Subject: Re: [PATCHv2 2/2] vhost_net: a kernel-level virtio server

From: Gregory Haskins <[email-masked]>
Date: Wed, 12 Aug 2009 09:01:35 -0400

Michael S. Tsirkin wrote:
> On Tue, Aug 11, 2009 at 08:06:02PM -0400, Gregory Haskins wrote:
>> Michael S. Tsirkin wrote:
>>> What it is: vhost net is a character device that can be used to reduce
>>> the number of system calls involved in virtio networking.
>>> Existing virtio net code is used in the guest without modification.
>>>
>>> There's similarity with vringfd, with some differences and reduced scope
>>> - uses eventfd for signalling
>>> - structures can be moved around in memory at any time (good for migration)
>>> - support memory table and not just an offset (needed for kvm)
>>>
>>> common virtio related code has been put in a separate file vhost.c and
>>> can be made into a separate module if/when more backends appear.  I used
>>> Rusty's lguest.c as the source for developing this part : this supplied
>>> me with witty comments I wouldn't be able to write myself.
>>>
>>> What it is not: vhost net is not a bus, and not a generic new system
>>> call. No assumptions are made on how guest performs hypercalls.
>>> Userspace hypervisors are supported as well as kvm.
>>>
>>> How it works: Basically, we connect virtio frontend (configured by
>>> userspace) to a backend. The backend could be a network device, or a
>>> tun-like device. In this version I only support raw socket as a backend,
>>> which can be bound to e.g. SR IOV, or to macvlan device.  Backend is
>>> also configured by userspace, including vlan/mac etc.
>>>
>>> Status:
>>> This works for me, and I haven't see any crashes.
>>> I have not run any benchmarks yet, compared to userspace, I expect to
>>> see improved latency (as I save up to 4 system calls per packet) but not
>>> bandwidth/CPU (as TSO and interrupt mitigation are not supported).
>>>
>>> Features that I plan to look at in the future:
>>> - TSO
>>> - interrupt mitigation
>>> - zero copy
>> Only a quick review for now.  Will look closer later.
>>
>> (see inline)
>>
>>> Signed-off-by: Michael S. Tsirkin <[email-masked]>
>>>
>>> v2
>>> ---
>>>  MAINTAINERS                |   10 +
>>>  arch/x86/kvm/Kconfig       |    1 +
>>>  drivers/Makefile           |    1 +
>>>  drivers/vhost/Kconfig      |   11 +
>>>  drivers/vhost/Makefile     |    2 +
>>>  drivers/vhost/net.c        |  411 +++++++++++++++++++++++++++
>>>  drivers/vhost/vhost.c      |  663 ++++++++++++++++++++++++++++++++++++++++++++
>>>  drivers/vhost/vhost.h      |  108 +++++++
>>>  include/linux/Kbuild       |    1 +
>>>  include/linux/miscdevice.h |    1 +
>>>  include/linux/vhost.h      |  100 +++++++
>>>  11 files changed, 1309 insertions(+), 0 deletions(-)
>>>  create mode 100644 drivers/vhost/Kconfig
>>>  create mode 100644 drivers/vhost/Makefile
>>>  create mode 100644 drivers/vhost/net.c
>>>  create mode 100644 drivers/vhost/vhost.c
>>>  create mode 100644 drivers/vhost/vhost.h
>>>  create mode 100644 include/linux/vhost.h
>>>
>>> diff --git a/MAINTAINERS b/MAINTAINERS
>>> index ebc2691..eb0c1da 100644
>>> --- a/MAINTAINERS
>>> +++ b/MAINTAINERS
>>> @@ -6312,6 +6312,16 @@ S:	Maintained
>>>  F:	Documentation/filesystems/vfat.txt
>>>  F:	fs/fat/
>>>  
>>> +VIRTIO HOST (VHOST)
>>> +P:	Michael S. Tsirkin
>>> +M:	[email-masked]
>>> +L:	[email-masked]
>>> +L:	[email-masked]
>>> +L:	[email-masked]
>>> +S:	Maintained
>>> +F:	drivers/vhost/
>>> +F:	include/linux/vhost.h
>>> +
>>>  VIA RHINE NETWORK DRIVER
>>>  P:	Roger Luethi
>>>  M:	[email-masked]
>>> diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
>>> index b84e571..94f44d9 100644
>>> --- a/arch/x86/kvm/Kconfig
>>> +++ b/arch/x86/kvm/Kconfig
>>> @@ -64,6 +64,7 @@ config KVM_AMD
>>>  
>>>  # OK, it's a little counter-intuitive to do this, but it puts it neatly under
>>>  # the virtualization menu.
>>> +source drivers/vhost/Kconfig
>>>  source drivers/lguest/Kconfig
>>>  source drivers/virtio/Kconfig
>>>  
>>> diff --git a/drivers/Makefile b/drivers/Makefile
>>> index bc4205d..1551ae1 100644
>>> --- a/drivers/Makefile
>>> +++ b/drivers/Makefile
>>> @@ -105,6 +105,7 @@ obj-$(CONFIG_HID)		+= hid/
>>>  obj-$(CONFIG_PPC_PS3)		+= ps3/
>>>  obj-$(CONFIG_OF)		+= of/
>>>  obj-$(CONFIG_SSB)		+= ssb/
>>> +obj-$(CONFIG_VHOST_NET)		+= vhost/
>>>  obj-$(CONFIG_VIRTIO)		+= virtio/
>>>  obj-$(CONFIG_VLYNQ)		+= vlynq/
>>>  obj-$(CONFIG_STAGING)		+= staging/
>>> diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
>>> new file mode 100644
>>> index 0000000..d955406
>>> --- /dev/null
>>> +++ b/drivers/vhost/Kconfig
>>> @@ -0,0 +1,11 @@
>>> +config VHOST_NET
>>> +	tristate "Host kernel accelerator for virtio net"
>>> +	depends on NET && EVENTFD
>>> +	---help---
>>> +	  This kernel module can be loaded in host kernel to accelerate
>>> +	  guest networking with virtio_net. Not to be confused with virtio_net
>>> +	  module itself which needs to be loaded in guest kernel.
>>> +
>>> +	  To compile this driver as a module, choose M here: the module will
>>> +	  be called vhost_net.
>>> +
>>> diff --git a/drivers/vhost/Makefile b/drivers/vhost/Makefile
>>> new file mode 100644
>>> index 0000000..72dd020
>>> --- /dev/null
>>> +++ b/drivers/vhost/Makefile
>>> @@ -0,0 +1,2 @@
>>> +obj-$(CONFIG_VHOST_NET) += vhost_net.o
>>> +vhost_net-y := vhost.o net.o
>>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
>>> new file mode 100644
>>> index 0000000..a04ffd0
>>> --- /dev/null
>>> +++ b/drivers/vhost/net.c
>>> @@ -0,0 +1,411 @@
>>> +/* Copyright (C) 2009 Red Hat, Inc.
>>> + * Author: Michael S. Tsirkin <[email-masked]>
>>> + *
>>> + * This work is licensed under the terms of the GNU GPL, version 2.
>>> + *
>>> + * virtio-net server in host kernel.
>>> + */
>>> +
>>> +#include <linux/eventfd.h>
>>> +#include <linux/vhost.h>
>>> +#include <linux/virtio_net.h>
>>> +#include <linux/mmu_context.h>
>>> +#include <linux/miscdevice.h>
>>> +#include <linux/module.h>
>>> +#include <linux/mutex.h>
>>> +#include <linux/workqueue.h>
>>> +#include <linux/rcupdate.h>
>>> +#include <linux/file.h>
>>> +
>>> +#include <linux/net.h>
>>> +#include <linux/if_packet.h>
>>> +#include <linux/if_arp.h>
>>> +
>>> +#include <net/sock.h>
>>> +
>>> +#include <asm/mmu_context.h>
>>> +
>>> +#include "vhost.h"
>>> +
>>> +enum {
>>> +	VHOST_NET_VQ_RX = 0,
>>> +	VHOST_NET_VQ_TX = 1,
>>> +	VHOST_NET_VQ_MAX = 2,
>>> +};
>>> +
>>> +struct vhost_net {
>>> +	struct vhost_dev dev;
>>> +	struct vhost_virtqueue vqs[VHOST_NET_VQ_MAX];
>>> +	/* We use a kind of RCU to access sock pointer.
>>> +	 * All readers access it from workqueue,
>>> +	 * which makes it possible to flush the workqueue
>>> +	 * instead of synchronize_rcu. Therefore readers
>>> +	 * do not need rcu_read_lock/rcu_read_unlock.
>>> +	 * Writers use device mutex. */
>> This seems odd.  If you have the flush to act as a sync-barrier, why do
>> you also need rcu_dereference(sock)?
> 
> It inserts memory barriers on architectures that require them
> (currently only the Alpha),

This makes sense

> and, more importantly, documents that the pointer is protected
> by RCU.

Is it?  I don't see where RCU actually enters the equation if you are
not bracketing the dereference with rcu_read_lock/unlock.  I'm sure you
have this correct, its just that it goes against my understanding of how
to use RCU properly so I am trying to understand what you did.  I'm
CC'ing Paul to see if he has any thoughts.

> 
>> At first blush, it seems
>> gratuitous.
> 
> I don't think so. sync-barrier has nothing to do with it as it comes
> after pointer assignment.

I am not sure if we are talking about the same thing, here.

> 
>>  Can you talk about this aspect of the design in more detail?
> 
> This is the same as usual rcu. See linux/rcupdate.h

I still am not seeing it.  Even rcupdate.h says:

/**
 * rcu_dereference - fetch an RCU-protected pointer in an
 * RCU read-side critical section.  This pointer may later
 * be safely dereferenced.

note the "in an RCU read-side critical section".  See inline for more
comments.

> 
>>> +	struct socket *sock;
>>> +	struct vhost_poll poll[VHOST_NET_VQ_MAX];
>>> +};
>>> +
>>> +static void handle_tx(struct vhost_net *net)
>>> +{
>>> +	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_TX];
>>> +	unsigned head, out, in;
>>> +	struct msghdr msg = {
>>> +		.msg_name = NULL,
>>> +		.msg_namelen = 0,
>>> +		.msg_control = NULL,
>>> +		.msg_controllen = 0,
>>> +		.msg_iov = (struct iovec *)vq->iov + 1,
>>> +		.msg_flags = MSG_DONTWAIT,
>>> +	};
>>> +	size_t len;
>>> +	int err;
>>> +	struct socket *sock = rcu_dereference(net->sock);

Normally you do this inside a read-side critical section, denoted by
something like rcu_read_lock/unlock.  I do not see this critical section
occurring (or am I missing it?).

If this truly is not a critical section, either the rcu_dereference is
gratuitous, or you have some funky trick up your sleeve ;)  If its a
trick, please explain it so us mere mortals can understand.

I think I understand what your comment above meant:  You don't need to
do synchronize_rcu() because you can flush the workqueue instead to
ensure that all readers have completed.  But if thats true, to me, the
rcu_dereference itself is gratuitous, and that pointer is *not* actually
RCU protected (nor does it need to be).

>>> +	if (!sock || !sock_writeable(sock->sk))
>>> +		return;
>>> +
>>> +	use_mm(net->dev.mm);
>>> +	mutex_lock(&vq->mutex);
>>> +	for (;;) {
>>> +		head = vhost_get_vq_desc(&net->dev, vq, vq->iov, &out, &in);
>>> +		if (head == vq->num)
>>> +			break;
>>> +		if (out <= 1 || in) {
>>> +			vq_err(vq, "Unexpected descriptor format for TX: "
>>> +			       "out %d, int %d\n", out, in);
>>> +			break;
>>> +		}
>>> +		/* Sanity check */
>>> +		if (vq->iov->iov_len != sizeof(struct virtio_net_hdr)) {
>>> +			vq_err(vq, "Unexpected header len for TX: "
>>> +			       "%ld expected %zd\n", vq->iov->iov_len,
>>> +			       sizeof(struct virtio_net_hdr));
>>> +			break;
>>> +		}
>>> +		/* Skip header. TODO: support TSO. */
>>> +		msg.msg_iovlen = out - 1;
>>> +		len = iov_length(vq->iov + 1, out - 1);
>>> +		/* TODO: Check specific error and bomb out unless ENOBUFS? */
>>> +		err = sock->ops->sendmsg(NULL, sock, &msg, len);
>>> +		if (err < 0) {
>>> +			vhost_discard_vq_desc(vq);
>>> +			break;
>>> +		}
>>> +		if (err != len)
>>> +			pr_err("Truncated TX packet: "
>>> +			       " len %d != %zd\n", err, len);
>>> +		vhost_add_used_and_trigger(vq, head,
>>> +				     len + sizeof(struct virtio_net_hdr));
>>> +	}
>>> +
>>> +	mutex_unlock(&vq->mutex);
>>> +	unuse_mm(net->dev.mm);
>>> +}
>>> +
>>> +static void handle_rx(struct vhost_net *net)
>>> +{
>>> +	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_RX];
>>> +	unsigned head, out, in;
>>> +	struct msghdr msg = {
>>> +		.msg_name = NULL,
>>> +		.msg_namelen = 0,
>>> +		.msg_control = NULL, /* FIXME: get and handle RX aux data. */
>>> +		.msg_controllen = 0,
>>> +		.msg_iov = vq->iov + 1,
>>> +		.msg_flags = MSG_DONTWAIT,
>>> +	};
>>> +
>>> +	struct virtio_net_hdr hdr = {
>>> +		.flags = 0,
>>> +		.gso_type = VIRTIO_NET_HDR_GSO_NONE
>>> +	};
>>> +
>>> +	size_t len;
>>> +	int err;
>>> +	struct socket *sock = rcu_dereference(net->sock);
>>> +	if (!sock || skb_queue_empty(&sock->sk->sk_receive_queue))
>>> +		return;
>>> +
>>> +	use_mm(net->dev.mm);
>>> +	mutex_lock(&vq->mutex);
>>> +
>>> +	for (;;) {
>>> +		head = vhost_get_vq_desc(&net->dev, vq, vq->iov, &out, &in);
>>> +		if (head == vq->num)
>>> +			break;
>>> +		if (in <= 1 || out) {
>>> +			vq_err(vq, "Unexpected descriptor format for RX: out %d, int %d\n",
>>> +			       out, in);
>>> +			break;
>>> +		}
>>> +		/* Sanity check */
>>> +		if (vq->iov->iov_len != sizeof(struct virtio_net_hdr)) {
>>> +			vq_err(vq, "Unexpected header len for RX: %ld expected %zd\n",
>>> +			       vq->iov->iov_len, sizeof(struct virtio_net_hdr));
>>> +			break;
>>> +		}
>>> +		/* Skip header. TODO: support TSO/mergeable rx buffers. */
>>> +		msg.msg_iovlen = in - 1;
>>> +		len = iov_length(vq->iov + 1, in - 1);
>>> +		err = sock->ops->recvmsg(NULL, sock, &msg,
>>> +					 len, MSG_DONTWAIT | MSG_TRUNC);
>>> +		/* TODO: Check specific error and bomb out unless EAGAIN? */
>>> +		if (err < 0) {
>>> +			vhost_discard_vq_desc(vq);
>>> +			break;
>>> +		}
>>> +		/* TODO: Should check and handle checksum. */
>>> +		if (err > len) {
>>> +			pr_err("Discarded truncated rx packet: "
>>> +			       " len %d > %zd\n", err, len);
>>> +			vhost_discard_vq_desc(vq);
>>> +			continue;
>>> +		}
>>> +		len = err;
>>> +		err = copy_to_user(vq->iov->iov_base, &hdr, sizeof hdr);
>>> +		if (err) {
>>> +			vq_err(vq, "Unable to write vnet_hdr at addr %p: %d\n",
>>> +			       vq->iov->iov_base, err);
>>> +			break;
>>> +		}
>>> +		vhost_add_used_and_trigger(vq, head, len + sizeof hdr);
>>> +	}
>>> +
>>> +	mutex_unlock(&vq->mutex);
>>> +	unuse_mm(net->dev.mm);
>>> +}
>>> +
>>> +static void handle_tx_kick(struct work_struct *work)
>>> +{
>>> +	struct vhost_virtqueue *vq;
>>> +	struct vhost_net *net;
>>> +	vq = container_of(work, struct vhost_virtqueue, poll.work);
>>> +	net = container_of(vq->dev, struct vhost_net, dev);
>>> +	handle_tx(net);
>>> +}
>>> +
>>> +static void handle_rx_kick(struct work_struct *work)
>>> +{
>>> +	struct vhost_virtqueue *vq;
>>> +	struct vhost_net *net;
>>> +	vq = container_of(work, struct vhost_virtqueue, poll.work);
>>> +	net = container_of(vq->dev, struct vhost_net, dev);
>>> +	handle_rx(net);
>>> +}
>>> +
>>> +static void handle_tx_net(struct work_struct *work)
>>> +{
>>> +	struct vhost_net *net;
>>> +	net = container_of(work, struct vhost_net, poll[VHOST_NET_VQ_TX].work);
>>> +	handle_tx(net);
>>> +}
>>> +
>>> +static void handle_rx_net(struct work_struct *work)
>>> +{
>>> +	struct vhost_net *net;
>>> +	net = container_of(work, struct vhost_net, poll[VHOST_NET_VQ_RX].work);
>>> +	handle_rx(net);
>>> +}
>>> +
>>> +static int vhost_net_open(struct inode *inode, struct file *f)
>>> +{
>>> +	struct vhost_net *n = kzalloc(sizeof *n, GFP_KERNEL);
>>> +	int r;
>>> +	if (!n)
>>> +		return -ENOMEM;
>>> +	f->private_data = n;
>>> +	n->vqs[VHOST_NET_VQ_TX].handle_kick = handle_tx_kick;
>>> +	n->vqs[VHOST_NET_VQ_RX].handle_kick = handle_rx_kick;
>>> +	r = vhost_dev_init(&n->dev, n->vqs, VHOST_NET_VQ_MAX);
>>> +	if (r < 0) {
>>> +		kfree(n);
>>> +		return r;
>>> +	}
>>> +
>>> +	vhost_poll_init(n->poll + VHOST_NET_VQ_TX, handle_tx_net, POLLOUT);
>>> +	vhost_poll_init(n->poll + VHOST_NET_VQ_RX, handle_rx_net, POLLIN);
>>> +	return 0;
>>> +}
>>> +
>>> +static struct socket *vhost_net_stop(struct vhost_net *n)
>>> +{
>>> +	struct socket *sock = n->sock;
>>> +	rcu_assign_pointer(n->sock, NULL);
>>> +	if (sock) {
>>> +		vhost_poll_flush(n->poll + VHOST_NET_VQ_TX);
>>> +		vhost_poll_flush(n->poll + VHOST_NET_VQ_RX);
>>> +	}
>>> +	return sock;
>>> +}
>>> +
>>> +static int vhost_net_release(struct inode *inode, struct file *f)
>>> +{
>>> +	struct vhost_net *n = f->private_data;
>>> +	struct socket *sock;
>>> +
>>> +	sock = vhost_net_stop(n);
>>> +	vhost_dev_cleanup(&n->dev);
>>> +	if (sock)
>>> +		fput(sock->file);
>>> +	kfree(n);
>>> +	return 0;
>>> +}
>>> +
>>> +static long vhost_net_set_socket(struct vhost_net *n, int fd)
>>> +{
>>> +	struct {
>>> +		struct sockaddr_ll sa;
>>> +		char  buf[MAX_ADDR_LEN];
>>> +	} uaddr;
>>> +	struct socket *sock, *oldsock = NULL;
>>> +	int uaddr_len = sizeof uaddr, r;
>>> +
>>> +	mutex_lock(&n->dev.mutex);
>>> +	r = vhost_dev_check_owner(&n->dev);
>>> +	if (r)
>>> +		goto done;
>>> +
>>> +	if (fd == -1) {
>>> +		/* Disconnect from socket and device. */
>>> +		oldsock = vhost_net_stop(n);
>>> +		goto done;
>>> +	}
>>> +	
>>> +	sock = sockfd_lookup(fd, &r);
>>> +	if (!sock) {
>>> +		r = -ENOTSOCK;
>>> +		goto done;
>>> +	}
>>> +
>>> +	/* Parameter checking */
>>> +	if (sock->sk->sk_type != SOCK_RAW) {
>>> +		r = -ESOCKTNOSUPPORT;
>>> +		goto done;
>>> +	}
>>> +
>>> +	r = sock->ops->getname(sock, (struct sockaddr *)&uaddr.sa,
>>> +			       &uaddr_len, 0);
>>> +	if (r)
>>> +		goto done;
>>> +
>>> +	if (uaddr.sa.sll_family != AF_PACKET) {
>>> +		r = -EPFNOSUPPORT;
>>> +		goto done;
>>> +	}
>>> +
>>> +	/* start polling new socket */
>>> +	if (sock == oldsock)
>>> +		goto done;
>>> +
>>> +	if (oldsock) {
>>> +		vhost_poll_stop(n->poll + VHOST_NET_VQ_TX);
>>> +		vhost_poll_stop(n->poll + VHOST_NET_VQ_RX);
>>> +	}
>>> +	oldsock = n->sock;
>>> +	rcu_assign_pointer(n->sock, sock);
>>> +	vhost_poll_start(n->poll + VHOST_NET_VQ_TX, sock->file);
>>> +	vhost_poll_start(n->poll + VHOST_NET_VQ_RX, sock->file);
>>> +done:
>>> +	mutex_unlock(&n->dev.mutex);
>>> +	if (oldsock) {
>>> +		vhost_poll_flush(n->poll + VHOST_NET_VQ_TX);
>>> +		vhost_poll_flush(n->poll + VHOST_NET_VQ_RX);
>>> +		vhost_poll_flush(&n->dev.vqs[VHOST_NET_VQ_TX].poll);
>>> +		vhost_poll_flush(&n->dev.vqs[VHOST_NET_VQ_RX].poll);
>>> +		fput(oldsock->file);
>>> +	}
>>> +	return r;
>>> +}
>>> +
>>> +static long vhost_net_reset_owner(struct vhost_net *n)
>>> +{
>>> +	struct socket *sock = NULL;
>>> +	long r;
>>> +	mutex_lock(&n->dev.mutex);
>>> +	r = vhost_dev_check_owner(&n->dev);
>>> +	if (r)
>>> +		goto done;
>>> +	sock = vhost_net_stop(n);
>>> +	r = vhost_dev_reset_owner(&n->dev);
>>> +done:
>>> +	mutex_unlock(&n->dev.mutex);
>>> +	if (sock)
>>> +		fput(sock->file);
>>> +	return r;
>>> +}
>>> +
>>> +static long vhost_net_ioctl(struct file *f, unsigned int ioctl,
>>> +			    unsigned long arg)
>>> +{
>>> +	struct vhost_net *n = f->private_data;
>>> +	void __user *argp = (void __user *)arg;
>>> +	u32 __user *featurep = argp;
>>> +	int __user *fdp = argp;
>>> +	u32 features;
>>> +	int fd, r;
>>> +	switch (ioctl) {
>>> +	case VHOST_NET_SET_SOCKET:
>>> +		r = get_user(fd, fdp);
>>> +		if (r < 0)
>>> +			return r;
>>> +		return vhost_net_set_socket(n, fd);
>>> +	case VHOST_GET_FEATURES:
>>> +		/* No features for now */
>>> +		features = 0;
>>> +		return put_user(features, featurep);
>>> +	case VHOST_ACK_FEATURES:
>>> +		r = get_user(features, featurep);
>>> +		/* No features for now */
>>> +		if (r < 0)
>>> +			return r;
>>> +		if (features)
>>> +			return -EOPNOTSUPP;
>>> +		return 0;
>>> +	case VHOST_RESET_OWNER:
>>> +		return vhost_net_reset_owner(n);
>>> +	default:
>>> +		return vhost_dev_ioctl(&n->dev, ioctl, arg);
>>> +	}
>>> +}
>>> +
>>> +static struct file_operations vhost_net_fops = {
>>> +	.owner          = THIS_MODULE,
>>> +	.release        = vhost_net_release,
>>> +	.unlocked_ioctl = vhost_net_ioctl,
>>> +	.open           = vhost_net_open,
>>> +};
>>> +
>>> +static struct miscdevice vhost_net_misc = {
>>> +	VHOST_NET_MINOR,
>>> +	"vhost-net",
>>> +	&vhost_net_fops,
>>> +};
>>> +
>>> +int vhost_net_init(void)
>>> +{
>>> +	int r = vhost_init();
>>> +	if (r)
>>> +		goto err_init;
>>> +	r = misc_register(&vhost_net_misc);
>>> +	if (r)
>>> +		goto err_reg;
>>> +	return 0;
>>> +err_reg:
>>> +	vhost_cleanup();
>>> +err_init:
>>> +	return r;
>>> +
>>> +}
>>> +module_init(vhost_net_init);
>>> +
>>> +void vhost_net_exit(void)
>>> +{
>>> +	misc_deregister(&vhost_net_misc);
>>> +	vhost_cleanup();
>>> +}
>>> +module_exit(vhost_net_exit);
>>> +
>>> +MODULE_VERSION("0.0.1");
>>> +MODULE_LICENSE("GPL v2");
>>> +MODULE_AUTHOR("Michael S. Tsirkin");
>>> +MODULE_DESCRIPTION("Host kernel accelerator for virtio net");
>>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>>> new file mode 100644
>>> index 0000000..6178ec1
>>> --- /dev/null
>>> +++ b/drivers/vhost/vhost.c
>>> @@ -0,0 +1,663 @@
>>> +/* Copyright (C) 2009 Red Hat, Inc.
>>> + * Copyright (C) 2006 Rusty Russell IBM Corporation
>>> + *
>>> + * Author: Michael S. Tsirkin <[email-masked]>
>>> + *
>>> + * Inspiration, some code, and most witty comments come from
>>> + * Documentation/lguest/lguest.c, by Rusty Russell
>>> + *
>>> + * This work is licensed under the terms of the GNU GPL, version 2.
>>> + *
>>> + * Generic code for virtio server in host kernel.
>>> + */
>>> +
>>> +#include <linux/eventfd.h>
>>> +#include <linux/vhost.h>
>>> +#include <linux/virtio_net.h>
>>> +#include <linux/mm.h>
>>> +#include <linux/miscdevice.h>
>>> +#include <linux/mutex.h>
>>> +#include <linux/workqueue.h>
>>> +#include <linux/rcupdate.h>
>>> +#include <linux/poll.h>
>>> +#include <linux/file.h>
>>> +
>>> +#include <linux/net.h>
>>> +#include <linux/if_packet.h>
>>> +#include <linux/if_arp.h>
>>> +
>>> +#include <net/sock.h>
>>> +
>>> +#include <asm/mmu_context.h>
>>> +
>>> +#include "vhost.h"
>>> +
>>> +enum {
>>> +	VHOST_MEMORY_MAX_NREGIONS = 64,
>>> +};
>>> +
>>> +struct workqueue_struct *vhost_workqueue;
>>> +
>>> +static void vhost_poll_func(struct file *file, wait_queue_head_t *wqh,
>>> +			    poll_table *pt)
>>> +{
>>> +	struct vhost_poll *poll;
>>> +	poll = container_of(pt, struct vhost_poll, table);
>>> +
>>> +	poll->wqh = wqh;
>>> +	add_wait_queue(wqh, &poll->wait);
>>> +}
>>> +
>>> +static int vhost_poll_wakeup(wait_queue_t *wait, unsigned mode, int sync, void *key)
>>> +{
>>> +	struct vhost_poll *poll;
>>> +	poll = container_of(wait, struct vhost_poll, wait);
>>> +	if (!((unsigned long)key & poll->mask))
>>> +		return 0;
>>> +
>>> +	queue_work(vhost_workqueue, &poll->work);
>>> +	return 0;
>>> +}
>>> +
>>> +/* Init poll structure */
>>> +void vhost_poll_init(struct vhost_poll *poll, work_func_t func,
>>> +		     unsigned long mask)
>>> +{
>>> +	INIT_WORK(&poll->work, func);
>>> +	init_waitqueue_func_entry(&poll->wait, vhost_poll_wakeup);
>>> +	init_poll_funcptr(&poll->table, vhost_poll_func);
>>> +	poll->mask = mask;
>>> +}
>>> +
>>> +/* Start polling a file. We add ourselves to file's wait queue. The user must
>>> + * keep a reference to a file until after vhost_poll_stop is called. */
>>> +void vhost_poll_start(struct vhost_poll *poll, struct file *file)
>>> +{
>>> +	unsigned long mask;
>>> +	mask = file->f_op->poll(file, &poll->table);
>>> +	if (mask)
>>> +		vhost_poll_wakeup(&poll->wait, 0, 0, (void *)mask);
>>> +}
>>> +
>>> +/* Stop polling a file. After this function returns, it becomes safe to drop the
>>> + * file reference. You must also flush afterwards. */
>>> +void vhost_poll_stop(struct vhost_poll *poll)
>>> +{
>>> +	remove_wait_queue(poll->wqh, &poll->wait);
>>> +}
>>> +
>>> +/* Flush any work that has been scheduled. When calling this, don't hold any
>>> + * locks that are also used by the callback. */
>>> +void vhost_poll_flush(struct vhost_poll *poll)
>>> +{
>>> +	flush_work(&poll->work);
>>> +}
>>> +
>>> +long vhost_dev_init(struct vhost_dev *dev, struct vhost_virtqueue *vqs, int nvqs)
>>> +{
>>> +	int i;
>>> +	dev->vqs = vqs;
>>> +	dev->nvqs = nvqs;
>>> +	mutex_init(&dev->mutex);
>>> +
>>> +	for(i = 0; i < dev->nvqs; ++i) {
>>> +		dev->vqs[i].dev = dev;
>>> +		mutex_init(&dev->vqs[i].mutex);
>>> +		if (dev->vqs[i].handle_kick)
>>> +			vhost_poll_init(&dev->vqs[i].poll,
>>> +					dev->vqs[i].handle_kick,
>>> +					POLLIN);
>>> +	}
>>> +	return 0;
>>> +}
>>> +
>>> +/* User should have device mutex */
>>> +long vhost_dev_check_owner(struct vhost_dev *dev)
>>> +{
>>> +	return dev->mm == current->mm ? 0 : -EPERM;
>>> +}
>>> +
>>> +/* User should have device mutex */
>>> +static long vhost_dev_set_owner(struct vhost_dev *dev)
>>> +{
>>> +	if (dev->mm)
>>> +		return -EBUSY;
>>> +	dev->mm = get_task_mm(current);
>>> +	return 0;
>>> +}
>>> +
>>> +/* User should have device mutex */
>>> +long vhost_dev_reset_owner(struct vhost_dev *dev)
>>> +{
>>> +	struct vhost_memory *memory;
>>> +
>>> +	/* Restore memory to default 1:1 mapping. */
>>> +	memory = kmalloc(offsetof(struct vhost_memory, regions) +
>>> +			 2 * sizeof *memory->regions, GFP_KERNEL);
>>> +	if (!memory)
>>> +		return -ENOMEM;
>>> +
>>> +	vhost_dev_cleanup(dev);
>>> +
>>> +	memory->nregions = 2;
>>> +	memory->regions[0].guest_phys_addr = 1;
>>> +	memory->regions[0].userspace_addr = 1;
>>> +	memory->regions[0].memory_size = ~0ULL;
>>> +	memory->regions[1].guest_phys_addr = 0;
>>> +	memory->regions[1].userspace_addr = 0;
>>> +	memory->regions[1].memory_size = 1;
>>> +	dev->memory = memory;
>>> +	return 0;
>>> +}
>>> +
>>> +/* User should have device mutex */
>>> +void vhost_dev_cleanup(struct vhost_dev *dev)
>>> +{
>>> +	int i;
>>> +	for(i = 0; i < dev->nvqs; ++i) {
>>> +		if (dev->vqs[i].kick && dev->vqs[i].handle_kick) {
>>> +			vhost_poll_stop(&dev->vqs[i].poll);
>>> +			vhost_poll_flush(&dev->vqs[i].poll);
>>> +		}
>>> +		if (dev->vqs[i].error_ctx)
>>> +			eventfd_ctx_put(dev->vqs[i].error_ctx);
>>> +		if (dev->vqs[i].error)
>>> +			fput(dev->vqs[i].error);
>>> +		if (dev->vqs[i].kick)
>>> +			fput(dev->vqs[i].kick);
>>> +		if (dev->vqs[i].call_ctx)
>>> +			eventfd_ctx_put(dev->vqs[i].call_ctx);
>>> +		if (dev->vqs[i].call)
>>> +			fput(dev->vqs[i].call);
>>> +		dev->vqs[i].error_ctx = NULL;
>>> +		dev->vqs[i].error = NULL;
>>> +		dev->vqs[i].kick = NULL;
>>> +		dev->vqs[i].call_ctx = NULL;
>>> +		dev->vqs[i].call = NULL;
>>> +	}
>>> +	/* No one will access memory at this point */
>>> +	kfree(dev->memory);
>>> +	dev->memory = NULL;
>>> +	if (dev->mm)
>>> +		mmput(dev->mm);
>>> +	dev->mm = NULL;
>>> +}
>>> +
>>> +static long vhost_set_memory(struct vhost_dev *d, struct vhost_memory __user *m)
>>> +{
>>> +	struct vhost_memory mem, *newmem, *oldmem;
>>> +	unsigned long size = offsetof(struct vhost_memory, regions);
>>> +	long r;
>>> +	r = copy_from_user(&mem, m, size);
>>> +	if (r)
>>> +		return r;
>>> +	if (mem.padding)
>>> +		return -EOPNOTSUPP;
>>> +	if (mem.nregions > VHOST_MEMORY_MAX_NREGIONS)
>>> +		return -E2BIG;
>>> +	newmem = kmalloc(size + mem.nregions * sizeof *m->regions, GFP_KERNEL);
>>> +	if (!newmem)
>>> +		return -ENOMEM;
>>> +
>>> +	memcpy(newmem, &mem, size);
>>> +	r = copy_from_user(newmem->regions, m->regions,
>>> +			   mem.nregions * sizeof *m->regions);
>>> +	if (r) {
>>> +		kfree(newmem);
>>> +		return r;
>>> +	}
>>> +	oldmem = d->memory;
>>> +	rcu_assign_pointer(d->memory, newmem);
>>> +	synchronize_rcu();
>>> +	kfree(oldmem);
>>> +	return 0;
>>> +}
>>> +
>>> +static int init_used(struct vhost_virtqueue *vq)
>>> +{
>>> +	u16 flags = 0;
>>> +	int r = put_user(flags, &vq->used->flags);
>>> +	if (r)
>>> +		return r;
>>> +	return get_user(vq->last_used_idx, &vq->used->idx);
>>> +}
>>> +
>>> +static long vhost_set_vring(struct vhost_dev *d, int ioctl, void __user *argp)
>>> +{
>>> +	struct file *eventfp, *filep = NULL, *pollstart = NULL, *pollstop = NULL;
>>> +	struct eventfd_ctx *ctx = NULL;
>>> +	u32 __user *idxp = argp;
>>> +	struct vhost_virtqueue *vq;
>>> +	struct vhost_vring_state s;
>>> +	struct vhost_vring_file f;
>>> +	struct vhost_vring_addr a;
>>> +	u32 idx;
>>> +	long r;
>>> +
>>> +	r = get_user(idx, idxp);
>>> +	if (r < 0)
>>> +		return r;
>>> +	if (idx > d->nvqs)
>>> +		return -ENOBUFS;
>>> +
>>> +	vq = d->vqs + idx;
>>> +
>>> +	mutex_lock(&vq->mutex);
>>> +
>>> +	switch (ioctl) {
>>> +	case VHOST_SET_VRING_NUM:
>>> +		r = copy_from_user(&s, argp, sizeof s);
>>> +		if (r < 0)
>>> +			break;
>>> +		if (s.num > 0xffff) {
>>> +			r = -EINVAL;
>>> +			break;
>>> +		}
>>> +		vq->num = s.num;
>>> +		break;
>>> +	case VHOST_SET_VRING_BASE:
>>> +		r = copy_from_user(&s, argp, sizeof s);
>>> +		if (r < 0)
>>> +			break;
>>> +		if (s.num > 0xffff) {
>>> +			r = -EINVAL;
>>> +			break;
>>> +		}
>>> +		vq->last_avail_idx = s.num;
>>> +		break;
>>> +	case VHOST_GET_VRING_BASE:
>>> +		s.index = idx;
>>> +		s.num = vq->last_avail_idx;
>>> +		r = copy_to_user(argp, &s, sizeof s);
>>> +		break;
>>> +	case VHOST_SET_VRING_DESC:
>>> +		r = copy_from_user(&a, argp, sizeof a);
>>> +		if (r < 0)
>>> +			break;
>>> +		if (a.padding) {
>>> +			r = -EOPNOTSUPP;
>>> +			break;
>>> +		}
>>> +		if ((u64)(long)a.user_addr != a.user_addr) {
>>> +			r = -EFAULT;
>>> +			break;
>>> +		}
>>> +		vq->desc = (void __user *)(long)a.user_addr;
>>> +		break;
>>> +	case VHOST_SET_VRING_AVAIL:
>>> +		r = copy_from_user(&a, argp, sizeof a);
>>> +		if (r < 0)
>>> +			break;
>>> +		if (a.padding) {
>>> +			r = -EOPNOTSUPP;
>>> +			break;
>>> +		}
>>> +		if ((u64)(long)a.user_addr != a.user_addr) {
>>> +			r = -EFAULT;
>>> +			break;
>>> +		}
>>> +		vq->avail = (void __user *)(long)a.user_addr;
>>> +		break;
>>> +	case VHOST_SET_VRING_USED:
>>> +		r = copy_from_user(&a, argp, sizeof a);
>>> +		if (r < 0)
>>> +			break;
>>> +		if (a.padding) {
>>> +			r = -EOPNOTSUPP;
>>> +			break;
>>> +		}
>>> +		if ((u64)(long)a.user_addr != a.user_addr) {
>>> +			r = -EFAULT;
>>> +			break;
>>> +		}
>>> +		vq->used = (void __user *)(long)a.user_addr;
>>> +		r = init_used(vq);
>>> +		if (r)
>>> +			break;
>>> +		break;
>>> +	case VHOST_SET_VRING_KICK:
>>> +		r = copy_from_user(&f, argp, sizeof f);
>>> +		if (r < 0)
>>> +			break;
>>> +		eventfp = f.fd == -1 ? NULL: eventfd_fget(f.fd);
>>> +		if (IS_ERR(eventfp))
>>> +			return PTR_ERR(eventfp);
>>> +		if (eventfp != vq->kick) {
>>> +			pollstop = filep = vq->kick;
>>> +			pollstart = vq->kick = eventfp;
>>> +		} else
>>> +			filep = eventfp;
>>> +		break;
>>> +	case VHOST_SET_VRING_CALL:
>>> +		r = copy_from_user(&f, argp, sizeof f);
>>> +		if (r < 0)
>>> +			break;
>>> +		eventfp = f.fd == -1 ? NULL: eventfd_fget(f.fd);
>>> +		if (IS_ERR(eventfp))
>>> +			return PTR_ERR(eventfp);
>>> +		if (eventfp != vq->call) {
>>> +			filep = vq->call;
>>> +			ctx = vq->call_ctx;
>>> +			vq->call = eventfp;
>>> +			vq->call_ctx = eventfp ?
>>> +				eventfd_ctx_fileget(eventfp) : NULL;
>>> +		} else
>>> +			filep = eventfp;
>>> +		break;
>>> +	case VHOST_SET_VRING_ERR:
>>> +		r = copy_from_user(&f, argp, sizeof f);
>>> +		if (r < 0)
>>> +			break;
>>> +		eventfp = f.fd == -1 ? NULL: eventfd_fget(f.fd);
>>> +		if (IS_ERR(eventfp))
>>> +			return PTR_ERR(eventfp);
>>> +		if (eventfp != vq->error) {
>>> +			filep = vq->error;
>>> +			vq->error = eventfp;
>>> +			ctx = vq->error_ctx;
>>> +			vq->error_ctx = eventfp ?
>>> +				eventfd_ctx_fileget(eventfp) : NULL;
>>> +		} else
>>> +			filep = eventfp;
>>> +		break;
>>> +	default:
>>> +		r = -ENOTTY;
>>> +	}
>>> +
>>> +	if (pollstop && vq->handle_kick)
>>> +		vhost_poll_stop(&vq->poll);
>>> +
>>> +	if (ctx)
>>> +		eventfd_ctx_put(ctx);
>>> +	if (filep)
>>> +		fput(filep);
>>> +
>>> +	if (pollstart && vq->handle_kick)
>>> +		vhost_poll_start(&vq->poll, vq->kick);
>>> +
>>> +	mutex_unlock(&vq->mutex);
>>> +
>>> +	if (pollstop && vq->handle_kick)
>>> +		vhost_poll_flush(&vq->poll);
>>> +	return 0;
>>> +}
>>> +
>>> +long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, unsigned long arg)
>>> +{
>>> +	void __user *argp = (void __user *)arg;
>>> +	long r;
>>> +
>>> +	mutex_lock(&d->mutex);
>>> +	if (ioctl == VHOST_SET_OWNER) {
>>> +		r = vhost_dev_set_owner(d);
>>> +		goto done;
>>> +	}
>>> +
>>> +	r = vhost_dev_check_owner(d);
>>> +	if (r)
>>> +		goto done;
>>> +		
>>> +	switch (ioctl) {
>>> +	case VHOST_SET_MEM_TABLE:
>>> +		r = vhost_set_memory(d, argp);
>>> +		break;
>>> +	default:
>>> +		r = vhost_set_vring(d, ioctl, argp);
>>> +		break;
>>> +	}
>>> +done:
>>> +	mutex_unlock(&d->mutex);
>>> +	return r;
>>> +}
>>> +
>>> +static const struct vhost_memory_region *find_region(struct vhost_memory *mem,
>>> +						     __u64 addr, __u32 len)
>>> +{
>>> +	struct vhost_memory_region *reg;
>>> +	int i;
>>> +	/* linear search is not brilliant, but we really have on the order of 6
>>> +	 * regions in practice */
>>> +	for (i = 0; i < mem->nregions; ++i) {
>>> +		reg = mem->regions + i;
>>> +		if (reg->guest_phys_addr <= addr &&
>>> +		    reg->guest_phys_addr + reg->memory_size - 1 >= addr)
>>> +			return reg;
>>> +	}
>>> +	return NULL;
>>> +}
>>> +
>>> +/* FIXME: this does not handle a region that spans multiple
>>> + * address/len pairs */
>>> +int translate_desc(struct vhost_dev *dev, u64 addr, u32 len,
>>> +		   struct iovec iov[], int iov_count, int iov_size,
>>> +		   unsigned *num)
>>> +{
>>> +	const struct vhost_memory_region *reg;
>>> +	struct vhost_memory *mem;
>>> +	struct iovec *_iov;
>>> +	u64 s = 0;
>>> +	int ret = 0;
>>> +
>>> +	rcu_read_lock();
>>> +
>>> +	mem = rcu_dereference(dev->memory);
>>> +	while ((u64)len > s) {
>>> +		u64 size;
>>> +		if (*num + iov_count >= iov_size) {
>>> +			ret = -ENOBUFS;
>>> +			break;
>>> +		}
>>> +		reg = find_region(mem, addr, len);
>>> +		if (!reg) {
>>> +			ret = -EFAULT;
>>> +			break;
>>> +		}
>>> +		_iov = iov + iov_count + *num;
>>> +		size = reg->memory_size - addr + reg->guest_phys_addr;
>>> +		_iov->iov_len = min((u64)len, size);
>>> +		_iov->iov_base = (void *)
>>> +			(reg->userspace_addr + addr - reg->guest_phys_addr);
>>> +		s += size;
>>> +		addr += size;
>>> +		++*num;
>>> +	}
>>> +	
>>> +	rcu_read_unlock();
>>> +	return ret;
>>> +}
>>> +
>>> +/* Each buffer in the virtqueues is actually a chain of descriptors.  This
>>> + * function returns the next descriptor in the chain, or vq->vring.num if we're
>>> + * at the end. */
>>> +static unsigned next_desc(struct vhost_virtqueue *vq, struct vring_desc *desc)
>>> +{
>>> +	unsigned int next;
>>> +
>>> +	/* If this descriptor says it doesn't chain, we're done. */
>>> +	if (!(desc->flags & VRING_DESC_F_NEXT))
>>> +		return vq->num;
>>> +
>>> +	/* Check they're not leading us off end of descriptors. */
>>> +	next = desc->next;
>>> +	/* Make sure compiler knows to grab that: we don't want it changing! */
>>> +	/* We will use the result as an index in an array, so most
>>> +	 * architectures only need a compiler barrier here. */
>>> +	read_barrier_depends();
>>> +
>>> +	if (next >= vq->num) {
>>> +		vq_err(vq, "Desc next is %u > %u", next, vq->num);
>>> +		return vq->num;
>>> +	}
>>> +
>>> +	return next;
>>> +}
>>> +
>>> +/* This looks in the virtqueue and for the first available buffer, and converts
>>> + * it to an iovec for convenient access.  Since descriptors consist of some
>>> + * number of output then some number of input descriptors, it's actually two
>>> + * iovecs, but we pack them into one and note how many of each there were.
>>> + *
>>> + * This function returns the descriptor number found, or vq->num (which
>>> + * is never a valid descriptor number) if none was found. */
>>> +unsigned vhost_get_vq_desc(struct vhost_dev *dev, struct vhost_virtqueue *vq,
>>> +			   struct iovec iov[],
>>> +			   unsigned int *out_num, unsigned int *in_num)
>>> +{
>>> +	struct vring_desc desc;
>>> +	unsigned int i, head;
>>> +	u16 last_avail_idx, idx;
>>> +
>>> +	/* Check it isn't doing very strange things with descriptor numbers. */
>>> +	last_avail_idx = vq->last_avail_idx;
>>> +	if (get_user(idx, &vq->avail->idx)) {
>>> +		vq_err(vq, "Failed to access avail idx at %p\n",
>>> +		       &vq->avail->idx);
>>> +		return vq->num;
>>> +	}
>>> +	
>>> +	if ((u16)(idx - last_avail_idx) > vq->num) {
>>> +		vq_err(vq, "Guest moved used index from %u to %u",
>>> +		       last_avail_idx, idx);
>>> +		return vq->num;
>>> +	}
>>> +
>>> +	/* If there's nothing new since last we looked, return invalid. */
>>> +	if (idx == last_avail_idx)
>>> +		return vq->num;
>>> +
>>> +	/* Grab the next descriptor number they're advertising, and increment
>>> +	 * the index we've seen. */
>>> +	if (get_user(head, &vq->avail->ring[last_avail_idx % vq->num])) {
>>> +		vq_err(vq, "Failed to read head: idx %d address %p\n",
>>> +		       idx, &vq->avail->ring[last_avail_idx % vq->num]);
>>> +		return vq->num;
>>> +	}
>>> +
>>> +	/* If their number is silly, that's a fatal mistake. */
>>> +	if (head >= vq->num) {
>>> +		vq_err(vq, "Guest says index %u > %u is available",
>>> +		       head, vq->num);
>>> +		return vq->num;
>>> +	}
>>> +
>>> +	vq->last_avail_idx++;
>>> +
>>> +	/* When we start there are none of either input nor output. */
>>> +	*out_num = *in_num = 0;
>>> +
>>> +	i = head;
>>> +	do {
>>> +		unsigned *num;
>>> +		unsigned iov_count;
>>> +		if (copy_from_user(&desc, vq->desc + i, sizeof desc)) {
>>> +			vq_err(vq, "Failed to get descriptor: idx %d addr %p\n",
>>> +			       i, vq->desc + i);
>>> +			return vq->num;
>>> +		}
>>> +		/* If this is an input descriptor, increment that count. */
>>> +		if (desc.flags & VRING_DESC_F_WRITE) {
>>> +			num = in_num;
>>> +			iov_count = *out_num;
>>> +		} else {
>>> +			/* If it's an output descriptor, they're all supposed
>>> +			 * to come before any input descriptors. */
>>> +			if (*in_num) {
>>> +				vq_err(vq, "Descriptor has out after in: "
>>> +				       "idx %d\n", i);
>>> +				return vq->num;
>>> +			}
>>> +			num = out_num;
>>> +			iov_count = *in_num;
>>> +		}
>>> +		if (translate_desc(dev, desc.addr, desc.len, iov, iov_count,
>>> +				   VHOST_NET_MAX_SG, num)) {
>>> +			vq_err(vq, "Failed to translate descriptor: idx %d\n",
>>> +			       i);
>>> +			return vq->num;
>>> +		}
>>> +
>>> +		/* If we've got too many, that implies a descriptor loop. */
>>> +		if (*out_num + *in_num > vq->num) {
>>> +			vq_err(vq, "Looped descriptor: idx %d\n", i);
>>> +			return vq->num;
>>> +		}
>>> +	} while ((i = next_desc(vq, &desc)) != vq->num);
>>> +
>>> +	vq->inflight++;
>>> +	return head;
>>> +}
>>> +
>>> +/* Reverse the effect of vhost_get_vq_desc. Useful for error handling. */
>>> +void vhost_discard_vq_desc(struct vhost_virtqueue *vq)
>>> +{
>>> +	vq->last_avail_idx--;
>>> +	vq->inflight--;
>>> +}
>>> +
>>> +/* After we've used one of their buffers, we tell them about it.  We'll then
>>> + * want to send them an interrupt, using vq->call. */
>>> +int vhost_add_used(struct vhost_virtqueue *vq,
>>> +			  unsigned int head, int len)
>>> +{
>>> +	struct vring_used_elem *used;
>>> +
>>> +	/* The virtqueue contains a ring of used buffers.  Get a pointer to the
>>> +	 * next entry in that used ring. */
>>> +	used = &vq->used->ring[vq->last_used_idx % vq->num];
>>> +	if (put_user(head, &used->id)) {
>>> +		vq_err(vq, "Failed to write used id");
>>> +		return -EFAULT;
>>> +	}
>>> +	if (put_user(len, &used->len)) {
>>> +		vq_err(vq, "Failed to write used len");
>>> +		return -EFAULT;
>>> +	}
>>> +	/* Make sure buffer is written before we update index. */
>>> +	wmb();
>>> +	if (put_user(vq->last_used_idx + 1, &vq->used->idx)) {
>>> +		vq_err(vq, "Failed to increment used idx");
>>> +		return -EFAULT;
>>> +	}
>>> +	vq->last_used_idx++;
>>> +	vq->inflight--;
>>> +	return 0;
>>> +}
>>> +
>>> +/* This actually sends the interrupt for this virtqueue */
>>> +void vhost_trigger_irq(struct vhost_virtqueue *vq)
>>> +{
>>> +	__u16 flags = 0;
>>> +	if (get_user(flags, &vq->avail->flags)) {
>>> +		vq_err(vq, "Failed to get flags");
>>> +		return;
>>> +	}
>>> +
>>> +	/* If they don't want an interrupt, don't send one, unless empty. */
>>> +	if ((flags & VRING_AVAIL_F_NO_INTERRUPT) && vq->inflight)
>>> +		return;
>>> +
>>> +	/* Send the Guest an interrupt tell them we used something up. */
>>> +	if (vq->call_ctx)
>>> +		eventfd_signal(vq->call_ctx, 1);
>>> +}
>>> +
>>> +/* And here's the combo meal deal.  Supersize me! */
>>> +void vhost_add_used_and_trigger(struct vhost_virtqueue *vq,
>>> +				unsigned int head, int len)
>>> +{
>>> +	vhost_add_used(vq, head, len);
>>> +	vhost_trigger_irq(vq);
>>> +}
>>> +
>>> +int vhost_init(void)
>>> +{
>>> +	vhost_workqueue = create_workqueue("vhost");
>>> +	if (!vhost_workqueue)
>>> +		return -ENOMEM;
>>> +	return 0;
>>> +}
>>> +
>>> +void vhost_cleanup(void)
>>> +{
>>> +	destroy_workqueue(vhost_workqueue);
>>> +}
>>> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
>>> new file mode 100644
>>> index 0000000..7f7ffcd
>>> --- /dev/null
>>> +++ b/drivers/vhost/vhost.h
>>> @@ -0,0 +1,108 @@
>>> +#ifndef _VHOST_H
>>> +#define _VHOST_H
>>> +
>>> +#include <linux/eventfd.h>
>>> +#include <linux/vhost.h>
>>> +#include <linux/mm.h>
>>> +#include <linux/mutex.h>
>>> +#include <linux/workqueue.h>
>>> +#include <linux/poll.h>
>>> +#include <linux/file.h>
>>> +#include <linux/skbuff.h>
>>> +
>>> +struct vhost_device;
>>> +
>>> +enum {
>>> +	VHOST_NET_MAX_SG = MAX_SKB_FRAGS + 2,
>>> +};
>>> +
>>> +/* Poll a file (eventfd or socket) */
>>> +/* Note: there's nothing vhost specific about this structure. */
>>> +struct vhost_poll {
>>> +	poll_table                table;
>>> +	wait_queue_head_t        *wqh;
>>> +	wait_queue_t              wait;
>>> +	/* struct which will handle all actual work. */
>>> +	struct work_struct        work;
>>> +	unsigned long		  mask;
>>> +};
>>> +
>>> +void vhost_poll_init(struct vhost_poll *poll, work_func_t func,
>>> +		     unsigned long mask);
>>> +void vhost_poll_start(struct vhost_poll *poll, struct file *file);
>>> +void vhost_poll_stop(struct vhost_poll *poll);
>>> +void vhost_poll_flush(struct vhost_poll *poll);
>>> +
>>> +/* The virtqueue structure describes a queue attached to a device. */
>>> +struct vhost_virtqueue {
>>> +	struct vhost_dev *dev;
>>> +
>>> +	/* The actual ring of buffers. */
>>> +	struct mutex mutex;
>>> +	unsigned int num;
>>> +	struct vring_desc __user *desc;
>>> +	struct vring_avail __user *avail;
>>> +	struct vring_used __user *used;
>>> +	struct file *kick;
>>> +	struct file *call;
>>> +	struct file *error;
>>> +	struct eventfd_ctx *call_ctx;
>>> +	struct eventfd_ctx *error_ctx;
>>> +
>>> +	struct vhost_poll poll;
>>> +
>>> +	/* The routine to call when the Guest pings us, or timeout. */
>>> +	work_func_t handle_kick;
>>> +
>>> +	/* Last available index we saw. */
>>> +	u16 last_avail_idx;
>>> +
>>> +	/* Last index we used. */
>>> +	u16 last_used_idx;
>>> +
>>> +	/* Outstanding buffers */
>>> +	unsigned int inflight;
>>> +
>>> +	/* Is this blocked? */
>>> +	bool blocked;
>>> +
>>> +	struct iovec iov[VHOST_NET_MAX_SG];
>>> +
>>> +} ____cacheline_aligned;
>>> +
>>> +struct vhost_dev {
>>> +	/* Readers use RCU to access memory table pointer.
>>> +	 * Writers use mutex below.*/
>>> +	struct vhost_memory *memory;
>>> +	struct mm_struct *mm;
>>> +	struct vhost_virtqueue *vqs;
>>> +	int nvqs;
>>> +	struct mutex mutex;
>>> +};
>>> +
>>> +long vhost_dev_init(struct vhost_dev *, struct vhost_virtqueue *vqs, int nvqs);
>>> +long vhost_dev_check_owner(struct vhost_dev *);
>>> +long vhost_dev_reset_owner(struct vhost_dev *);
>>> +void vhost_dev_cleanup(struct vhost_dev *);
>>> +long vhost_dev_ioctl(struct vhost_dev *, unsigned int ioctl, unsigned long arg);
>>> +
>>> +unsigned vhost_get_vq_desc(struct vhost_dev *, struct vhost_virtqueue *,
>>> +			   struct iovec iov[],
>>> +			   unsigned int *out_num, unsigned int *in_num);
>>> +void vhost_discard_vq_desc(struct vhost_virtqueue *);
>>> +
>>> +int vhost_add_used(struct vhost_virtqueue *, unsigned int head, int len);
>>> +void vhost_trigger_irq(struct vhost_virtqueue *);
>>> +void vhost_add_used_and_trigger(struct vhost_virtqueue *,
>>> +				unsigned int head, int len);
>>> +
>>> +int vhost_init(void);
>>> +void vhost_cleanup(void);
>>> +
>>> +#define vq_err(vq, fmt, ...) do {                                  \
>>> +		printk(KERN_ERR pr_fmt(fmt), ##__VA_ARGS__);       \
>>> +		if ((vq)->error_ctx)                               \
>>> +				eventfd_signal((vq)->error_ctx, 1);\
>>> +	} while (0)
>>> +
>>> +#endif
>>> diff --git a/include/linux/Kbuild b/include/linux/Kbuild
>>> index dec2f18..975df9a 100644
>>> --- a/include/linux/Kbuild
>>> +++ b/include/linux/Kbuild
>>> @@ -360,6 +360,7 @@ unifdef-y += uio.h
>>>  unifdef-y += unistd.h
>>>  unifdef-y += usbdevice_fs.h
>>>  unifdef-y += utsname.h
>>> +unifdef-y += vhost.h
>>>  unifdef-y += videodev2.h
>>>  unifdef-y += videodev.h
>>>  unifdef-y += virtio_config.h
>>> diff --git a/include/linux/miscdevice.h b/include/linux/miscdevice.h
>>> index 0521177..781a8bb 100644
>>> --- a/include/linux/miscdevice.h
>>> +++ b/include/linux/miscdevice.h
>>> @@ -30,6 +30,7 @@
>>>  #define HPET_MINOR		228
>>>  #define FUSE_MINOR		229
>>>  #define KVM_MINOR		232
>>> +#define VHOST_NET_MINOR		233
>> Would recommend using DYNAMIC-MINOR.
>>
>>>  #define MISC_DYNAMIC_MINOR	255
>>>  
>>>  struct device;
>>> diff --git a/include/linux/vhost.h b/include/linux/vhost.h
>>> new file mode 100644
>>> index 0000000..9ec6d5f
>>> --- /dev/null
>>> +++ b/include/linux/vhost.h
>>> @@ -0,0 +1,100 @@
>>> +#ifndef _LINUX_VHOST_H
>>> +#define _LINUX_VHOST_H
>>> +/* Userspace interface for in-kernel virtio accelerators. */
>>> +
>>> +/* vhost is used to reduce the number of system calls involved in virtio.
>>> + *
>>> + * Existing virtio net code is used in the guest without modification.
>>> + *
>>> + * This header includes interface used by userspace hypervisor for
>>> + * device configuration.
>>> + */
>>> +
>>> +#include <linux/types.h>
>>> +#include <linux/compiler.h>
>>> +#include <linux/ioctl.h>
>>> +#include <linux/virtio_config.h>
>>> +#include <linux/virtio_ring.h>
>>> +
>>> +struct vhost_vring_state {
>>> +	unsigned int index;
>>> +	unsigned int num;
>>> +};
>>> +
>>> +struct vhost_vring_file {
>>> +	unsigned int index;
>>> +	int fd;
>>> +};
>>> +
>>> +struct vhost_vring_addr {
>>> +	unsigned int index;
>>> +	unsigned int padding;
>>> +	__u64 user_addr;
>>> +};
>>> +
>>> +struct vhost_memory_region {
>>> +	__u64 guest_phys_addr;
>>> +	__u64 memory_size; /* bytes */
>>> +	__u64 userspace_addr;
>>> +	__u64 padding; /* read/write protection? */
>>> +};
>>> +
>>> +struct vhost_memory {
>>> +	__u32 nregions;
>>> +	__u32 padding;
>>> +	struct vhost_memory_region regions[0];
>>> +};
>>> +
>>> +/* ioctls */
>>> +
>>> +#define VHOST_VIRTIO 0xAF
>>> +
>>> +/* Features bitmask for forward compatibility. Transport bits must be zero. */
>>> +#define VHOST_GET_FEATURES	_IOR(VHOST_VIRTIO, 0x00, __u32)
>>> +#define VHOST_ACK_FEATURES	_IOW(VHOST_VIRTIO, 0x00, __u32)
>>> +
>>> +/* Set current process as the (exclusive) owner of this file descriptor.  This
>>> + * must be called before any other vhost command.  Further calls to
>>> + * VHOST_OWNER_SET fail until VHOST_OWNER_RESET is called. */
>>> +#define VHOST_SET_OWNER _IO(VHOST_VIRTIO, 0x01)
>>> +/* Give up ownership, and reset the device to default values.
>>> + * Allows subsequent call to VHOST_OWNER_SET to succeed. */
>>> +#define VHOST_RESET_OWNER _IO(VHOST_VIRTIO, 0x02)
>>> +
>>> +/* Set up/modify memory layout */
>>> +#define VHOST_SET_MEM_TABLE	_IOW(VHOST_VIRTIO, 0x03, struct vhost_memory)
>>> +
>>> +/* Ring setup. These parameters can not be modified while ring is running
>>> + * (bound to a device). */
>>> +/* Set number of descriptors in ring */
>>> +#define VHOST_SET_VRING_NUM _IOW(VHOST_VIRTIO, 0x10, struct vhost_vring_state)
>>> +/* Start of array of descriptors (virtually contiguous) */
>>> +#define VHOST_SET_VRING_DESC _IOW(VHOST_VIRTIO, 0x11, struct vhost_vring_addr)
>>> +/* Used structure address */
>>> +#define VHOST_SET_VRING_USED _IOW(VHOST_VIRTIO, 0x12, struct vhost_vring_addr)
>>> +/* Available structure address */
>>> +#define VHOST_SET_VRING_AVAIL _IOW(VHOST_VIRTIO, 0x13, struct vhost_vring_addr)
>>> +/* Base value where queue looks for available descriptors */
>>> +#define VHOST_SET_VRING_BASE _IOW(VHOST_VIRTIO, 0x14, struct vhost_vring_state)
>>> +/* Get accessor: reads index, writes value in num */
>>> +#define VHOST_GET_VRING_BASE _IOWR(VHOST_VIRTIO, 0x14, struct vhost_vring_state)
>>> +
>>> +/* The following ioctls use eventfd file descriptors to signal and poll
>>> + * for events. */
>>> +
>>> +/* Set eventfd to poll for added buffers */
>>> +#define VHOST_SET_VRING_KICK _IOW(VHOST_VIRTIO, 0x20, struct vhost_vring_file)
>>> +/* Set eventfd to signal when buffers have beed used */
>>> +#define VHOST_SET_VRING_CALL _IOW(VHOST_VIRTIO, 0x21, struct vhost_vring_file)
>>> +/* Set eventfd to signal an error */
>>> +#define VHOST_SET_VRING_ERR _IOW(VHOST_VIRTIO, 0x22, struct vhost_vring_file)
>>> +
>>> +/* VHOST_NET specific defines */
>>> +
>>> +/* Attach virtio net device to a raw socket. The socket must be already
>>> + * bound to an ethernet device, this device will be used for transmit.
>>> + * Pass -1 to unbind from the socket and the transmit device.
>>> + * This can be used to stop the device (e.g. for migration). */
>>> +#define VHOST_NET_SET_SOCKET _IOW(VHOST_VIRTIO, 0x30, int)
>>> +
>>> +#endif
>>
> 
>