On Fri, 18 Apr 2008 14:39:48 +1000 Rusty Russell <rusty@xxxxxxxxxxxxxxx> wrote:
> virtio introduced a ring structure ABI for guest-host communications
> (currently used by lguest and kvm). Using this same ABI, we can
> create a nice fd version.
>
> This is useful for efficiently passing packets to and from the tun,
> for example.
>
> ...
>
> +static int vring_mmap(struct file *filp, struct vm_area_struct *vma)
> +{
> + unsigned long size, num_descs;
> + struct vring_info *vr = filp->private_data;
> + int err;
> +
> + /* We overload mmap's offset to hold the ring number. */
> + num_descs = vma->vm_pgoff;
> +
> + /* Must be a power of two, and limit indices to a u16. */
> + if (!num_descs || (num_descs & (num_descs-1)) || num_descs > 65536)
We have an is_power_of_2().
> + return -EINVAL;
> +
> + /* mmap size must be what we expect for such a ring. */
> + size = vma->vm_end - vma->vm_start;
> + if (size != ALIGN(vring_size(num_descs, PAGE_SIZE), PAGE_SIZE))
> + return -EINVAL;
> +
> + /* We only let them map this in one place. */
> + mutex_lock(&vr->lock);
> + if (vr->ring.num != 0) {
> + err = -EBUSY;
> + goto unlock;
> + }
> +
> + vring_init(&vr->ring, num_descs, (void *)vma->vm_start, PAGE_SIZE);
> +
> + vr->mask = num_descs - 1;
> + err = 0;
> +
> +unlock:
> + mutex_unlock(&vr->lock);
> + return err;
> +}
>
> ...
>
> +/**
> + * vring_get - check out a vring file descriptor
> + * @filp: the file structure to attach to (eg. from fget()).
> + *
> + * Userspace opens /dev/vring and mmaps it, then hands that fd to the
> + * kernel subsystem it wants to communicate with. That subsystem uses
> + * this routine and vring_set_ops() to attach to it.
> + *
> + * This simply checks that it really is a vring fd (otherwise it
> + * returns NULL), the other routine checks that it's not already
> + * attached.
> + */
hm, I don't understand the big picture here yet.
Isn't this kinda-sorta like what a relayfs file does? The oprofile
buffers? etc? Nothing in common at all, no hope?
> +struct vring_info *vring_get(struct file *filp)
> +{
> + /* Must be one of ours. */
> + if (filp->f_op != &vring_fops)
> + return NULL;
> +
> + return filp->private_data;
> +}
> +EXPORT_SYMBOL_GPL(vring_get);
> +
> +/**
> + * vring_set_ops - attach operations to a vring file descriptor.
> + * @vr: the vring_info returned from vring_get.
> + * @ops: the operations to attach.
> + * @ops_data: the argument to the ops callbacks.
> + *
> + * This is called after vring_get(): the reason for the two-part
> + * process is that the ops can be called before vring_set_ops returns
> + * (we don't do locking), so you really need to set things up before
> + * this call.
> + *
> + * This simply checks that the ring is not already attached to something,
> + * then sets the ops.
> + */
> +int vring_set_ops(struct vring_info *vr,
> + const struct vring_ops *ops, void *ops_data)
> +{
> + int err;
> +
> + mutex_lock(&vr->lock);
> + if (vr->ops) {
> + err = -EBUSY;
> + goto unlock;
> + }
> +
> + /* We don't lock, so make sure we get this in the right order. */
> + vr->ops_data = ops_data;
> + wmb();
> + vr->ops = ops;
> +
> + err = 0;
> +unlock:
> + mutex_unlock(&vr->lock);
> + local_irq_enable();
what's this doing here?
> + return err;
> +}
> +EXPORT_SYMBOL_GPL(vring_set_ops);
> +
> +/**
> + * vring_unset_ops - remove operations to a vring file descriptor.
> + * @vr: the vring_info previously successfully vring_set_ops'd
> + */
> +void vring_unset_ops(struct vring_info *vr)
> +{
> + BUG_ON(!vr->ops);
> + mutex_lock(&vr->lock);
> + vr->ops = NULL;
> + mutex_unlock(&vr->lock);
> +}
> +EXPORT_SYMBOL_GPL(vring_unset_ops);
Isn't this just vring_set_ops(vr, NULL, NULL)?
> +static struct miscdevice vring_dev = {
> + .minor = MISC_DYNAMIC_MINOR,
> + .name = KBUILD_MODNAME,
> + .fops = &vring_fops,
> +};
> +
> +static int __init init(void)
> +{
> + return misc_register(&vring_dev);
> +}
> +
> +static void __exit fini(void)
> +{
> + misc_deregister(&vring_dev);
> +}
> +
> +module_init(init);
> +module_exit(fini);
> diff -r b2d9869d338f include/linux/vring.h
> --- /dev/null Thu Jan 01 00:00:00 1970 +0000
> +++ b/include/linux/vring.h Fri Apr 18 13:35:16 2008 +1000
> @@ -0,0 +1,58 @@
> +/* 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
> + */
ponders #include <copyright.h>
> +#ifndef _LINUX_VRING_H
> +#define _LINUX_VRING_H
> +
> +/**
> + * vring_ops - operations for a vring fd.
> + * @needs_pull: more data is pending, need to call pull.
> + * @pull: callback when read() is called to report used buffers.
> + * @push: callback when write() is called to notify of added buffers.
> + *
> + * Any of these callbacks can be NULL, if you don't need them.
> + */
> +struct vring_ops {
> + bool (*needs_pull)(void *ops_data);
> +
> + /* Returns 0 or negative errno. */
> + int (*pull)(void *ops_data);
> +
> + /* Returns 0 or negative errno. */
> + int (*push)(void *ops_data);
> +};
> +
> +struct file;
> +
> +struct vring_info *vring_get(struct file *filp);
> +int vring_set_ops(struct vring_info *,
> + const struct vring_ops *ops, void *ops_data);
the first arg to vring_set_ops() lost its name.
> +void vring_unset_ops(struct vring_info *vr);
> +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_wake(struct vring_info *vr);
> +#endif /* _LINUX_VRING_H */
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