UBUNTU: SAUCE: KVM: PPC: Book3S HV: Fix soft lockups in KVM on HMI for time base errors

BugLink: http://bugs.launchpad.net/bugs/1537881
https://lists.ozlabs.org/pipermail/linuxppc-dev/2016-January/138011.html

When secondaries are napping in kvm_unsplit_nap() with hwthread_req = 1,
the HMI goes ignored even though subcores are already exited the guest.
Hence HMI keeps waking up secondaries from nap in a loop and secondaries
always go back to nap since no vcore is assigned to them. This makes
impossible for primary thread to get hold of secondary threads resulting
into a soft lockup in KVM path.

This patch fixes this by adding a HMI check just before the thread goes
to unsplit nap.
Signed-off-by: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

arch/powerpc/kvm/book3s_hv_rmhandlers.S

@@ -440,6 +440,12 @@ kvm_no_guest:
  * whole-core mode, so we need to nap.
  */
 kvm_unsplit_nap:
+	/* Before we head down to nap, check if HMI is pending and handle it */
+	cmpwi	r12, BOOK3S_INTERRUPT_HMI
+	bne	55f
+	li	r3, 0			/* NULL argument */
+	bl	hmi_exception_realmode
+55:
 	/*
 	 * Ensure that secondary doesn't nap when it has
 	 * its vcore pointer set.