[libvirt] virsh capabilities, kvm with apic and toggle

[Chris Lalancette]

Garry Dolley wrote:
> When I run 'virsh capabilities' on a libvirt 0.6.4 system with qemu
> 0.10 and kvm-84, one of the results I get is:
> 
>   <guest>        
>     <os_type>hvm</os_type>
>     <arch name='x86_64'>
>       <wordsize>64</wordsize>
>       <emulator>/usr/bin/qemu-system-x86_64</emulator>
>       <machine>pc</machine>
>       <machine>isapc</machine>
>       <domain type='qemu'>
>       </domain>
>       <domain type='kvm'>
>         <emulator>/usr/bin/kvm</emulator>
>       </domain>
>     </arch>
>     <features>
>       <acpi default='on' toggle='yes'/>
>       <apic default='on' toggle='no'/>
>     </features>
>   </guest>
> 
> I'm wondering why the "apic" feature can't be toggled.  Passing
> -no-kvm-irqchip to kvm would turn off "PIC/IOAPIC/LAPIC"
> 

No, that's not what -no-kvm-irqchip does.  By default, KVM will emulate the PIC,
IOAPIC, and LAPIC in the kernel for performance reasons.  However, due to bugs
sometimes in the emulation, it's useful to be able to fall back to Qemu
emulation for these pieces of hardware.  -no-kvm-irqchip means to use the Qemu
versions of the PIC/IOAPIC/LAPIC instead of the in-kernel ones.

-- 
Chris Lalancette