From bcaacec1ca714224807728a63eccb37e3d685cd6 Mon Sep 17 00:00:00 2001
From: Glenn Elliott <gelliott@cs.unc.edu>
Date: Fri, 8 Mar 2013 00:43:41 -0500
Subject: uncachedev: mmap memory that is not cached by CPUs
This patch creates a new character device, uncachedev.
Pages of RAM allocated by this device are not cached by
CPUs.
Uses for such pages:
1) Determining *very* pessimistic emperical worst-
case execution times.
2) Compare against performance with caches (quantify
the avg. case benefit).
3) Deterministic memory accesses (access cannot cause a
cache eviction.)
4) Theoretically, increased performance can be achieved
by storing infrequently accessed data in uncache pages.
uncachedev allocates pages with the pgprot_noncached() page
attribute for user applications. Since pages allocated by
uncachedev are not locked in memory by default, applications
with any access level may mmap pages with uncachedev.
Limitations:
1) Uncache pages must be MAP_PRIVATE.
2) Remapping not supported.
Usage (user level):
int size = NR_PAGES*PAGE_SIZE;
int fd = open("/dev/litmus/uncache", O_RDWR);
char *data = mmap(NULL, size, PROT_READ | PROT_WRITE,
MAP_PRIVATE, fd, 0);
<...do stuff...>
munmap(data, size);
---
litmus/Makefile | 1 +
litmus/uncachedev.c | 102 ++++++++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 103 insertions(+)
create mode 100644 litmus/uncachedev.c
diff --git a/litmus/Makefile b/litmus/Makefile
index d26ca7076b62..2bddc94a399f 100644
--- a/litmus/Makefile
+++ b/litmus/Makefile
@@ -18,6 +18,7 @@ obj-y = sched_plugin.o litmus.o \
bheap.o \
binheap.o \
ctrldev.o \
+ uncachedev.o \
sched_gsn_edf.o \
sched_psn_edf.o \
sched_pfp.o
diff --git a/litmus/uncachedev.c b/litmus/uncachedev.c
new file mode 100644
index 000000000000..06a6a7c17983
--- /dev/null
+++ b/litmus/uncachedev.c
@@ -0,0 +1,102 @@
+#include <linux/sched.h>
+#include <linux/kernel.h>
+#include <linux/mm.h>
+#include <linux/fs.h>
+#include <linux/errno.h>
+#include <linux/highmem.h>
+#include <asm/page.h>
+#include <linux/miscdevice.h>
+#include <linux/module.h>
+
+#include <litmus/litmus.h>
+
+/* device for allocating pages not cached by the CPU */
+
+#define UNCACHE_NAME "litmus/uncache"
+
+void litmus_uncache_vm_open(struct vm_area_struct *vma)
+{
+}
+
+void litmus_uncache_vm_close(struct vm_area_struct *vma)
+{
+}
+
+int litmus_uncache_vm_fault(struct vm_area_struct* vma,
+ struct vm_fault* vmf)
+{
+ /* modeled after SG DMA video4linux, but without DMA. */
+ /* (see drivers/media/video/videobuf-dma-sg.c) */
+ struct page *page;
+
+ page = alloc_page(GFP_USER);
+ if (!page)
+ return VM_FAULT_OOM;
+
+ clear_user_highpage(page, (unsigned long)vmf->virtual_address);
+ vmf->page = page;
+
+ return 0;
+}
+
+static struct vm_operations_struct litmus_uncache_vm_ops = {
+ .open = litmus_uncache_vm_open,
+ .close = litmus_uncache_vm_close,
+ .fault = litmus_uncache_vm_fault,
+};
+
+static int litmus_uncache_mmap(struct file* filp, struct vm_area_struct* vma)
+{
+ /* first make sure mapper knows what he's doing */
+
+ /* 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;
+
+ /* cannot be expanded, and is not a "normal" page. */
+ vma->vm_flags |= VM_DONTEXPAND;
+
+ /* noncached pages are not explicitly locked in memory (for now). */
+ vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
+
+ vma->vm_ops = &litmus_uncache_vm_ops;
+
+ return 0;
+}
+
+static struct file_operations litmus_uncache_fops = {
+ .owner = THIS_MODULE,
+ .mmap = litmus_uncache_mmap,
+};
+
+static struct miscdevice litmus_uncache_dev = {
+ .name = UNCACHE_NAME,
+ .minor = MISC_DYNAMIC_MINOR,
+ .fops = &litmus_uncache_fops,
+ /* pages are not locked, so there is no reason why
+ anyone cannot allocate an uncache pages */
+ .mode = (S_IRUGO | S_IWUGO),
+};
+
+static int __init init_litmus_uncache_dev(void)
+{
+ int err;
+
+ printk("Initializing LITMUS^RT uncache device.\n");
+ err = misc_register(&litmus_uncache_dev);
+ if (err)
+ printk("Could not allocate %s device (%d).\n", UNCACHE_NAME, err);
+ return err;
+}
+
+static void __exit exit_litmus_uncache_dev(void)
+{
+ misc_deregister(&litmus_uncache_dev);
+}
+
+module_init(init_litmus_uncache_dev);
+module_exit(exit_litmus_uncache_dev);
--
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