Re: [Qemu-ppc] [Qemu-devel] [PATCH] spapr: Explicitly check machine support before using EVENT_CLASS_HOT_PLUG

[Greg Kurz]

On Fri, 01 Mar 2019 10:20:16 -0600
Michael Roth <address@hidden> wrote:

> Quoting Greg Kurz (2019-03-01 08:17:53)
> > Recent commit b8165118f52c "spapr: support memory unplug for qtest" broke
> > CPU hotplug tests for old machine types:
> > 
> > $ QTEST_QEMU_BINARY=ppc64-softmmu/qemu-system-ppc64 ./tests/cpu-plug-test 
> > -m=slow
> > /ppc64/cpu-plug/pseries-3.1/device-add/2x3x1&maxcpus=6: OK
> > /ppc64/cpu-plug/pseries-2.12-sxxm/device-add/2x3x1&maxcpus=6: OK
> > /ppc64/cpu-plug/pseries-3.0/device-add/2x3x1&maxcpus=6: OK
> > /ppc64/cpu-plug/pseries-2.10/device-add/2x3x1&maxcpus=6: OK
> > /ppc64/cpu-plug/pseries-2.11/device-add/2x3x1&maxcpus=6: OK
> > /ppc64/cpu-plug/pseries-2.12/device-add/2x3x1&maxcpus=6: OK
> > /ppc64/cpu-plug/pseries-2.9/device-add/2x3x1&maxcpus=6: OK
> > /ppc64/cpu-plug/pseries-2.7/device-add/2x3x1&maxcpus=6: **
> > ERROR:/home/thuth/devel/qemu/hw/ppc/spapr_events.c:313:rtas_event_log_to_source:
> >  assertion failed: (source->enabled)
> > Broken pipe
> > /home/thuth/devel/qemu/tests/libqtest.c:143: kill_qemu() detected QEMU 
> > death from signal 6 (Aborted) (core dumped)
> > Aborted (core dumped)
> > 
> > The rtas_event_log_to_source() function is supposed to return the event
> > source to be used for a given event type. In the case of hotplug, it first
> > tries EVENT_CLASS_HOT_PLUG and, if the guest doesn't support it, it falls
> > back to EVENT_CLASS_EPOW.
> > 
> > This works well for machine types that enable EVENT_CLASS_HOT_PLUG. For
> > older machine types, this happened to work because they don't set the
> > OV5_HP_EVT bit in spapr->ov5. CAS hence logically keeps the bit cleared
> > in spapr->ov5_cas and we avoid the assert.
> > 
> > As shown by commit b8165118f52c, the logic is fragile and we need a
> > bigger hammer to ensure that rtas_event_log_to_source() doesn't go
> > down the EVENT_CLASS_HOT_PLUG path with older machine types.  
> 
> I think that fact that we set spapr->ov5 based on relevant machine
> options/support being there, and the fact that this means they won't
> show up in ov5_cas, is a nice thing to assert and rely on, since being
> able to rely on that keeps keeps the logic throughout the code tied to
> ov5/ov5_cas and avoids the need to keep re-checking things like
> spapr->use_hotplug_event_source (and various other options tied to an
> OV5 capability) after init time.
> 

True.

> I think this particular case is purely the result of us violating (via
> b8165118f52c) the condition that bits that aren't ever set in spapr->ov5
> should never end up in spapr->ov5_cas, and losing that precondition might
> make the code harder to reason about since now we have to check both
> ov5_cas and the related options (rather than 1 necessarilly following
> from the other).
> 
> Another approach might be to revert b8165118f52c, and then add a hook for
> qtest_enabled() that does a fake CAS negotiation (which I guess would
> essentially involve copying spapr->ov5 into spapr->ov5_cas). This would
> maintain the flow of machine options -> spapr->ov5 -> spapr->ov5_cas and
> still let qtest do it's thing (AFAICT).
> 

Yeah, that looks nice. I'll try that right away.

> > 
> > Signed-off-by: Greg Kurz <address@hidden>
> > ---
> >  hw/ppc/spapr_events.c |   12 +++++++-----
> >  1 file changed, 7 insertions(+), 5 deletions(-)
> > 
> > diff --git a/hw/ppc/spapr_events.c b/hw/ppc/spapr_events.c
> > index ab9a1f0063d5..1a09dab6857d 100644
> > --- a/hw/ppc/spapr_events.c
> > +++ b/hw/ppc/spapr_events.c
> > @@ -307,11 +307,13 @@ rtas_event_log_to_source(sPAPRMachineState *spapr, 
> > int log_type)
> > 
> >      switch (log_type) {
> >      case RTAS_LOG_TYPE_HOTPLUG:
> > -        source = spapr_event_sources_get_source(spapr->event_sources,
> > -                                                EVENT_CLASS_HOT_PLUG);
> > -        if (spapr_ovec_test(spapr->ov5_cas, OV5_HP_EVT)) {
> > -            g_assert(source->enabled);
> > -            break;
> > +        if (spapr->use_hotplug_event_source) {
> > +            source = spapr_event_sources_get_source(spapr->event_sources,
> > +                                                    EVENT_CLASS_HOT_PLUG);
> > +            if (spapr_ovec_test(spapr->ov5_cas, OV5_HP_EVT)) {
> > +                g_assert(source->enabled);
> > +                break;
> > +            }
> >          }
> >          /* fall back to epow for legacy hotplug interrupt source */
> >      case RTAS_LOG_TYPE_EPOW:
> > 
> >