KVM: Add a comment describing the /dev/kvm no_compat handling

Add a comment explaining the rational behind having both
no_compat open and ioctl callbacks to fend off compat tasks.
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/kvm_main.c

@@ -122,6 +122,13 @@ static long kvm_vcpu_compat_ioctl(struct file *file, unsigned int ioctl,
 				  unsigned long arg);
 #define KVM_COMPAT(c)	.compat_ioctl	= (c)
 #else
+/*
+ * For architectures that don't implement a compat infrastructure,
+ * adopt a double line of defense:
+ * - Prevent a compat task from opening /dev/kvm
+ * - If the open has been done by a 64bit task, and the KVM fd
+ *   passed to a compat task, let the ioctls fail.
+ */
 static long kvm_no_compat_ioctl(struct file *file, unsigned int ioctl,
 				unsigned long arg) { return -EINVAL; }