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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 05.06.2020 13:25, Paul Durrant wrote:
>> -----Original Message-----
>> From: Paul Durrant <xadimgnik@xxxxxxxxx>
>> Sent: 05 June 2020 12:06
>> To: 'Jan Beulich' <jbeulich@xxxxxxxx>; 'Marek Marczykowski-Górecki' 
>> <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
>> Cc: 'Andrew Cooper' <andrew.cooper3@xxxxxxxxxx>; 'xen-devel' 
>> <xen-devel@xxxxxxxxxxxxxxxxxxxx>
>> Subject: RE: handle_pio looping during domain shutdown, with qemu 4.2.0 in 
>> stubdom
>>
>> Sorry, only just catching up with this...
>>
>>> -----Original Message-----
>>> From: Jan Beulich <jbeulich@xxxxxxxx>
>>> Sent: 05 June 2020 10:09
>>> To: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
>>> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>; xen-devel 
>>> <xen-devel@xxxxxxxxxxxxxxxxxxxx>; Paul
>>> Durrant <paul@xxxxxxx>
>>> Subject: Re: handle_pio looping during domain shutdown, with qemu 4.2.0 in 
>>> stubdom
>>>
>>> On 04.06.2020 16:25, Marek Marczykowski-Górecki wrote:
>>>> On Thu, Jun 04, 2020 at 02:36:26PM +0200, Jan Beulich wrote:
>>>>> On 04.06.2020 13:13, Andrew Cooper wrote:
>>>>>> On 04/06/2020 08:08, Jan Beulich wrote:
>>>>>>> On 04.06.2020 03:46, Marek Marczykowski-Górecki wrote:
>>>>>>>> Then, we get the main issue:
>>>>>>>>
>>>>>>>>     (XEN) d3v0 handle_pio port 0xb004 read 0x0000
>>>>>>>>     (XEN) d3v0 Weird PIO status 1, port 0xb004 read 0xffff
>>>>>>>>     (XEN) domain_crash called from io.c:178
>>>>>>>>
>>>>>>>> Note, there was no XEN_DOMCTL_destroydomain for domain 3 nor its 
>>>>>>>> stubdom
>>>>>>>> yet. But XEN_DMOP_remote_shutdown for domain 3 was called already.
>>>>>>> I'd guess an issue with the shutdown deferral logic. Did you / can
>>>>>>> you check whether XEN_DMOP_remote_shutdown managed to pause all
>>>>>>> CPUs (I assume it didn't, since once they're paused there shouldn't
>>>>>>> be any I/O there anymore, and hence no I/O emulation)?
>>>>>>
>>>>>> The vcpu in question is talking to Qemu, so will have v->defer_shutdown
>>>>>> intermittently set, and skip the pause in domain_shutdown()
>>>>>>
>>>>>> I presume this lack of pause is to allow the vcpu in question to still
>>>>>> be scheduled to consume the IOREQ reply?  (Its fairly opaque logic with
>>>>>> 0 clarifying details).
>>>>>>
>>>>>> What *should* happen is that, after consuming the reply, the vcpu should
>>>>>> notice and pause itself, at which point it would yield to the
>>>>>> scheduler.  This is the purpose of vcpu_{start,end}_shutdown_deferral().
>>>>>>
>>>>>> Evidentially, this is not happening.
>>>>>
>>>>> We can't tell yet, until ...
>>>>>
>>>>>> Marek: can you add a BUG() after the weird PIO printing?  That should
>>>>>> confirm whether we're getting into handle_pio() via the
>>>>>> handle_hvm_io_completion() path, or via the vmexit path (at which case,
>>>>>> we're fully re-entering the guest).
>>>>>
>>>>> ... we know this. handle_pio() gets called from handle_hvm_io_completion()
>>>>> after having called hvm_wait_for_io() -> hvm_io_assist() ->
>>>>> vcpu_end_shutdown_deferral(), so the issue may be that we shouldn't call
>>>>> handle_pio() (etc) at all anymore in this state. IOW perhaps
>>>>> hvm_wait_for_io() should return "!sv->vcpu->domain->is_shutting_down"
>>>>> instead of plain "true"?
>>>>>
>>>>> Adding Paul to Cc, as being the maintainer here.
>>>>
>>>> Got it, by sticking BUG() just before that domain_crash() in
>>>> handle_pio(). And also vcpu 0 of both HVM domains do have
>>>> v->defer_shutdown.
>>>
>>> As per the log they did get it set. I'd be curious of the flag's
>>> value (as well as v->paused_for_shutdown's) at the point of the
>>> problematic handle_pio() invocation (see below). It may be
>>> worthwhile to instrument vcpu_check_shutdown() (inside its if())
>>> - before exiting to guest context (in order to then come back
>>> and call handle_pio()) the vCPU ought to be getting through
>>> there. No indication of it doing so would be a sign that there's
>>> a code path bypassing the call to vcpu_end_shutdown_deferral().
>>>
>>>> (XEN) hvm.c:1620:d6v0 All CPUs offline -- powering off.
>>>> (XEN) d3v0 handle_pio port 0xb004 read 0x0000
>>>> (XEN) d3v0 handle_pio port 0xb004 read 0x0000
>>>> (XEN) d3v0 handle_pio port 0xb004 write 0x0001
>>>> (XEN) d3v0 handle_pio port 0xb004 write 0x2001
>>>> (XEN) d4v0 XEN_DMOP_remote_shutdown domain 3 reason 0
>>>> (XEN) d4v0 domain 3 domain_shutdown vcpu_id 0 defer_shutdown 1
>>>> (XEN) d4v0 XEN_DMOP_remote_shutdown domain 3 done
>>>> (XEN) hvm.c:1620:d5v0 All CPUs offline -- powering off.
>>>> (XEN) d1v0 handle_pio port 0xb004 read 0x0000
>>>> (XEN) d1v0 handle_pio port 0xb004 read 0x0000
>>>> (XEN) d1v0 handle_pio port 0xb004 write 0x0001
>>>> (XEN) d1v0 handle_pio port 0xb004 write 0x2001
>>>> (XEN) d2v0 XEN_DMOP_remote_shutdown domain 1 reason 0
>>>> (XEN) d2v0 domain 1 domain_shutdown vcpu_id 0 defer_shutdown 1
>>>> (XEN) d2v0 XEN_DMOP_remote_shutdown domain 1 done
>>>> (XEN) grant_table.c:3702:d0v0 Grant release 0x3 ref 0x11d flags 0x2 d6
>>>> (XEN) grant_table.c:3702:d0v0 Grant release 0x4 ref 0x11e flags 0x2 d6
>>>> (XEN) d3v0 handle_pio port 0xb004 read 0x0000
>>>
>>> Perhaps in this message could you also log
>>> v->domain->is_shutting_down, v->defer_shutdown, and
>>> v->paused_for_shutdown? (Would be nice if, after having made
>>> changes to your debugging patch, you could point again at the
>>> precise version you've used for the log provided.)
>>>
>>>> (XEN) d3v0 Unexpected PIO status 1, port 0xb004 read 0xffff
>>>> (XEN) Xen BUG at io.c:178
>>>
>>> Btw, instead of BUG(), WARN() or dump_execution_state() would
>>> likely also do, keeping Xen alive.
>>>
>>
>> A shutdown deferral problem would result in X86EMUL_RETRY wouldn't it? That 
>> would mean we wouldn't be
>> seeing the "Unexpected PIO" message. From that message this clearly 
>> X86EMUL_UNHANDLEABLE which
>> suggests a race with ioreq server teardown, possibly due to selecting a 
>> server but then not finding a
>> vcpu match in ioreq_vcpu_list.
> 
> Actually looking at remote_shutdown... the test of ( reason == SHUTDOWN_crash 
> ) and then clearing defer_shutdown looks a bit odd... Just because the domain 
> shutdown code has been set that way doesn't mean that a vcpu is not deferred 
> in emulation; SCHEDOP_shutdown_code could easily be called from one vcpu 
> whilst another has emulation pending.

I'm confused: The deferral is of shutting down the domain, not of
a specific instance of emulation. When a guest crashed I understand
this code is intended to make sure shutting down because of the
crash won't get deferred because of in-progress emulation anywhere.

Marek didn't provide any hints so far that the guest may be crashing,
so I think if there is an issue here, it's likely only a tangential
one anyway.

Jan