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, 150 insertions, 0 deletions

diff --git a/litmus/ctrldev.c b/litmus/ctrldev.c
new file mode 100644
index 000000000000..238da453228d
--- /dev/null
+++ b/litmus/ctrldev.c
@@ -0,0 +1,150 @@
+#include <linux/sched.h>
+#include <linux/mm.h>
+#include <linux/fs.h>
+#include <linux/miscdevice.h>
+#include <linux/module.h>
+
+#include <litmus/litmus.h>
+
+/* only one page for now, but we might want to add a RO version at some point */
+#define CTRL_MINOR_COUNT 1
+#define CTRL_NAME        "litmus/ctrl"
+
+/* allocate t->rt_param.ctrl_page*/
+static int alloc_ctrl_page(struct task_struct *t)
+{
+        int err = 0;
+
+        /* only allocate if the task doesn't have one yet */
+        if (!tsk_rt(t)->ctrl_page) {
+                tsk_rt(t)->ctrl_page = (void*) get_zeroed_page(GFP_KERNEL);
+                if (!tsk_rt(t)->ctrl_page)
+                        err = -ENOMEM;
+                /* will get de-allocated in task teardown */
+                TRACE_TASK(t, "%s ctrl_page = %p\n", __FUNCTION__,
+                           tsk_rt(t)->ctrl_page);
+        }
+        return err;
+}
+
+static int map_ctrl_page(struct task_struct *t, struct vm_area_struct* vma)
+{
+        int err;
+        unsigned long pfn;
+
+        struct page* ctrl = virt_to_page(tsk_rt(t)->ctrl_page);
+
+        /* Increase ref count. Is decreased when vma is destroyed. */
+        get_page(ctrl);
+
+        /* compute page frame number */
+        pfn = page_to_pfn(ctrl);
+
+        TRACE_CUR(CTRL_NAME
+                  ": mapping %p (pfn:%lx, %lx) to 0x%lx (prot:%lx)\n",
+                  tsk_rt(t)->ctrl_page, pfn, page_to_pfn(ctrl), vma->vm_start,
+                  vma->vm_page_prot);
+
+        /* Map it into the vma. Make sure to use PAGE_SHARED, otherwise
+         * userspace actually gets a copy-on-write page. */
+        err = remap_pfn_range(vma, vma->vm_start, pfn, PAGE_SIZE, PAGE_SHARED);
+
+        if (err)
+                TRACE_CUR(CTRL_NAME ": remap_pfn_range() failed (%d)\n", err);
+
+        return err;
+}
+
+static void litmus_ctrl_vm_close(struct vm_area_struct* vma)
+{
+        TRACE_CUR("%s flags=0x%x prot=0x%x\n", __FUNCTION__,
+                  vma->vm_flags, vma->vm_page_prot);
+
+        TRACE_CUR(CTRL_NAME
+                  ": %p:%p vma:%p vma->vm_private_data:%p closed.\n",
+                  (void*) vma->vm_start, (void*) vma->vm_end, vma,
+                  vma->vm_private_data, current->comm,
+                  current->pid);
+}
+
+static int litmus_ctrl_vm_fault(struct vm_area_struct* vma,
+                                      struct vm_fault* vmf)
+{
+        /* This function should never be called, since
+         * all pages should have been mapped by mmap()
+         * already. */
+        TRACE_CUR("%s flags=0x%x\n", __FUNCTION__, vma->vm_flags);
+
+        /* nope, you only get one page */
+        return VM_FAULT_SIGBUS;
+}
+
+static struct vm_operations_struct litmus_ctrl_vm_ops = {
+        .close = litmus_ctrl_vm_close,
+        .fault = litmus_ctrl_vm_fault,
+};
+
+static int litmus_ctrl_mmap(struct file* filp, struct vm_area_struct* vma)
+{
+        int err = 0;
+
+        /* first make sure mapper knows what he's doing */
+
+        /* you can only get one page */
+        if (vma->vm_end - vma->vm_start != PAGE_SIZE)
+                return -EINVAL;
+
+        /* you can only map the "first" page */
+        if (vma->vm_pgoff != 0)
+                return -EINVAL;
+
+        /* you can't share it with anyone */
+        if (vma->vm_flags & (VM_MAYSHARE | VM_SHARED))
+                return -EINVAL;
+
+        vma->vm_ops = &litmus_ctrl_vm_ops;
+        /* this mapping should not be kept across forks,
+         * and cannot be expanded */
+        vma->vm_flags |= VM_DONTCOPY | VM_DONTEXPAND;
+
+        err = alloc_ctrl_page(current);
+        if (!err)
+                err = map_ctrl_page(current, vma);
+
+        TRACE_CUR("%s flags=0x%x prot=0x%lx\n",
+                  __FUNCTION__, vma->vm_flags, vma->vm_page_prot);
+
+        return err;
+}
+
+static struct file_operations litmus_ctrl_fops = {
+        .owner = THIS_MODULE,
+        .mmap  = litmus_ctrl_mmap,
+};
+
+static struct miscdevice litmus_ctrl_dev = {
+        .name  = CTRL_NAME,
+        .minor = MISC_DYNAMIC_MINOR,
+        .fops  = &litmus_ctrl_fops,
+};
+
+static int __init init_litmus_ctrl_dev(void)
+{
+        int err;
+
+        BUILD_BUG_ON(sizeof(struct control_page) > PAGE_SIZE);
+
+        printk("Initializing LITMUS^RT control device.\n");
+        err = misc_register(&litmus_ctrl_dev);
+        if (err)
+                printk("Could not allocate %s device (%d).\n", CTRL_NAME, err);
+        return err;
+}
+
+static void __exit exit_litmus_ctrl_dev(void)
+{
+        misc_deregister(&litmus_ctrl_dev);
+}
+
+module_init(init_litmus_ctrl_dev);
+module_exit(exit_litmus_ctrl_dev);