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

[Paul Durrant <xadimgnik@xxxxxxxxx>]

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.

  Paul

> Jan