powerpc/fsl: Emulate SPRN_BUCSR register

commit 98518c4d upstream.

In order to flush the branch predictor the guest kernel performs
writes to the BUCSR register which is hypervisor privilleged. However,
the branch predictor is flushed at each KVM entry, so the branch
predictor has been already flushed, so just return as soon as possible
to guest.
Signed-off-by: Diana Craciun <diana.craciun@nxp.com>
[mpe: Tweak comment formatting]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/powerpc/kvm/e500_emulate.c

@@ -277,6 +277,13 @@ int kvmppc_core_emulate_mtspr_e500(struct kvm_vcpu *vcpu, int sprn, ulong spr_va
 		vcpu->arch.pwrmgtcr0 = spr_val;
 		break;
 
+	case SPRN_BUCSR:
+		/*
+		 * If we are here, it means that we have already flushed the
+		 * branch predictor, so just return to guest.
+		 */
+		break;
+
 	/* extra exceptions */
 #ifdef CONFIG_SPE_POSSIBLE
 	case SPRN_IVOR32: