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: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Avi Kivity <avi@redhat.com>

1 files changed, 14 insertions, 9 deletions

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index ba4275d06fb..2d6ca7968d6 100644
--- a/virt/kvm/kvm_main.c
+++ b/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;
 }