==========================
> > 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.
>
> I think I know what's happening. We've asked Windows to use synthetic
> MSRs to access APIC (HV_APIC_ACCESS_RECOMMENDED) and this can't be
> accelerated in hardware.
>
> Could you please try the following hack (KVM):
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index c8f2592ccc99..66ee85a83e9a 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -145,6 +145,13 @@ void kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu)
> vcpu->arch.ia32_misc_enable_msr &
> MSR_IA32_MISC_ENABLE_MWAIT);
> }
> +
> + /* Dirty hack: force HV_DEPRECATING_AEOI_RECOMMENDED. Not to be merged! */
> + best = kvm_find_cpuid_entry(vcpu, HYPERV_CPUID_ENLIGHTMENT_INFO, 0);
> + if (best) {
> + best->eax &= ~HV_X64_APIC_ACCESS_RECOMMENDED;
> + best->eax |= HV_DEPRECATING_AEOI_RECOMMENDED;
> + }
> }
> EXPORT_SYMBOL_GPL(kvm_update_cpuid_runtime);
>
> > It seems turning on hyper V related features is not always the best
> > choice for a windows guest.
>
> Generally it is, we'll just need to make QEMU smarter when setting
> 'recommendation' bits.
>
Hi Vitaly,
I have tried your patch and found it can help to reduce the overhead.
it works as well as
the option "<feature policy='disable' name='hypervisor'/>" is set in
libvirt xml.
=======with your patch and stimer enabled=====
Analyze events for all VMs, all VCPUs:
VM-EXIT Samples Samples% Time% Min Time Max
Time Avg time
APIC_WRITE 172232 78.36% 68.99% 0.70us
47.71us 1.48us ( +- 0.18% )
DR_ACCESS 19136 8.71% 4.42% 0.55us
4.42us 0.85us ( +- 0.32% )
EXTERNAL_INTERRUPT 15921 7.24% 13.84% 0.87us
55.28us 3.21us ( +- 0.55% )
VMCALL 6971 3.17% 10.34% 1.16us
12.02us 5.48us ( +- 0.49%
Total Samples:219802, Total events handled time:369310.30us.
===========with hypervisor disabled=========
Analyze events for all VMs, all VCPUs:
VM-EXIT Samples Samples% Time% Min Time Max
Time Avg time
APIC_WRITE 200482 78.51% 68.62% 0.64us
49.51us 1.37us ( +- 0.16% )
DR_ACCESS 24235 9.49% 4.92% 0.55us
3.65us 0.81us ( +- 0.26% )
EXTERNAL_INTERRUPT 17084 6.69% 13.20% 0.89us
56.38us 3.09us ( +- 0.53% )
VMCALL 7124 2.79% 9.87% 1.26us
12.39us 5.54us ( +- 0.49% )
EOI_INDUCED 5066 1.98% 1.36% 0.66us
2.64us 1.07us ( +- 0.25% )
IO_INSTRUCTION 591 0.23% 1.27% 3.37us
673.23us 8.59us ( +- 13.69% )
Total Samples:255363, Total events handled time:399954.27us.
Thanks!
Liang
