powerpc/perf/imc: Fix nest-imc cpuhotplug callback failure

Oops is observed during boot:

  Faulting instruction address: 0xc000000000248340
  cpu 0x0: Vector: 380 (Data Access Out of Range) at [c000000ff66fb850]
      pc: c000000000248340: event_function_call+0x50/0x1f0
      lr: c00000000024878c: perf_remove_from_context+0x3c/0x100
      sp: c000000ff66fbad0
     msr: 9000000000009033
     dar: 7d20e2a6f92d03c0
    pid = 14, comm = cpuhp/0

While registering the cpuhotplug callbacks for nest-imc, if we fail in
the cpuhotplug online path for any random node in a multi node
system (because the opal call to stop nest-imc counters fails for that
node), ppc_nest_imc_cpu_offline() will get invoked for other nodes who
successfully returned from cpuhotplug online path.

This call trace is generated since in the ppc_nest_imc_cpu_offline()
path we are trying to migrate the event context, when nest-imc
counters are not even initialized.

Patch to add a check to ensure that nest-imc is registered before
migrating the event context.

Fixes: 885dcd70 ("powerpc/perf: Add nest IMC PMU support")
Signed-off-by: Anju T Sudhakar <anju@linux.vnet.ibm.com>
Reviewed-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/perf/imc-pmu.c

@@ -309,6 +309,19 @@ static int ppc_nest_imc_cpu_offline(unsigned int cpu)
 	if (!cpumask_test_and_clear_cpu(cpu, &nest_imc_cpumask))
 		return 0;
 
+	/*
+	 * Check whether nest_imc is registered. We could end up here if the
+	 * cpuhotplug callback registration fails. i.e, callback invokes the
+	 * offline path for all successfully registered nodes. At this stage,
+	 * nest_imc pmu will not be registered and we should return here.
+	 *
+	 * We return with a zero since this is not an offline failure. And
+	 * cpuhp_setup_state() returns the actual failure reason to the caller,
+	 * which in turn will call the cleanup routine.
+	 */
+	if (!nest_pmus)
+		return 0;
+
 	/*
 	 * Now that this cpu is one of the designated,
 	 * find a next cpu a) which is online and b) in same chip.