[PATCH RFC v3 6/9] KVM: rework kvm_vcpu_on_spin loop

[Radim Krčmář]

The original code managed to obfuscate a straightforward idea:
start iterating from the selected index and reset the index to 0 when
reaching the end of online vcpus, then iterate until reaching the index
that we started at.

The resulting code is a bit better, IMO.  (Still horrible, though.)

Signed-off-by: Radim Krčmář <rkrcmar at redhat.com>
---
 include/linux/kvm_host.h | 13 +++++++++++++
 virt/kvm/kvm_main.c      | 47 ++++++++++++++++++-----------------------------
 2 files changed, 31 insertions(+), 29 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index abd5cb1feb9e..cfb3c0efdd51 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -498,6 +498,19 @@ static inline struct kvm_vcpu *kvm_get_vcpu(struct kvm *kvm, int i)
 	     (vcpup = kvm_get_vcpu(kvm, idx)) != NULL; \
 	     idx++)
 
+#define kvm_for_each_vcpu_from(idx, vcpup, from, kvm) \
+	for (idx = from, vcpup = kvm_get_vcpu(kvm, idx); \
+	     vcpup; \
+	     ({ \
+		idx++; \
+		if (idx >= atomic_read(&kvm->online_vcpus)) \
+			idx = 0; \
+		if (idx == from) \
+			vcpup = NULL; \
+		else \
+			vcpup = kvm_get_vcpu(kvm, idx); \
+	      }))
+
 static inline struct kvm_vcpu *kvm_get_vcpu_by_id(struct kvm *kvm, int id)
 {
 	struct kvm_vcpu *vcpu = NULL;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index d89261d0d8c6..33a15e176927 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2333,8 +2333,7 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
 	struct kvm_vcpu *vcpu;
 	int last_boosted_vcpu = me->kvm->last_boosted_vcpu;
 	int yielded = 0;
-	int try = 3;
-	int pass;
+	int try = 2;
 	int i;
 
 	kvm_vcpu_set_in_spin_loop(me, true);
@@ -2345,34 +2344,24 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
 	 * VCPU is holding the lock that we need and will release it.
 	 * We approximate round-robin by starting at the last boosted VCPU.
 	 */
-	for (pass = 0; pass < 2 && !yielded && try; pass++) {
-		kvm_for_each_vcpu(i, vcpu, kvm) {
-			if (!pass && i <= last_boosted_vcpu) {
-				i = last_boosted_vcpu;
-				continue;
-			} else if (pass && i > last_boosted_vcpu)
-				break;
-			if (!ACCESS_ONCE(vcpu->preempted))
-				continue;
-			if (vcpu == me)
-				continue;
-			if (swait_active(&vcpu->wq) && !kvm_arch_vcpu_runnable(vcpu))
-				continue;
-			if (yield_to_kernel_mode && !kvm_arch_vcpu_in_kernel(vcpu))
-				continue;
-			if (!kvm_vcpu_eligible_for_directed_yield(vcpu))
-				continue;
+	kvm_for_each_vcpu_from(i, vcpu, last_boosted_vcpu, kvm) {
+		if (!ACCESS_ONCE(vcpu->preempted))
+			continue;
+		if (vcpu == me)
+			continue;
+		if (swait_active(&vcpu->wq) && !kvm_arch_vcpu_runnable(vcpu))
+			continue;
+		if (yield_to_kernel_mode && !kvm_arch_vcpu_in_kernel(vcpu))
+			continue;
+		if (!kvm_vcpu_eligible_for_directed_yield(vcpu))
+			continue;
 
-			yielded = kvm_vcpu_yield_to(vcpu);
-			if (yielded > 0) {
-				kvm->last_boosted_vcpu = i;
-				break;
-			} else if (yielded < 0) {
-				try--;
-				if (!try)
-					break;
-			}
-		}
+		yielded = kvm_vcpu_yield_to(vcpu);
+		if (yielded > 0) {
+			kvm->last_boosted_vcpu = i;
+			break;
+		} else if (yielded < 0 && !try--)
+			break;
 	}
 	kvm_vcpu_set_in_spin_loop(me, false);
 
-- 
2.13.3