About the performance of hyper-v

[Liang Li]

[resend for missing cc]

Hi Vitaly,

I found a case that the virtualization overhead was almost doubled
when turning on Hper-v related features compared to that without any
no hyper-v feature.  It happens when running a 3D game in windows
guest in qemu kvm environment.

By investigation, I found there are a lot of IPIs triggered by guest,
when turning on the hyer-v related features including stimer, for the
apicv is turned off, at least two vm exits are needed for processing a
single IPI.


perf stat will show something like below [recorded for 5 seconds]

---------

Analyze events for all VMs, all VCPUs:
             VM-EXIT    Samples  Samples%     Time%    Min Time    Max
Time         Avg time
  EXTERNAL_INTERRUPT     471831    59.89%    68.58%      0.64us
65.42us      2.34us ( +-   0.11% )
           MSR_WRITE     238932    30.33%    23.07%      0.48us
41.05us      1.56us ( +-   0.14% )

Total Samples:787803, Total events handled time:1611193.84us.

I tried turning off hyper-v for the same workload and repeat the test,
the overall virtualization overhead reduced by about of 50%:

-------

Analyze events for all VMs, all VCPUs:

             VM-EXIT    Samples  Samples%     Time%    Min Time    Max
Time         Avg time
          APIC_WRITE     255152    74.43%    50.72%      0.49us
50.01us      1.42us ( +-   0.14% )
       EPT_MISCONFIG      39967    11.66%    40.58%      1.55us
686.05us      7.27us ( +-   0.43% )
           DR_ACCESS      35003    10.21%     4.64%      0.32us
40.03us      0.95us ( +-   0.32% )
  EXTERNAL_INTERRUPT       6622     1.93%     2.08%      0.70us
57.38us      2.25us ( +-   1.42% )

Total Samples:342788, Total events handled time:715695.62us.

For this scenario,  hyper-v works really bad.  stimer works better
than hpet, but on the other hand, it relies on SynIC which has
negative effects for IPI intensive workloads.
Do you have any plans for improvement?


Thanks!
Liang