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Re: [Qemu-ppc] [PATCH v2 13/13] spapr/xive: fix device hotplug when VM i

From: David Gibson
Subject: Re: [Qemu-ppc] [PATCH v2 13/13] spapr/xive: fix device hotplug when VM is stopped
Date: Wed, 13 Mar 2019 15:11:11 +1100
User-agent: Mutt/1.11.3 (2019-02-01)

On Mon, Mar 11, 2019 at 09:59:32PM +0100, Cédric Le Goater wrote:
> On 2/26/19 5:17 AM, David Gibson wrote:
> > On Fri, Feb 22, 2019 at 02:13:22PM +0100, Cédric Le Goater wrote:
> >> Instead of switching off the sources, set their state to PENDING to
> >> possibly catch a hotplug event occuring while the VM is stopped. At
> >> resume, check the previous state and if an interrupt was queued,
> >> generate a trigger.
> > 
> > First, I think it would be better to fold this fix into the patch
> > introducing the state change handlers.> 
> > Second, IIUC this would handle any instance of an irq being triggered
> > while the VM is stopped. 
> yes.
> > Hotplug interrupts is one obvious case of that, but I'm not sure its 
> > the only one.  
> Do we really need to support device hotplug when the VM is stopped ? 
> Is that a libvirt requirement ?  

I'm not actually sure, but I think it's the right thing to do.
Interrupts are asynchronous by nature, so it makes sense that they can
occur while the machine is otherwise stopped.

> > VFIO devices could interrupt while the VM is stopped, I think. 
> If the guest has configured and mapped the IRQs, I would say yes. 
> > Maybe even emulated devices
> > depending on how their synchronization with the cpu run state works.
> The console is one example.
> > There might be other cases.  Does that sound right to you?
> yes.
> Supporting interrupts while a VM is stopped seems like a weird 
> test scenario to me. Should we or should we not ?

I think we should.

David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!

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