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

[Michael Roth <mdroth@linux.vnet.ibm.com>]

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.

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).

> 
> Signed-off-by: Greg Kurz <groug@kaod.org>
> ---
>  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:
> 
>