[PATCH v5 3/3] KVM: s390: Add opportunistic floating IRQ delivery

[Janosch Frank <frankja@linux.ibm.com>]

Try to distribute floating IRQs which haven't been taken by a VCPU yet
to sleeping VCPUs. This lowers the risk of floating IRQs not being
delivered.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
---
 arch/s390/include/asm/kvm_host_s390.h |  1 +
 arch/s390/kvm/s390/interrupt.c        | 34 +++++++++++++++++++++++++++
 arch/s390/kvm/s390/s390.c             |  1 +
 3 files changed, 36 insertions(+)

diff --git a/arch/s390/include/asm/kvm_host_s390.h b/arch/s390/include/asm/kvm_host_s390.h
index cd692f8fb764..a86863e621b7 100644
--- a/arch/s390/include/asm/kvm_host_s390.h
+++ b/arch/s390/include/asm/kvm_host_s390.h
@@ -412,6 +412,7 @@ struct kvm_vm_stat {
 	u64 gmap_shadow_r3_entry;
 	u64 gmap_shadow_sg_entry;
 	u64 gmap_shadow_pg_entry;
+	u64 inject_redist;
 };
 
 struct kvm_arch_memory_slot {
diff --git a/arch/s390/kvm/s390/interrupt.c b/arch/s390/kvm/s390/interrupt.c
index 3c0e6216eee3..1b06775e581b 100644
--- a/arch/s390/kvm/s390/interrupt.c
+++ b/arch/s390/kvm/s390/interrupt.c
@@ -372,6 +372,38 @@ static unsigned long deliverable_irqs(struct kvm_vcpu *vcpu)
 	return active_mask;
 }
 
+static void distribute_float_irqs(struct kvm *kvm)
+{
+	struct kvm_vcpu *dst_vcpu;
+	int sigcpu, online_vcpus;
+
+	if (!READ_ONCE(kvm->arch.float_int.pending_irqs))
+		return;
+
+	online_vcpus = atomic_read(&kvm->online_vcpus);
+
+	/*
+	 * Not too worried about synchronization for idle_mask. We
+	 * might burn too many cycles but apart from that waking a
+	 * vcpu is not harmful.
+	 */
+	sigcpu = find_first_bit(kvm->arch.idle_mask, online_vcpus);
+
+	/* Well nobody's sleeping so someone will likely take the IRQ soon */
+	if (sigcpu == online_vcpus)
+		return;
+
+	do {
+		dst_vcpu = kvm_get_vcpu(kvm, sigcpu);
+		if (deliverable_irqs(dst_vcpu)) {
+			kvm->stat.inject_redist++;
+			kvm_s390_vcpu_wakeup(dst_vcpu);
+			break;
+		}
+		sigcpu = find_next_bit(kvm->arch.idle_mask, online_vcpus, ++sigcpu);
+	} while (sigcpu < online_vcpus);
+}
+
 static void __set_cpu_idle(struct kvm_vcpu *vcpu)
 {
 	kvm_s390_set_cpuflags(vcpu, CPUSTAT_WAIT);
@@ -1294,6 +1326,8 @@ int kvm_s390_handle_wait(struct kvm_vcpu *vcpu)
 	if (!sltime)
 		return 0;
 
+	distribute_float_irqs(vcpu->kvm);
+
 	__set_cpu_idle(vcpu);
 	hrtimer_start(&vcpu->arch.ckc_timer, sltime, HRTIMER_MODE_REL);
 	VCPU_EVENT(vcpu, 4, "enabled wait: %llu ns", sltime);
diff --git a/arch/s390/kvm/s390/s390.c b/arch/s390/kvm/s390/s390.c
index efd77b9cd36c..252fde3abc9c 100644
--- a/arch/s390/kvm/s390/s390.c
+++ b/arch/s390/kvm/s390/s390.c
@@ -87,6 +87,7 @@ const struct kvm_stats_desc kvm_vm_stats_desc[] = {
 	STATS_DESC_COUNTER(VM, gmap_shadow_r3_entry),
 	STATS_DESC_COUNTER(VM, gmap_shadow_sg_entry),
 	STATS_DESC_COUNTER(VM, gmap_shadow_pg_entry),
+	STATS_DESC_COUNTER(VM, inject_redist),
 };
 
 const struct kvm_stats_header kvm_vm_stats_header = {
-- 
2.53.0