Re: A question about HTL VM-Exit handling time

[Paolo Bonzini <pbonzini@redhat.com>]

On 10/16/2014 10:15 AM, Wu, Feng wrote:
> 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!

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