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

[Paul Durrant <xadimgnik@xxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: 05 June 2020 14:47
> To: paul@xxxxxxx
> Cc: 'Marek Marczykowski-Górecki' <marmarek@xxxxxxxxxxxxxxxxxxxxxx>; '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
> 
> On 05.06.2020 15:43, Paul Durrant wrote:
> >> -----Original Message-----
> >> From: Jan Beulich <jbeulich@xxxxxxxx>
> >> Sent: 05 June 2020 14:37
> >> To: paul@xxxxxxx
> >> Cc: 'Marek Marczykowski-Górecki' <marmarek@xxxxxxxxxxxxxxxxxxxxxx>; 
> >> '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
> >>
> >> 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.
> >
> > Now I'm confused... defer_shutdown is per-vcpu.
> 
> Right - each vCPU can individually defer shutting down of the domain
> as a whole.
> 

Ok, I think we're only going to make more progress if we know exactly where the 
X86EMUL_UNHANDLEABLE is coming from.

  Paul