Add kvm_eventfds_enabled function

Add a function to check if the eventfd capability is present in KVM in
the host kernel.

Signed-off-by: Antonios Motakis <a.motakis@virtualopensystems.com>
Signed-off-by: Nikolay Nikolaev <n.nikolaev@virtualopensystems.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>

include/sysemu/kvm.h

@@ -43,6 +43,7 @@ extern bool kvm_allowed;
 extern bool kvm_kernel_irqchip;
 extern bool kvm_async_interrupts_allowed;
 extern bool kvm_halt_in_kernel_allowed;
+extern bool kvm_eventfds_allowed;
 extern bool kvm_irqfds_allowed;
 extern bool kvm_msi_via_irqfd_allowed;
 extern bool kvm_gsi_routing_allowed;
@@ -82,6 +83,15 @@ extern bool kvm_readonly_mem_allowed;
  */
 #define kvm_halt_in_kernel() (kvm_halt_in_kernel_allowed)
 
+/**
+ * kvm_eventfds_enabled:
+ *
+ * Returns: true if we can use eventfds to receive notifications
+ * from a KVM CPU (ie the kernel supports eventds and we are running
+ * with a configuration where it is meaningful to use them).
+ */
+#define kvm_eventfds_enabled() (kvm_eventfds_allowed)
+
 /**
  * kvm_irqfds_enabled:
  *
@@ -128,6 +138,7 @@ extern bool kvm_readonly_mem_allowed;
 #define kvm_irqchip_in_kernel() (false)
 #define kvm_async_interrupts_enabled() (false)
 #define kvm_halt_in_kernel() (false)
+#define kvm_eventfds_enabled() (false)
 #define kvm_irqfds_enabled() (false)
 #define kvm_msi_via_irqfd_enabled() (false)
 #define kvm_gsi_routing_allowed() (false)

kvm-all.c

@@ -113,6 +113,7 @@ KVMState *kvm_state;
 bool kvm_kernel_irqchip;
 bool kvm_async_interrupts_allowed;
 bool kvm_halt_in_kernel_allowed;
+bool kvm_eventfds_allowed;
 bool kvm_irqfds_allowed;
 bool kvm_msi_via_irqfd_allowed;
 bool kvm_gsi_routing_allowed;
@@ -1541,6 +1542,9 @@ int kvm_init(MachineClass *mc)
         (kvm_check_extension(s, KVM_CAP_READONLY_MEM) > 0);
 #endif
 
+    kvm_eventfds_allowed =
+        (kvm_check_extension(s, KVM_CAP_IOEVENTFD) > 0);
+
     ret = kvm_arch_init(s);
     if (ret < 0) {
         goto err;

kvm-stub.c

@@ -22,6 +22,7 @@
 KVMState *kvm_state;
 bool kvm_kernel_irqchip;
 bool kvm_async_interrupts_allowed;
+bool kvm_eventfds_allowed;
 bool kvm_irqfds_allowed;
 bool kvm_msi_via_irqfd_allowed;
 bool kvm_gsi_routing_allowed;