> >> > 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@xxxxxxxxxx/
> >>
> >> 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?
>
> AFAIR, 'Recommend deprecating AutoEOI' bit appeared in WS2012 so I'd
> expect WS2008 to ignore it completely (and thus SynIC will always be
> disabling APICv for it).
>
Hi Vitaly,
I tried your patchset and found it's not helpful to reduce the
virtualization overhead.
here are some perfdata with the same workload
===============================
Analyze events for all VMs, all VCPUs:
VM-EXIT Samples Samples% Time% Min Time Max
Time Avg time
MSR_WRITE 924045 89.96% 81.10% 0.42us
68.42us 1.26us ( +- 0.07% )
DR_ACCESS 44669 4.35% 2.36% 0.32us
50.74us 0.76us ( +- 0.32% )
EXTERNAL_INTERRUPT 29809 2.90% 6.42% 0.66us
70.75us 3.10us ( +- 0.54% )
VMCALL 17819 1.73% 5.21% 0.75us
15.64us 4.20us ( +- 0.33%
Total Samples:1027227, Total events handled time:1436343.94us.
===============================
The result shows the overhead increased. enable the apicv can help to
reduce the vm-exit
caused by interrupt injection, but on the other side, there are a lot
of vm-exit caused by APIC_EOI.
When turning off the hyper-v and using the kvm apicv, there is no such
overhead. It seems turning
on hyper V related features is not always the best choice for a windows guest.
Thanks!
Liang
