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Subject[PATCH RFC 1/5] vringfd syscall
Date
For virtualization, we've developed virtio_ring for efficient communication.
This would also work well for userspace-kernel communication, particularly
for things like the tun device. By using the same ABI, we can join guests
to the host kernel trivially.

These patches are fairly alpha; I've seen some network stalls I have to
track down and there are some fixmes.

Comments welcome!
Rusty.

diff -r 99132ad16999 Documentation/test_vring.c
--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/Documentation/test_vring.c Sat Apr 05 21:31:40 2008 +1100
@@ -0,0 +1,47 @@
+#include <unistd.h>
+#include <linux/virtio_ring.h>
+#include <stdio.h>
+#include <stdint.h>
+#include <err.h>
+#include <poll.h>
+
+#ifndef __NR_vringfd
+#define __NR_vringfd 327
+#endif
+
+int main()
+{
+ int fd, r;
+ struct vring vr;
+ uint16_t used = 0;
+ struct pollfd pfd;
+ void *buf = calloc(vring_size(256, getpagesize()), 0);
+
+ vring_init(&vr, 256, buf, getpagesize());
+
+ fd = syscall(__NR_vringfd, buf, 256, &used);
+ if (fd < 0)
+ err(1, "vringfd gave %i", fd);
+
+ pfd.fd = fd;
+ pfd.events = POLLIN;
+ r = poll(&pfd, 1, 0);
+
+ if (r != 0)
+ err(1, "poll gave %i", r);
+
+ vr.used->idx++;
+ r = poll(&pfd, 1, 0);
+
+ if (r != 1)
+ err(1, "poll after buf used gave %i", r);
+
+ used++;
+ r = poll(&pfd, 1, 0);
+
+ if (r != 0)
+ err(1, "poll after used incremented gave %i", r);
+
+ close(fd);
+ return 0;
+}
diff -r 99132ad16999 arch/x86/kernel/syscall_table_32.S
--- a/arch/x86/kernel/syscall_table_32.S Sat Apr 05 21:20:32 2008 +1100
+++ b/arch/x86/kernel/syscall_table_32.S Sat Apr 05 21:31:40 2008 +1100
@@ -326,3 +326,4 @@ ENTRY(sys_call_table)
.long sys_fallocate
.long sys_timerfd_settime /* 325 */
.long sys_timerfd_gettime
+ .long sys_vringfd
diff -r 99132ad16999 fs/Kconfig
--- a/fs/Kconfig Sat Apr 05 21:20:32 2008 +1100
+++ b/fs/Kconfig Sat Apr 05 21:31:40 2008 +1100
@@ -2135,4 +2135,14 @@ source "fs/nls/Kconfig"
source "fs/nls/Kconfig"
source "fs/dlm/Kconfig"

+config VRINGFD
+ bool "vring fd support (EXPERIMENTAL)"
+ depends on EXPERIMENTAL
+ help
+ vring is a ringbuffer implementation for efficient I/O. It is
+ currently used by virtualization hosts (lguest, kvm) for efficient
+ networking using the tun driver.
+
+ If unsure, say N.
+
endmenu
diff -r 99132ad16999 fs/Makefile
--- a/fs/Makefile Sat Apr 05 21:20:32 2008 +1100
+++ b/fs/Makefile Sat Apr 05 21:31:40 2008 +1100
@@ -119,3 +119,4 @@ obj-$(CONFIG_DEBUG_FS) += debugfs/
obj-$(CONFIG_DEBUG_FS) += debugfs/
obj-$(CONFIG_OCFS2_FS) += ocfs2/
obj-$(CONFIG_GFS2_FS) += gfs2/
+obj-$(CONFIG_VRINGFD) += vring.o
diff -r 99132ad16999 fs/vring.c
--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/fs/vring.c Sat Apr 05 21:31:40 2008 +1100
@@ -0,0 +1,376 @@
+/* Ring-buffer file descriptor implementation.
+ *
+ * Copyright 2008 Rusty Russell IBM Corporation
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+#include <linux/virtio_ring.h>
+#include <linux/vring.h>
+#include <linux/init.h>
+#include <linux/mutex.h>
+#include <linux/wait.h>
+#include <linux/fs.h>
+#include <linux/poll.h>
+#include <linux/highmem.h>
+#include <linux/file.h>
+#include <linux/mount.h>
+#include <linux/magic.h>
+#include <linux/module.h>
+
+static struct vfsmount *vring_mnt;
+static DEFINE_MUTEX(vring_lock);
+
+struct vring_info
+{
+ struct vring ring;
+ u16 mask;
+ u16 __user *last_used;
+ u16 last_avail;
+
+ const struct vring_ops *ops;
+ void *ops_data;
+
+ /* Waitqueue for poll() */
+ wait_queue_head_t poll_wait;
+
+ /* The mapped used ring. */
+ struct vring_used *used;
+ struct page *used_page;
+};
+
+static unsigned int vring_poll(struct file *filp,
+ struct poll_table_struct *poll)
+{
+ struct vring_info *vr = filp->private_data;
+ int err;
+ unsigned int mask;
+ u16 used, last_used;
+
+ /* Some uses of vrings require updating in user context. This
+ * is best done close to the caller, ie. here. */
+ if (vr->ops && vr->ops->pull) {
+ err = vr->ops->pull(vr->ops_data);
+ if (unlikely(err < 0))
+ return err;
+
+ if (err > 0) {
+ /* Buffers have been used, no need to check indices */
+ mask = POLLIN | POLLRDNORM;
+ goto poll_wait;
+ }
+ }
+
+ err = get_user(used, &vr->ring.used->idx);
+ if (unlikely(err))
+ return err;
+
+ err = get_user(last_used, vr->last_used);
+ if (unlikely(err))
+ return err;
+
+ /* More buffers have been used? It's 'readable'. */
+ if (used != last_used)
+ mask = POLLIN | POLLRDNORM;
+ else
+ mask = 0;
+
+poll_wait:
+ poll_wait(filp, &vr->poll_wait, poll);
+
+ return mask;
+}
+
+static ssize_t vring_write(struct file *filp, const char __user *buf,
+ size_t size, loff_t *off)
+{
+ struct vring_info *vr = filp->private_data;
+
+ if (vr->ops && vr->ops->push)
+ return vr->ops->push(vr->ops_data);
+
+ return -EINVAL;
+}
+
+static int vring_release(struct inode *inode, struct file *filp)
+{
+ struct vring_info *vr = filp->private_data;
+
+ /* Callback for other end. */
+ if (vr->ops && vr->ops->destroy)
+ vr->ops->destroy(vr->ops_data);
+
+ if (vr->used) {
+ kunmap(vr->used_page);
+ put_page(vr->used_page);
+ }
+
+ kfree(vr);
+ return 0;
+}
+
+static const struct file_operations vring_fops = {
+ .release = vring_release,
+ .write = vring_write,
+ .poll = vring_poll,
+};
+
+asmlinkage long sys_vringfd(void __user *addr,
+ unsigned num_descs,
+ u16 __user *last_used)
+{
+ int fd, err;
+ struct file *filp;
+ struct vring_info *vr;
+
+ /* Must be a power of two, and representable by u16 */
+ if (!num_descs || (num_descs & (num_descs-1)) || num_descs > 65536) {
+ err = -EINVAL;
+ goto out;
+ }
+
+ fd = get_unused_fd();
+ if (fd < 0) {
+ err = fd;
+ goto out;
+ }
+
+ filp = alloc_file(vring_mnt, dget(vring_mnt->mnt_root), FMODE_WRITE,
+ &vring_fops);
+ if (!filp) {
+ err = -ENFILE;
+ goto put_fd;
+ }
+
+ filp->private_data = vr = kmalloc(sizeof(*vr), GFP_KERNEL);
+ if (!vr) {
+ err = -ENOMEM;
+ goto put_filp;
+ }
+
+ /* Set up pointers into ring. */
+ vring_init(&vr->ring, num_descs, addr, PAGE_SIZE);
+ init_waitqueue_head(&vr->poll_wait);
+ vr->last_used = last_used;
+ vr->mask = num_descs - 1;
+ vr->ops = NULL;
+ vr->used = NULL;
+
+ err = get_user(vr->last_avail, &vr->ring.avail->idx);
+ if (err)
+ goto free_vr;
+
+ fd_install(fd, filp);
+ return fd;
+
+free_vr:
+ kfree(vr);
+put_filp:
+ put_filp(filp);
+put_fd:
+ put_unused_fd(fd);
+out:
+ return err;
+}
+
+/* Returns an error, or 0 (no buffers), or an id for vring_used_buffer() */
+int vring_get_buffer(struct vring_info *vr,
+ struct iovec *in_iov,
+ unsigned int *num_in, unsigned long *in_len,
+ struct iovec *out_iov,
+ unsigned int *num_out, unsigned long *out_len)
+{
+ unsigned int i, in = 0, out = 0;
+ unsigned long dummy;
+ u16 head;
+ struct vring_desc d;
+
+ if (unlikely(get_user(head, &vr->ring.avail->idx) != 0))
+ return -EFAULT;
+
+ if (vr->last_avail == head)
+ return 0;
+
+ if (!in_len)
+ in_len = &dummy;
+ if (!out_len)
+ out_len = &dummy;
+
+ *in_len = *out_len = 0;
+
+ if (unlikely(get_user(head, &vr->ring.avail->ring[head]) != 0))
+ return -EFAULT;
+
+ i = head;
+ do {
+ if (unlikely(i >= vr->ring.num)) {
+ pr_debug("vring: bad index: %u\n", i);
+ return -EINVAL;
+ }
+
+ if (copy_from_user(&d, &vr->ring.desc[i], sizeof(d)) != 0)
+ return -EFAULT;
+
+ if (d.flags & VRING_DESC_F_WRITE) {
+ /* Check for length and iovec overflows */
+ if (!num_in)
+ return -EINVAL;
+ if (in == *num_in || *in_len + d.len < *in_len)
+ return -E2BIG;
+ in_iov[in].iov_len = d.len;
+ *in_len += d.len;
+ in_iov[in].iov_base = (void __user*)(long)d.addr;
+ in++;
+ } else {
+ if (!num_out)
+ return -EINVAL;
+ if (out == *num_out || *out_len + d.len < *out_len)
+ return -E2BIG;
+ out_iov[out].iov_len = d.len;
+ *out_len += d.len;
+ out_iov[out].iov_base = (void __user*)(long)d.addr;
+ out++;
+ }
+
+ i = d.next;
+ } while (d.flags & VRING_DESC_F_NEXT);
+
+ if (num_in)
+ *num_in = in;
+ if (num_out)
+ *num_out = out;
+
+ /* 0 is a valid head, so add one. */
+ vr->last_avail++;
+ return head + 1;
+}
+EXPORT_SYMBOL_GPL(vring_get_buffer);
+
+void vring_used_buffer(struct vring_info *vr, int id, u32 len)
+{
+ struct vring_used_elem used;
+ u16 used_idx;
+
+ BUG_ON(id <= 0 || id > vr->ring.num);
+
+ used.id = id - 1;
+ used.len = len;
+ if (get_user(used_idx, &vr->ring.used->idx) != 0)
+ return;
+
+ copy_to_user(&vr->ring.used->ring[used_idx & vr->mask], &used,
+ sizeof(used));
+ wmb();
+ used_idx++;
+ put_user(used_idx, &vr->ring.used->idx);
+}
+EXPORT_SYMBOL_GPL(vring_used_buffer);
+
+void vring_used_buffer_atomic(struct vring_info *vr, int id, u32 len)
+{
+ struct vring_used_elem *used;
+
+ BUG_ON(id <= 0 || id > vr->ring.num);
+ BUG_ON(!vr->used);
+
+ used = &vr->used->ring[vr->used->idx & vr->mask];
+ used->id = id - 1;
+ used->len = len;
+ /* Make sure buffer is written before we update index. */
+ wmb();
+ vr->used->idx++;
+}
+EXPORT_SYMBOL_GPL(vring_used_buffer_atomic);
+
+void vring_wake(struct vring_info *vr)
+{
+ wake_up(&vr->poll_wait);
+}
+EXPORT_SYMBOL_GPL(vring_wake);
+
+struct vring_info *vring_attach(int fd, const struct vring_ops *ops,
+ void *data, bool atomic_use)
+{
+ struct file *filp;
+ struct vring_info *vr;
+
+ /* Must be a valid fd, and must be one of ours. */
+ filp = fget(fd);
+ if (!filp) {
+ vr = ERR_PTR(-EBADF);
+ goto out;
+ }
+
+ if (filp->f_op != &vring_fops) {
+ vr = ERR_PTR(-EBADF);
+ goto fput;
+ }
+
+ /* Mutex simply protects against parallel vring_attach. */
+ mutex_lock(&vring_lock);
+ vr = filp->private_data;
+ if (vr->ops) {
+ vr = ERR_PTR(-EBUSY);
+ goto unlock;
+ }
+
+ /* If they want to use atomically, we have to map the page. */
+ if (atomic_use) {
+ if (get_user_pages(current, current->mm,
+ (unsigned long)vr->ring.used, 1, 1, 1,
+ &vr->used_page, NULL) != 1) {
+ vr = ERR_PTR(-EFAULT);
+ goto unlock;
+ }
+ vr->used = kmap(vr->used_page);
+ if (!vr->used) {
+ put_page(vr->used_page);
+ vr = ERR_PTR(-ENOMEM);
+ goto unlock;
+ }
+ }
+
+ vr->ops = ops;
+ vr->ops_data = data;
+
+unlock:
+ mutex_unlock(&vring_lock);
+fput:
+ fput(filp);
+out:
+ return vr;
+}
+EXPORT_SYMBOL_GPL(vring_attach);
+
+static int vringfs_get_sb(struct file_system_type *fs_type,
+ int flags, const char *dev_name, void *data,
+ struct vfsmount *mnt)
+{
+ return get_sb_pseudo(fs_type, "vring", NULL, VRINGFS_SUPER_MAGIC, mnt);
+}
+
+static struct file_system_type vring_fs_type = {
+ .name = "vringfs",
+ .get_sb = vringfs_get_sb,
+ .kill_sb = kill_anon_super,
+};
+
+static int init(void)
+{
+ register_filesystem(&vring_fs_type);
+ vring_mnt = kern_mount(&vring_fs_type);
+ return 0;
+}
+
+module_init(init);
diff -r 99132ad16999 include/asm-x86/unistd_32.h
--- a/include/asm-x86/unistd_32.h Sat Apr 05 21:20:32 2008 +1100
+++ b/include/asm-x86/unistd_32.h Sat Apr 05 21:31:40 2008 +1100
@@ -332,6 +332,7 @@
#define __NR_fallocate 324
#define __NR_timerfd_settime 325
#define __NR_timerfd_gettime 326
+#define __NR_vringfd 327

#ifdef __KERNEL__

diff -r 99132ad16999 include/linux/magic.h
--- a/include/linux/magic.h Sat Apr 05 21:20:32 2008 +1100
+++ b/include/linux/magic.h Sat Apr 05 21:31:40 2008 +1100
@@ -41,5 +41,6 @@

#define FUTEXFS_SUPER_MAGIC 0xBAD1DEA
#define INOTIFYFS_SUPER_MAGIC 0x2BAD1DEA
+#define VRINGFS_SUPER_MAGIC 0xB1BBAD

#endif /* __LINUX_MAGIC_H__ */
diff -r 99132ad16999 include/linux/syscalls.h
--- a/include/linux/syscalls.h Sat Apr 05 21:20:32 2008 +1100
+++ b/include/linux/syscalls.h Sat Apr 05 21:31:40 2008 +1100
@@ -614,6 +614,7 @@ asmlinkage long sys_timerfd_gettime(int
asmlinkage long sys_timerfd_gettime(int ufd, struct itimerspec __user *otmr);
asmlinkage long sys_eventfd(unsigned int count);
asmlinkage long sys_fallocate(int fd, int mode, loff_t offset, loff_t len);
+asmlinkage long sys_vringfd(void __user *, unsigned num, u16 __user *);

int kernel_execve(const char *filename, char *const argv[], char *const envp[]);

diff -r 99132ad16999 include/linux/vring.h
--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/include/linux/vring.h Sat Apr 05 21:31:40 2008 +1100
@@ -0,0 +1,54 @@
+/* Ring-buffer file descriptor implementation.
+ *
+ * Copyright 2008 Rusty Russell IBM Corporation
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+#ifndef _LINUX_VRING_H
+#define _LINUX_VRING_H
+
+/* All members are optional */
+struct vring_ops
+{
+ /* Cleanup */
+ void (*destroy)(void *);
+
+ /* Returns number of used buffers, or negative errno. */
+ int (*pull)(void *);
+
+ /* Returns 0 or negative errno. */
+ int (*push)(void *);
+};
+
+/* If they want to call vring_used_buffer_atomic(), set atomic_use.
+ * This currently means that the userspace used buffer must fit in a page. */
+struct vring_info *vring_attach(int fd, const struct vring_ops *ops,
+ void *data, bool atomic_use);
+
+struct iovec;
+
+/* Returns an error, or 0 (no buffers), or an id for vring_used_buffer() */
+int vring_get_buffer(struct vring_info *vr,
+ struct iovec *in_iov,
+ unsigned int *num_in, unsigned long *in_len,
+ struct iovec *out_iov,
+ unsigned int *num_out, unsigned long *out_len);
+
+void vring_used_buffer(struct vring_info *vr, int id, u32 len);
+
+void vring_used_buffer_atomic(struct vring_info *vr, int id, u32 len);
+
+void vring_wake(struct vring_info *vr);
+#endif /* _LINUX_VRING_H */
diff -r 99132ad16999 kernel/sys_ni.c
--- a/kernel/sys_ni.c Sat Apr 05 21:20:32 2008 +1100
+++ b/kernel/sys_ni.c Sat Apr 05 21:31:40 2008 +1100
@@ -161,3 +161,4 @@ cond_syscall(compat_sys_timerfd_settime)
cond_syscall(compat_sys_timerfd_settime);
cond_syscall(compat_sys_timerfd_gettime);
cond_syscall(sys_eventfd);
+cond_syscall(sys_vringfd);

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