KVM: Allow kernel to select size of mmap() buffer

This allows us to store offsets in the kernel/user kvm_run area, and be
sure that userspace has them mapped.  As offsets can be outside the
kvm_run struct, userspace has no way of knowing how much to mmap.
Signed-off-by: Avi Kivity <avi@qumranet.com>

drivers/kvm/kvm_main.c

@@ -2436,7 +2436,7 @@ static long kvm_dev_ioctl(struct file *filp,
 			  unsigned int ioctl, unsigned long arg)
 {
 	void __user *argp = (void __user *)arg;
-	int r = -EINVAL;
+	long r = -EINVAL;
 
 	switch (ioctl) {
 	case KVM_GET_API_VERSION:
@@ -2478,6 +2478,12 @@ static long kvm_dev_ioctl(struct file *filp,
 		 */
 		r = 0;
 		break;
+	case KVM_GET_VCPU_MMAP_SIZE:
+		r = -EINVAL;
+		if (arg)
+			goto out;
+		r = PAGE_SIZE;
+		break;
 	default:
 		;
 	}

include/linux/kvm.h

@@ -253,6 +253,10 @@ struct kvm_signal_mask {
  * return is 1 (yes) or 0 (no, sorry).
  */
 #define KVM_CHECK_EXTENSION       _IO(KVMIO,   0x03)
+/*
+ * Get size for mmap(vcpu_fd)
+ */
+#define KVM_GET_VCPU_MMAP_SIZE    _IO(KVMIO,   0x04) /* in bytes */
 
 /*
  * ioctls for VM fds