[libvirt] [PATCHv3 1/6] conf: add <irqchip mode> to <features>

[Paolo Bonzini]

On 26/04/2017 11:36, Peter Xu wrote:
>> Possible values are off, split or on.
> Hi, Ján,
> 
> Thanks you for cced me. One tiny comment on irqchip mode...
> 
> Here could user specify irqchip=off from libvirt side? IIUC that might
> be dangerous since userspace APIC should only be for debugging purpose
> (when kernel-irqchip=off, we'll be using userspace APIC, not kernel
> one, and iirc Paolo mentioned known bugs in userspace APIC). So imho
> we'd better not allow user to use "off", but only "on" and "split".
> 
> CCing Paolo here in case he has any comment on this.

Allowing "off" may be okay as long as it taints the domain.

In the end, irqchip mode is a feature of the APIC/IOAPIC/PIC/PIT, and
even more specifically:

- for kernel-irqchip=off, the distinguishing feature is in-kernel vs.
userspace APIC.

- for kernel-irqchip=split, what we really care about is userspace IOAPIC.

So perhaps another choice for the XML is:

  <features>
    <apic driver='qemu|kvm'/>
    <ioapic driver='qemu|kvm'/>
  </features>

where (apic:qemu,ioapic:kvm) is invalid?

Paolo