Re: handle_pio looping during domain shutdown, with qemu 4.2.0 in stubdom

[Jan Beulich <jbeulich@xxxxxxxx>]

On 08.06.2020 14:38, Paul Durrant wrote:
>> From: Jürgen Groß <jgross@xxxxxxxx>
>> Sent: 08 June 2020 10:25
>>
>> On 08.06.20 11:15, Paul Durrant wrote:
>>>> From: Jan Beulich <jbeulich@xxxxxxxx>
>>>> Sent: 08 June 2020 09:14
>>>>
>>>> On 05.06.2020 18:18, 'Marek Marczykowski-Górecki' wrote:
>>>>> (XEN) d12v0 XEN_DMOP_remote_shutdown domain 11 reason 0
>>>>> (XEN) d12v0 domain 11 domain_shutdown vcpu_id 0 defer_shutdown 1
>>>>> (XEN) d12v0 XEN_DMOP_remote_shutdown domain 11 done
>>>>> (XEN) d12v0 hvm_destroy_ioreq_server called for 11, id 0
>>>>
>>>> Can either of you tell why this is? As said before, qemu shouldn't
>>>> start tearing down ioreq servers until the domain has made it out
>>>> of all shutdown deferrals, and all its vCPU-s have been paused.
>>>> For the moment I think the proposed changes, while necessary, will
>>>> mask another issue elsewhere. The @releaseDomain xenstore watch,
>>>> being the trigger I would consider relevant here, will trigger
>>>> only once XEN_DOMINF_shutdown is reported set for a domain, which
>>>> gets derived from d->is_shut_down (i.e. not mistakenly
>>>> d->is_shutting_down).
>>>
>>> I can't find anything that actually calls xendevicemodel_shutdown(). It was 
>>> added by:
>>
>> destroy_hvm_domain() in qemu does.
>>
> 
> Ah ok, thanks. So it looks like this should only normally be called when the 
> guest has written to the PIIX to request shutdown. Presumably the 
> hvm_destroy_ioreq_server call we see afterwards is QEMU then exiting.

If a shutdown request was received, qemu should still not exit as long
as there's in-flight I/O, or as long as there are CPUs running in the
guest. It would seem legitimate to "cancel" in-flight I/O (and perhaps
"reject" further requests arriving), but the device model needs to
remain there as long as there's at least one running CPU. That's no
different on "real" hardware - the machine doesn't "disappear" just
because shutdown was requested.

Jan