Re: About the performance of hyper-v

[Liang Li]

> > 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?
> >
>
> Hey,
>
> the above can be caused by the fact that when 'hv-synic' is enabled, KVM
> automatically disables APICv and this can explain the overhead and the
> fact that you're seeing more vmexits. KVM disables APICv because SynIC's
> 'AutoEOI' feature is incompatible with it. We can, however, tell Windows
> to not use AutoEOI ('Recommend deprecating AutoEOI' bit) and only
> inhibit APICv if the recommendation was ignored. This is implemented in
> the following KVM patch series:
> https://lore.kernel.org/kvm/20210518144339.1987982-1-vkuznets@redhat.com/
>
> It will, however, require a new 'hv-something' flag to QEMU. For now, it
> can be tested with 'hv-passthrough'.
>
> It would be great if you could give it a spin!
>
> --
> Vitaly

It's great to know that you already have a solution for this. :)

By the way,  is there any requirement for the version of windows or
windows updates for the new feature to work?

Thanks!

Liang