KVM: halt_polling: improve grow/shrink settings

Right now halt_poll_ns can be change during runtime. The
grow and shrink factors can only be set during module load.
Lets fix several aspects of grow shrink:
- make grow/shrink changeable by root
- make all variables unsigned int
- read the variables once to prevent races

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/trace/events/kvm.h

@@ -359,14 +359,15 @@ TRACE_EVENT(
 #endif
 
 TRACE_EVENT(kvm_halt_poll_ns,
-	TP_PROTO(bool grow, unsigned int vcpu_id, int new, int old),
+	TP_PROTO(bool grow, unsigned int vcpu_id, unsigned int new,
+		 unsigned int old),
 	TP_ARGS(grow, vcpu_id, new, old),
 
 	TP_STRUCT__entry(
 		__field(bool, grow)
 		__field(unsigned int, vcpu_id)
-		__field(int, new)
-		__field(int, old)
+		__field(unsigned int, new)
+		__field(unsigned int, old)
 	),
 
 	TP_fast_assign(
@@ -376,7 +377,7 @@ TRACE_EVENT(kvm_halt_poll_ns,
 		__entry->old            = old;
 	),
 
-	TP_printk("vcpu %u: halt_poll_ns %d (%s %d)",
+	TP_printk("vcpu %u: halt_poll_ns %u (%s %u)",
 			__entry->vcpu_id,
 			__entry->new,
 			__entry->grow ? "grow" : "shrink",

virt/kvm/kvm_main.c

@@ -72,11 +72,11 @@ module_param(halt_poll_ns, uint, S_IRUGO | S_IWUSR);
 
 /* Default doubles per-vcpu halt_poll_ns. */
 static unsigned int halt_poll_ns_grow = 2;
-module_param(halt_poll_ns_grow, int, S_IRUGO);
+module_param(halt_poll_ns_grow, uint, S_IRUGO | S_IWUSR);
 
 /* Default resets per-vcpu halt_poll_ns . */
 static unsigned int halt_poll_ns_shrink;
-module_param(halt_poll_ns_shrink, int, S_IRUGO);
+module_param(halt_poll_ns_shrink, uint, S_IRUGO | S_IWUSR);
 
 /*
  * Ordering of locks:
@@ -1943,14 +1943,15 @@ EXPORT_SYMBOL_GPL(kvm_vcpu_mark_page_dirty);
 
 static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
 {
-	int old, val;
+	unsigned int old, val, grow;
 
 	old = val = vcpu->halt_poll_ns;
+	grow = READ_ONCE(halt_poll_ns_grow);
 	/* 10us base */
-	if (val == 0 && halt_poll_ns_grow)
+	if (val == 0 && grow)
 		val = 10000;
 	else
-		val *= halt_poll_ns_grow;
+		val *= grow;
 
 	vcpu->halt_poll_ns = val;
 	trace_kvm_halt_poll_ns_grow(vcpu->vcpu_id, val, old);
@@ -1958,13 +1959,14 @@ static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
 
 static void shrink_halt_poll_ns(struct kvm_vcpu *vcpu)
 {
-	int old, val;
+	unsigned int old, val, shrink;
 
 	old = val = vcpu->halt_poll_ns;
-	if (halt_poll_ns_shrink == 0)
+	shrink = READ_ONCE(halt_poll_ns_shrink);
+	if (shrink == 0)
 		val = 0;
 	else
-		val /= halt_poll_ns_shrink;
+		val /= shrink;
 
 	vcpu->halt_poll_ns = val;
 	trace_kvm_halt_poll_ns_shrink(vcpu->vcpu_id, val, old);