Discussion:
A question about HTL VM-Exit handling time
Wu, Feng
2014-10-16 08:15:43 UTC
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Hi folks,

I run kernel build in the guest and use perf kvm to get some VM-Exit result as the following:

Analyze events for all VCPUs:

VM-EXIT Samples Samples% Time% Min Time Max Time A

MSR_WRITE 3613908 57.53% 18.97% 5us 1362us 9.73
HLT 1399747 22.28% 74.90% 5us 432448us 99.24
CR_ACCESS 961203 15.30% 3.28% 4us 188us 6.33
EXTERNAL_INTERRUPT 213821 3.40% 2.25% 4us 4089us 19.54
EXCEPTION_NMI 25152 0.40% 0.12% 4us 71us 9.05
EPT_MISCONFIG 20104 0.32% 0.15% 8us 5628us 13.74
CPUID 19904 0.32% 0.07% 4us 220us 6.90
IO_INSTRUCTION 17097 0.27% 0.20% 13us 1008us 22.08
PAUSE_INSTRUCTION 10737 0.17% 0.05% 4us 53us 8.33
MSR_READ 48 0.00% 0.00% 4us 8us 5.62

Total Samples:6281721, Total events handled time:185457820.41us.

I also do some other experiments with different workload in the guest, I got the same results in terms of
HLT VM-Exit handling time. Does anyone know why the handling time for HLT VM-Exit is so high? Appreciate
You help!

Thanks,
Feng
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Paolo Bonzini
2014-10-21 18:39:49 UTC
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Post by Wu, Feng
Hi folks,
VM-EXIT Samples Samples% Time% Min Time Max Time A
MSR_WRITE 3613908 57.53% 18.97% 5us 1362us 9.73
HLT 1399747 22.28% 74.90% 5us 432448us 99.24
CR_ACCESS 961203 15.30% 3.28% 4us 188us 6.33
EXTERNAL_INTERRUPT 213821 3.40% 2.25% 4us 4089us 19.54
EXCEPTION_NMI 25152 0.40% 0.12% 4us 71us 9.05
EPT_MISCONFIG 20104 0.32% 0.15% 8us 5628us 13.74
CPUID 19904 0.32% 0.07% 4us 220us 6.90
IO_INSTRUCTION 17097 0.27% 0.20% 13us 1008us 22.08
PAUSE_INSTRUCTION 10737 0.17% 0.05% 4us 53us 8.33
MSR_READ 48 0.00% 0.00% 4us 8us 5.62
Total Samples:6281721, Total events handled time:185457820.41us.
I also do some other experiments with different workload in the guest, I got the same results in terms of
HLT VM-Exit handling time. Does anyone know why the handling time for HLT VM-Exit is so high? Appreciate
You help!
432 ms sounds like a lot, but in general it is expected that HLT vmexits
take a long time. After an HLT vmexit, the VCPU will not be reentered
until the next interrupt comes. On hardware, the HLT instruction can
also take many milliseconds.

If this is an SMP guest, it's possible that the maximum time is
registered on the APs before Linux boots. With a UP guest I would
expect a shorter maximum time, but still longer than other vmexits.

Paolo
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Wu, Feng
2014-10-23 04:56:57 UTC
Permalink
-----Original Message-----
Bonzini
Sent: Wednesday, October 22, 2014 2:40 AM
Cc: Xiao Guangrong
Subject: Re: A question about HTL VM-Exit handling time
Post by Wu, Feng
Hi folks,
I run kernel build in the guest and use perf kvm to get some VM-Exit result as
VM-EXIT Samples Samples% Time% Min Time
Max Time A
Post by Wu, Feng
MSR_WRITE 3613908 57.53% 18.97%
5us 1362us 9.73
Post by Wu, Feng
HLT 1399747 22.28% 74.90% 5us
432448us 99.24
Post by Wu, Feng
CR_ACCESS 961203 15.30% 3.28%
4us 188us 6.33
Post by Wu, Feng
EXTERNAL_INTERRUPT 213821 3.40% 2.25%
4us 4089us 19.54
Post by Wu, Feng
EXCEPTION_NMI 25152 0.40% 0.12%
4us 71us 9.05
Post by Wu, Feng
EPT_MISCONFIG 20104 0.32% 0.15%
8us 5628us 13.74
Post by Wu, Feng
CPUID 19904 0.32% 0.07% 4us
220us 6.90
Post by Wu, Feng
IO_INSTRUCTION 17097 0.27% 0.20% 13us
1008us 22.08
Post by Wu, Feng
PAUSE_INSTRUCTION 10737 0.17% 0.05%
4us 53us 8.33
Post by Wu, Feng
MSR_READ 48 0.00% 0.00%
4us 8us 5.62
Post by Wu, Feng
Total Samples:6281721, Total events handled time:185457820.41us.
I also do some other experiments with different workload in the guest, I got
the same results in terms of
Post by Wu, Feng
HLT VM-Exit handling time. Does anyone know why the handling time for HLT
VM-Exit is so high? Appreciate
Post by Wu, Feng
You help!
432 ms sounds like a lot, but in general it is expected that HLT vmexits
take a long time. After an HLT vmexit, the VCPU will not be reentered
until the next interrupt comes. On hardware, the HLT instruction can
also take many milliseconds.
Thanks for your answer, Paolo.
There is a general question for perf kvm. Do you know how the vmexit
handling time is measured? Such as what is the start point, and the
end point? If I got this information, I think I can better understand the
value of the handling time. Thanks a lot!
If this is an SMP guest, it's possible that the maximum time is
registered on the APs before Linux boots. With a UP guest I would
expect a shorter maximum time, but still longer than other vmexits.
Yes, you are right, I get the data for an UP guest and the HLT vmexit
handling time is shorter. Here is the result:

Analyze events for all VCPUs:

VM-EXIT Samples Samples% Time% Min Time Max Time

EXTERNAL_INTERRUPT 90015 29.68% 41.92% 4us 660us 24.9
MSR_WRITE 89512 29.52% 14.25% 4us 162us 8.5
IO_INSTRUCTION 47823 15.77% 16.67% 11us 1907us 18.6
EPT_MISCONFIG 38504 12.70% 7.42% 7us 4848us 10.3
CPUID 32658 10.77% 3.61% 3us 304us 5.9
EXCEPTION_NMI 2232 0.74% 0.34% 4us 164us 8.2
PAUSE_INSTRUCTION 1594 0.53% 0.21% 3us 44us 6.9
HLT 547 0.18% 15.50% 8us 177098us 1516.5
CR_ACCESS 278 0.09% 0.05% 5us 26us 8.7
EPT_VIOLATION 71 0.02% 0.03% 8us 27us 20.3
MSR_READ 12 0.00% 0.00% 3us 4us 4.1

Total Samples:303246, Total events handled time:5350983.30us.

Thanks,
Feng
Paolo
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Paolo Bonzini
2014-10-23 12:23:30 UTC
Permalink
Post by Wu, Feng
Thanks for your answer, Paolo.
There is a general question for perf kvm. Do you know how the vmexit
handling time is measured? Such as what is the start point, and the
end point? If I got this information, I think I can better understand the
value of the handling time. Thanks a lot!
It is measured from kvm:kvm_exit to kvm:kvm_entry. See
exit_event_begin, exit_event_get_key, exit_event_end in
tools/perf/builtin-kvm.c.

Paolo
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Wu, Feng
2014-10-24 01:14:23 UTC
Permalink
-----Original Message-----
Sent: Thursday, October 23, 2014 8:23 PM
Cc: Xiao Guangrong
Subject: Re: A question about HTL VM-Exit handling time
Post by Wu, Feng
Thanks for your answer, Paolo.
There is a general question for perf kvm. Do you know how the vmexit
handling time is measured? Such as what is the start point, and the
end point? If I got this information, I think I can better understand the
value of the handling time. Thanks a lot!
It is measured from kvm:kvm_exit to kvm:kvm_entry. See
exit_event_begin, exit_event_get_key, exit_event_end in
tools/perf/builtin-kvm.c.
Got it, Thanks for the information!

Thanks,
Feng
Paolo
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