powerpc: fix hardware PMU exception bug on PowerVM compatibility mode systems

PowerVM systems running compatibility mode on a few Power8 revisions are
still vulnerable to the hardware defect that loses PMU exceptions arriving
prior to a context switch.

The software fix for this issue is enabled through the CPU_FTR_PMAO_BUG
cpu_feature bit, nevertheless this bit also needs to be set for PowerVM
compatibility mode systems.

Fixes: 68f2f0d ("powerpc: Add a cpu feature CPU_FTR_PMAO_BUG")
Signed-off-by: Desnes A. Nunes do Rosario <desnesn@linux.ibm.com>
Reviewed-by: Leonardo Bras <leonardo@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200227134715.9715-1-desnesn@linux.ibm.com

Author: Desnes A. Nunes do Rosario

arch/powerpc/kernel/cputable.c

@@ -2193,11 +2193,13 @@ static struct cpu_spec * __init setup_cpu_spec(unsigned long offset,
 		 * oprofile_cpu_type already has a value, then we are
 		 * possibly overriding a real PVR with a logical one,
 		 * and, in that case, keep the current value for
-		 * oprofile_cpu_type.
+		 * oprofile_cpu_type. Futhermore, let's ensure that the
+		 * fix for the PMAO bug is enabled on compatibility mode.
 		 */
 		if (old.oprofile_cpu_type != NULL) {
 			t->oprofile_cpu_type = old.oprofile_cpu_type;
 			t->oprofile_type = old.oprofile_type;
+			t->cpu_features |= old.cpu_features & CPU_FTR_PMAO_BUG;
 		}
 	}