powerpc/perf: Disable trace_imc pmu

When a root user or a user with CAP_SYS_ADMIN privilege uses any
trace_imc performance monitoring unit events, to monitor application
or KVM threads, it may result in a checkstop (System crash).

The cause is frequent switching of the "trace/accumulation" mode of
the In-Memory Collection hardware (LDBAR).

This patch disables the trace_imc PMU unit entirely to avoid
triggering the checkstop. A future patch will reenable it at a later
stage once a workaround has been developed.

Fixes: 012ae244 ("powerpc/perf: Trace imc PMU functions")
Cc: stable@vger.kernel.org # v5.2+
Signed-off-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Tested-by: Hariharan T.S. <hari@linux.ibm.com>
[mpe: Add pr_info_once() so dmesg shows the PMU has been disabled]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191118034452.9939-1-maddy@linux.vnet.ibm.com

arch/powerpc/platforms/powernv/opal-imc.c

@@ -278,7 +278,14 @@ static int opal_imc_counters_probe(struct platform_device *pdev)
 			domain = IMC_DOMAIN_THREAD;
 			break;
 		case IMC_TYPE_TRACE:
-			domain = IMC_DOMAIN_TRACE;
+			/*
+			 * FIXME. Using trace_imc events to monitor application
+			 * or KVM thread performance can cause a checkstop
+			 * (system crash).
+			 * Disable it for now.
+			 */
+			pr_info_once("IMC: disabling trace_imc PMU\n");
+			domain = -1;
 			break;
 		default:
 			pr_warn("IMC Unknown Device type \n");