KVM: use modern cpumask primitives, no cpumask_t on stack

We're getting rid on on-stack cpumasks for large NR_CPUS.

1) Use cpumask_var_t/alloc_cpumask_var.
2) smp_call_function_mask -> smp_call_function_many
3) cpus_clear, cpus_empty, cpu_set -> cpumask_clear, cpumask_empty,
   cpumask_set_cpu.

This actually generates slightly smaller code than the old one with
CONFIG_CPUMASKS_OFFSTACK=n.  (gcc knows that cpus cannot be NULL in
that case, where cpumask_var_t is cpumask_t[1]).
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>
Signed-off-by: NAvi Kivity <avi@redhat.com>

virt/kvm/kvm_main.c

@@ -555,12 +555,14 @@ static void ack_flush(void *_completed)
 static bool make_all_cpus_request(struct kvm *kvm, unsigned int req)
 {
 	int i, cpu, me;
-	cpumask_t cpus;
-	bool called = false;
+	cpumask_var_t cpus;
+	bool called = true;
 	struct kvm_vcpu *vcpu;
 
+	if (alloc_cpumask_var(&cpus, GFP_ATOMIC))
+		cpumask_clear(cpus);
+
 	me = get_cpu();
-	cpus_clear(cpus);
 	for (i = 0; i < KVM_MAX_VCPUS; ++i) {
 		vcpu = kvm->vcpus[i];
 		if (!vcpu)
@@ -568,14 +570,17 @@ static bool make_all_cpus_request(struct kvm *kvm, unsigned int req)
 		if (test_and_set_bit(req, &vcpu->requests))
 			continue;
 		cpu = vcpu->cpu;
-		if (cpu != -1 && cpu != me)
-			cpu_set(cpu, cpus);
-	}
-	if (!cpus_empty(cpus)) {
-		smp_call_function_mask(cpus, ack_flush, NULL, 1);
-		called = true;
+		if (cpus != NULL && cpu != -1 && cpu != me)
+			cpumask_set_cpu(cpu, cpus);
 	}
+	if (unlikely(cpus == NULL))
+		smp_call_function_many(cpu_online_mask, ack_flush, NULL, 1);
+	else if (!cpumask_empty(cpus))
+		smp_call_function_many(cpus, ack_flush, NULL, 1);
+	else
+		called = false;
 	put_cpu();
+	free_cpumask_var(cpus);
 	return called;
 }