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[Xen-devel] xen tsc problems?

To: <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: [Xen-devel] xen tsc problems?
From: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Date: Tue, 13 Jul 2010 15:37:37 +0100
Cc: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
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User-agent: Alpine 2.00 (DEB 1167 2008-08-23)
Hi all,
I get this warning from the HVM DomU kernel (both PV on HVM or normal

checking TSC synchronization [CPU#0 -> CPU#1]:
Measured 116836520 cycles TSC warp between CPUs, turning off TSC clock.
Marking TSC unstable due to check_tsc_sync_source failed

the host cpu is the following:

processor       : 0                                                             
vendor_id       : GenuineIntel                                                  
cpu family      : 15
model           : 6
model name      : Genuine Intel(R) CPU 3.00GHz
stepping        : 2
cpu MHz         : 3000.050
cache size      : 2048 KB
fdiv_bug        : no
hlt_bug         : no
f00f_bug        : no
coma_bug        : no
fpu             : yes
fpu_exception   : yes
cpuid level     : 6
wp              : yes
flags           : fpu de tsc msr pae mce cx8 apic sep mtrr mca cmov pat clflush 
acpi mmx fxsr sse sse2 ss ht nx constant_tsc pni est cid hypervisor arat
bogomips        : 6000.10
clflush size    : 64
cache_alignment : 128
address sizes   : 36 bits physical, 48 bits virtual
power management:

it happens on both xen 4.0 (pre 21236) and 4.1.

if I specify tsc_mode=2, first I get:

checking TSC synchronization [CPU#0 -> CPU#1]: passed.

but a little bit afterwards:

Clocksource tsc unstable (delta = 116372610 ns)

If I use a PV on HVM kernel (pv timer enabled) and tsc_mode=1,
besides these messages I get about 20-30 messages like the following:

CE: xen increased min_delta_ns to 506250 nsec

tracing them back to xen I found out that they happen when the guest
kernel tries to set the next timer event in the past.

Does this mean that the host has some serious tsc issues?
Can this be a symptom of a bug in xen?
Suggestion are welcome.



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