Re: [PATCH] KVM: add KVM_CAP_VMX_APICV to advertise hardware apic-v support

[Paolo Bonzini <pbonzini@redhat.com>]

On 11/12/2014 13:53, zhanghailiang wrote:
> 
> 
>> 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,

hv-vapic MSRs doesn't provide any performance improvement over x2apic
MSRs.  So Windows should use x2apic MSRs if both are available.  Windows
can use x2apic MSRs together with its EOI optimization, like Linux does.

There definitely are Windows versions that know how to use x2apic (e.g.
2008R2).

> but apic-v (need hardware support, i.e. Haswell cpu).

APICv can use the x2apic MSRs.

> When we don't passthough host cpu model to Guest os,
> it has no idea about whether it supports apic-v in host,

The presence of APICv should be totally transparent to the guest.

> 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.

... and cause the CPUID to change under the guest's feet if you migrate
from a non-APICv to an APICv machines, or vice versa.

Paolo