the drift (*not* update the offset). But if we're
definitely only talking about fixed, relatively small time offsets then
it is fine.
Can you by any chance run ingo's time warp test on those machines?
You need to define TOD to 1, and leave out the TSC test.
For me, warps exists on every machine out there, but the nehalems, so far
Or apply this patch.
diff -rup a/time-warp-test.c b/time-warp-test.c
--- a/time-warp-test.c 2010-04-15 16:30:13.955981607 -1000
+++ b/time-warp-test.c 2010-04-15 16:35:37.777982377 -1000
@@ -91,7 +91,7 @@ static inline unsigned long long __rdtsc
{
DECLARE_ARGS(val, low, high);
- asm volatile("cpuid; rdtsc" : EAX_EDX_RET(val, low, high));
+ asm volatile("cpuid; rdtsc" : EAX_EDX_RET(val, low, high) :: "ebx", "ecx");
return EAX_EDX_VAL(val, low, high);
}
