[PATCH v2 4/7] x86/hyper-v: redirect reenlightment notifications on CPU offlining

[Vitaly Kuznetsov]

It is very unlikely for CPUs to get offlined when we run on Hyper-V as
we have a protection in vmbus module which prevents it when we have 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 of have a CPU with no assigned channels.

Reassign reenlightenment notifications to some other active CPU when
the CPU which is assigned to get them goes offline.

Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
---
 arch/x86/hyperv/hv_init.c | 32 +++++++++++++++++++++++++++++++-
 1 file changed, 31 insertions(+), 1 deletion(-)

diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index 347be4de3edd..38118ca38d97 100644
--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c
@@ -191,6 +191,36 @@ 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;
+	int i;
+	static DEFINE_SPINLOCK(lock);
+
+	if (hv_reenlightenment_cb == NULL)
+		return 0;
+
+	/* Make sure the CPU we migrate to is not going away too */
+	spin_lock(&lock);
+	rdmsrl(HV_X64_MSR_REENLIGHTENMENT_CONTROL, *((u64 *)&re_ctrl));
+	if (re_ctrl.target_vp == hv_vp_index[cpu]) {
+		/* Find some other online CPU */
+		for_each_online_cpu(i) {
+			if (i == cpu)
+				continue;
+
+			re_ctrl.target_vp = hv_vp_index[i];
+			wrmsrl(HV_X64_MSR_REENLIGHTENMENT_CONTROL,
+			       *((u64 *)&re_ctrl));
+
+			break;
+		}
+	}
+	spin_unlock(&lock);
+
+	return 0;
+}
+
 /*
  * This function is to be invoked early in the boot sequence after the
  * hypervisor has been detected.
@@ -220,7 +250,7 @@ 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;
 
 	/*
-- 
2.14.3