x86/hyperv: Redirect reenlightment notifications on CPU offlining

It is very unlikely for CPUs to get offlined when running on Hyper-V as
there is a protection in the vmbus module which prevents it when the guest
has any VMBus devices assigned. This, however, may change in future if an
option to reassign an already active channel will be added. It is also
possible to run without any Hyper-V devices or to have a CPU with no
assigned channels.

Reassign reenlightenment notifications to some other active CPU when the
CPU which is assigned to them goes offline.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephen Hemminger <sthemmin@microsoft.com>
Cc: kvm@vger.kernel.org
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: "Michael Kelley (EOSG)" <Michael.H.Kelley@microsoft.com>
Cc: Roman Kagan <rkagan@virtuozzo.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: devel@linuxdriverproject.org
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Cathy Avery <cavery@redhat.com>
Cc: Mohammed Gamal <mmorsy@redhat.com>
Link: https://lkml.kernel.org/r/20180124132337.30138-5-vkuznets@redhat.com

arch/x86/hyperv/hv_init.c

@@ -191,6 +191,26 @@ void clear_hv_tscchange_cb(void)
 }
 EXPORT_SYMBOL_GPL(clear_hv_tscchange_cb);
 
+static int hv_cpu_die(unsigned int cpu)
+{
+	struct hv_reenlightenment_control re_ctrl;
+	unsigned int new_cpu;
+
+	if (hv_reenlightenment_cb == NULL)
+		return 0;
+
+	rdmsrl(HV_X64_MSR_REENLIGHTENMENT_CONTROL, *((u64 *)&re_ctrl));
+	if (re_ctrl.target_vp == hv_vp_index[cpu]) {
+		/* Reassign to some other online CPU */
+		new_cpu = cpumask_any_but(cpu_online_mask, cpu);
+
+		re_ctrl.target_vp = hv_vp_index[new_cpu];
+		wrmsrl(HV_X64_MSR_REENLIGHTENMENT_CONTROL, *((u64 *)&re_ctrl));
+	}
+
+	return 0;
+}
+
 /*
  * This function is to be invoked early in the boot sequence after the
  * hypervisor has been detected.
@@ -220,7 +240,7 @@ void hyperv_init(void)
 		return;
 
 	if (cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "x86/hyperv_init:online",
-			      hv_cpu_init, NULL) < 0)
+			      hv_cpu_init, hv_cpu_die) < 0)
 		goto free_vp_index;
 
 	/*