Add virtual LITMUS^RT control device.

This device only supports mmap()'ing a single page.
This page is shared RW between the kernel and userspace.
It is inteded to allow near-zero-overhead communication
between the kernel and userspace. It's first use will be a
proper implementation of user-signaled
non-preemptable section support.

1 files changed, 27 insertions, 0 deletions

diff --git a/include/litmus/rt_param.h b/include/litmus/rt_param.h
index e20427846273..9353251fb30e 100644
--- a/include/litmus/rt_param.h
+++ b/include/litmus/rt_param.h
@@ -35,6 +35,30 @@ struct rt_task {
         task_class_t    cls;
 };
 
+/* The definition of the data that is shared between the kernel and real-time
+ * tasks via a shared page (see litmus/ctrldev.c).
+ *
+ * WARNING: User space can write to this, so don't trust
+ * the correctness of the fields!
+ *
+ * This servees two purposes: to enable efficient signaling
+ * of non-preemptive sections (user->kernel) and
+ * delayed preemptions (kernel->user), and to export
+ * some real-time relevant statistics such as preemption and
+ * migration data to user space. We can't use a device to export
+ * statistics because we want to avoid system call overhead when
+ * determining preemption/migration overheads).
+ */
+struct control_page {
+        /* Is the task currently in a non-preemptive section? */
+        int np_flag;
+        /* Should the task call into the kernel when it leaves
+         * its non-preemptive section? */
+        int delayed_preemption;
+
+        /* to be extended */
+};
+
 /* don't export internal data structures to user space (liblitmus) */
 #ifdef __KERNEL__
 
@@ -163,6 +187,9 @@ struct rt_param {
         /* Used by rt_domain to queue task in release list.
          */
         struct list_head list;
+
+        /* Pointer to the page shared between userspace and kernel. */
+        struct control_page * ctrl_page;
 };
 
 /*      Possible RT flags       */