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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Jan