Re: why guests show "Clocksource tsc unstable" on bootup?

[Michael Tokarev <mjt@tls.msk.ru>]

Tomasz Chmielewski wrote:
> Avi Kivity schrieb:
>> Tomasz Chmielewski wrote:
>>> Why do my guests show "Clocksource tsc unstable" on bootup?

The same happens here as well.

>> Linux expects the tsc to be monotonic and to have a 1:1 correspondence 
>> with real time, which isn't easy to achieve with virtualization.
> 
> But the clocksource is kvm-clock, so why does the guest probe tsc at all?
> 
> 
>>> dmesg shows that kvm-clock was set as the primary cpu clock.
>>> Yet a bit later kernel says "Clocksource tsc unstable".
>>>
>>> Is it something to worry about, or perhaps calculating tsc is hardcoded?
>>> And as such, will be always checked?
>>> Or, is it host CPU related?
>>>
>>> [    0.000000] kvm-clock: cpu 0, msr 0:3baf81, boot clock
>>> [    0.000000] kvm-clock: cpu 0, msr 0:1208f81, primary cpu clock
>>> [    0.000000] Kernel command line: root=/dev/vda1 ro quiet 
>>> clocksource=kvm-clock
>>> [    1.253602] rtc_cmos 00:01: setting system clock to 2009-03-09 
>>> 11:41:30 UTC (1236598890)
>>> [   41.500623] Clocksource tsc unstable (delta = -153498948 ns)
>>>
>>
>> What host cpu and kvm version are you using?
> 
> I pasted a part of /proc/cpuinfo below.
> I saw these with kvm-83 and kvm-84 (with cpufreq disabled, as it perhaps 
> can matter).
> 
> processor       : 3
> vendor_id       : AuthenticAMD
> cpu family      : 15
> model           : 65
> model name      : Dual-Core AMD Opteron(tm) Processor 2212
> stepping        : 2
> cpu MHz         : 2000.000
> cache size      : 1024 KB
> physical id     : 1
> siblings        : 2
> core id         : 1
> cpu cores       : 2
> fpu             : yes
> fpu_exception   : yes
> cpuid level     : 1
> wp              : yes
> flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge 
> mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext 
> fxsr_opt rdtscp lm 3dnowext 3dnow rep_good pni cx16 lahf_lm cmp_legacy 
> svm extapic cr8_legacy
> bogomips        : 3993.03
> TLB size        : 1024 4K pages
> clflush size    : 64
> cache_alignment : 64
> address sizes   : 40 bits physical, 48 bits virtual
> power management: ts fid vid ttp tm stc

Here, it shows unstable tsc on this machine since kvm-72 or even earlier,
up to current kvm-84.

The CPU on the host is this one (4 cores):

vendor_id	: AuthenticAMD
cpu family	: 16
model		: 2
model name	: AMD Phenom(tm) 9750 Quad-Core Processor
stepping	: 3
cpu MHz		: 2400.000
cache size	: 512 KB
physical id	: 0
siblings	: 4
core id		: 3
cpu cores	: 4
apicid		: 3
initial apicid	: 3
fpu		: yes
fpu_exception	: yes
cpuid level	: 5
wp		: yes
flags		: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb rdtscp lm 3dnowext 3dnow constant_tsc rep_good pni monitor cx16 lahf_lm cmp_legacy svm extapic cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw ibs
bogomips	: 4812.60
TLB size	: 1024 4K pages
clflush size	: 64
cache_alignment	: 64
address sizes	: 48 bits physical, 48 bits virtual
power management: ts ttp tm stc 100mhzsteps hwpstate

Host kernel is 2.6.28.8 amd64, guest is running 2.6.27.19 i686,
kvm-84 userspace, kvm modules are from host kernel.

Here how it looks like from the guest:

# dmesg | egrep -i 'clock|tsc|time'
kvm-clock: cpu 0, msr 0:3cb001, boot clock
ACPI: PM-Timer IO Port: 0xb008
kvm-clock: cpu 0, msr 0:1409001, primary cpu clock
TSC: Unable to calibrate against PIT
TSC: using PMTIMER reference calibration
Calibrating delay loop (skipped), value calculated using timer frequency.. 4800.87 BogoMIPS (lpj=24004390)
..TIMER: vector=0x31 apic1=0 pin1=0 apic2=-1 pin2=-1
Machine check exception polling timer started.
Real Time Clock Driver v1.12ac
Clocksource tsc unstable (delta = -69824578 ns)

# cat /sys/devices/system/clocksource/clocksource0/available_clocksource
kvm-clock acpi_pm jiffies tsc

Thanks!

/mjt