KVM: add basic paravirt support

Add basic KVM paravirt support. Avoid vm-exits on IO delays.
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>

arch/x86/kvm/x86.c

@@ -820,6 +820,7 @@ int kvm_dev_ioctl_check_extension(long ext)
 	case KVM_CAP_EXT_CPUID:
 	case KVM_CAP_CLOCKSOURCE:
 	case KVM_CAP_PIT:
+	case KVM_CAP_NOP_IO_DELAY:
 		r = 1;
 		break;
 	case KVM_CAP_VAPIC:

include/asm-x86/kvm_para.h

@@ -10,7 +10,8 @@
  * paravirtualization, the appropriate feature bit should be checked.
  */
 #define KVM_CPUID_FEATURES	0x40000001
-#define KVM_FEATURE_CLOCKSOURCE 0
+#define KVM_FEATURE_CLOCKSOURCE		0
+#define KVM_FEATURE_NOP_IO_DELAY	1
 
 #define MSR_KVM_WALL_CLOCK  0x11
 #define MSR_KVM_SYSTEM_TIME 0x12

include/linux/kvm.h

@@ -237,6 +237,7 @@ struct kvm_vapic_addr {
 #define KVM_CAP_NR_VCPUS 9       /* returns max vcpus per vm */
 #define KVM_CAP_NR_MEMSLOTS 10   /* returns max memory slots per vm */
 #define KVM_CAP_PIT 11
+#define KVM_CAP_NOP_IO_DELAY 12
 
 /*
  * ioctls for VM fds