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

[Ján Tomko]

On Wed, Apr 26, 2017 at 04:19:07PM +0200, Paolo Bonzini wrote:
>
>
>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.
>

If it's only useful for debugging, I can drop "off" from the XML
completely.

>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?

And if apic:qemu,ioapic:qemu is only for debugging, the <apic> element
can be dropped too.

'ioapic' is also more specific (so if kernel-irqchip starts to be useful
on other archs, libvirt might need a new element?).

I'm considering sticking the attribute to the already-existing apic
element:
<apic mode='kernel|split'/>

But I don't really like any of the options.

Jan
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