2014-12-11 20:53+0800, zhanghailiang: > On 2014/12/11 19:48, Paolo Bonzini wrote: > >On 11/12/2014 09:07, zhanghailiang wrote: > >>User space (i.e. QEMU) should be able to check whether KVM > >>supports apic-v. User space will use this to decide whether enable > >>emulated MSR-based APIC (i.e. hyperv-vapic). > > > >Did you see an improvement in some workload? > > > > Yes, compared with using hyperv-vapic, using apic-v reduces lots of VM exit. > > (1) Using Hyperv-vapic > 693288 kvm:kvm_apic [100.00%] > 1228026 kvm:kvm_exit [100.00%] > 536450 kvm:kvm_msr [100.00%] > 164441 kvm:kvm_apic_ipi [100.00%] > 377100 kvm:kvm_eoi [100.00%] > 901 kvm:kvm_mmio [100.00%] > (2) Using apic-v > 324926 kvm:kvm_apic [100.00%] > 898841 kvm:kvm_exit [100.00%] > 60 kvm:kvm_msr [100.00%] > 164641 kvm:kvm_apic_ipi [100.00%] > 665 kvm:kvm_eoi [100.00%] > 949 kvm:kvm_mmio [100.00%] > > >I think it's a Windows bug---it should prefer x2apic to hv-vapic if both > >are available. > > > > No, i don't think it is a windows bug, it has nothing to do with x2apic, but apic-v > (need hardware support, i.e. Haswell cpu). When we don't passthough host cpu model to Guest os, > it has no idea about whether it supports apic-v in host, so the choose should be done by qemu. Nice, thank you. > Actually, qemu has a option 'hv_vapic' for -cpu, we can choose not to configure it if we know > there is apic-v support in host. But IMHO, we'd better to do it automatically. hv-vapic has a legitimate use case with xapic without apic-v, so it makes sense to disable it there. (I'd dislike if QEMU disabled it without giving a control about it.) > PS: > This is the perf data when we configure x2apic but without configure hv-vapic and disable apic-v in kvm. > It is the worst result, compared with the above two cases. I'm not aware of any benefit of using the hyper-v MSR over x2apic MSR, so if Windows does that, I'd call it a bug too ... > 1616665 kvm:kvm_entry [100.00%] > 1634073 kvm:kvm_exit [100.00%] > 0 kvm:kvm_msr [100.00%] > 181544 kvm:kvm_apic_ipi [100.00%] > 368745 kvm:kvm_eoi [100.00%] > 568249 kvm:kvm_mmio [100.00%] and it's simple to see why, Windows doesn't do x2apic. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
