[PATCH 4/6] x86/hyper-v: redirect reenlightment notifications on CPU offlining

[Vitaly Kuznetsov <vkuznets@redhat.com>]

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@redhat.com>
---
RFC -> v1:
- protect hv_cpu_die() from reentrance (when multiple CPUs go offline
  in parallel. I don't think it happens nowdays though but things tend
  to change).
---
 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 bb62839ede81..fbf8e372e261 100644
--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c
@@ -173,6 +173,36 @@ __irq_entry smp_hyperv_reenlightenment_intr(struct pt_regs *regs)
 	exiting_irq();
 }
 
+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.
@@ -202,7 +232,7 @@ 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