Subject: Re: [PATCH v1 1/3] A device for zero-copy based on KVM virtio-net.

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 8 Mar 2010 13:28:49 +0200

On Sat, Mar 06, 2010 at 05:38:36PM +0800, [email-masked] wrote:
> From: Xin Xiaohui <[email-masked]>
> 
> Add a device to utilize the vhost-net backend driver for
> copy-less data transfer between guest FE and host NIC.
> It pins the guest user space to the host memory and
> provides proto_ops as sendmsg/recvmsg to vhost-net.
> 
> Signed-off-by: Xin Xiaohui <[email-masked]>
> Signed-off-by: Zhao Yu <[email-masked]>
> Sigend-off-by: Jeff Dike <[email-masked]>

I think some of the comments below are repeated.
Do you plan addressing them?

> ---
>  drivers/vhost/Kconfig     |    5 +
>  drivers/vhost/Makefile    |    2 +
>  drivers/vhost/mpassthru.c | 1202 +++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/mpassthru.h |   29 ++

I'm not sure it's wise to limit the device to
vhost even if that's the only mode that you are going to
support in the first version.
How about locating the char device under drivers/net/?

>  4 files changed, 1238 insertions(+), 0 deletions(-)
>  create mode 100644 drivers/vhost/mpassthru.c
>  create mode 100644 include/linux/mpassthru.h
> 
> diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
> index 9f409f4..ee32a3b 100644
> --- a/drivers/vhost/Kconfig
> +++ b/drivers/vhost/Kconfig
> @@ -9,3 +9,8 @@ config VHOST_NET
>  	  To compile this driver as a module, choose M here: the module will
>  	  be called vhost_net.
>  
> +config VHOST_PASSTHRU
> +	tristate "Zerocopy network driver (EXPERIMENTAL)"
> +	depends on VHOST_NET
> +	---help---
> +	  zerocopy network I/O support
> diff --git a/drivers/vhost/Makefile b/drivers/vhost/Makefile
> index 72dd020..3f79c79 100644
> --- a/drivers/vhost/Makefile
> +++ b/drivers/vhost/Makefile
> @@ -1,2 +1,4 @@
>  obj-$(CONFIG_VHOST_NET) += vhost_net.o
>  vhost_net-y := vhost.o net.o
> +
> +obj-$(CONFIG_VHOST_PASSTHRU) += mpassthru.o
> diff --git a/drivers/vhost/mpassthru.c b/drivers/vhost/mpassthru.c
> new file mode 100644
> index 0000000..744d6cd
> --- /dev/null
> +++ b/drivers/vhost/mpassthru.c
> @@ -0,0 +1,1202 @@
> +/*
> + *  MPASSTHRU - Mediate passthrough device.
> + *  Copyright (C) 2009 ZhaoYu, XinXiaohui, Dike, Jeffery G
> + *
> + *  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.
> + *
> + */
> +
> +#define DRV_NAME        "mpassthru"
> +#define DRV_DESCRIPTION "Mediate passthru device driver"
> +#define DRV_COPYRIGHT   "(C) 2009 ZhaoYu, XinXiaohui, Dike, Jeffery G"
> +
> +#include <linux/module.h>
> +#include <linux/errno.h>
> +#include <linux/kernel.h>
> +#include <linux/major.h>
> +#include <linux/slab.h>
> +#include <linux/smp_lock.h>
> +#include <linux/poll.h>
> +#include <linux/fcntl.h>
> +#include <linux/init.h>
> +#include <linux/skbuff.h>
> +#include <linux/netdevice.h>
> +#include <linux/etherdevice.h>
> +#include <linux/miscdevice.h>
> +#include <linux/ethtool.h>
> +#include <linux/rtnetlink.h>
> +#include <linux/if.h>
> +#include <linux/if_arp.h>
> +#include <linux/if_ether.h>
> +#include <linux/crc32.h>
> +#include <linux/nsproxy.h>
> +#include <linux/uaccess.h>
> +#include <linux/virtio_net.h>
> +#include <linux/mpassthru.h>
> +#include <net/net_namespace.h>
> +#include <net/netns/generic.h>
> +#include <net/rtnetlink.h>
> +#include <net/sock.h>
> +
> +#include <asm/system.h>
> +
> +#include "vhost.h"
> +
> +/* Uncomment to enable debugging */
> +/* #define MPASSTHRU_DEBUG 1 */
> +
> +#ifdef MPASSTHRU_DEBUG
> +static int debug;
> +
> +#define DBG  if (mp->debug) printk
> +#define DBG1 if (debug == 2) printk
> +#else
> +#define DBG(a...)
> +#define DBG1(a...)
> +#endif
> +
> +#define COPY_THRESHOLD (L1_CACHE_BYTES * 4)
> +#define COPY_HDR_LEN   (L1_CACHE_BYTES < 64 ? 64 : L1_CACHE_BYTES)
> +
> +struct frag {
> +	u16     offset;
> +	u16     size;
> +};
> +
> +struct page_ctor {
> +	struct list_head        readq;
> +	int 			w_len;
> +	int 			r_len;
> +	spinlock_t      	read_lock;
> +	atomic_t        	refcnt;
> +	struct kmem_cache   	*cache;
> +	struct net_device   	*dev;
> +	struct mpassthru_port	port;
> +	void 			*sendctrl;
> +	void 			*recvctrl;
> +};
> +
> +struct page_info {
> +	struct list_head    	list;
> +	int         		header;
> +	/* indicate the actual length of bytes
> +	 * send/recv in the user space buffers
> +	 */
> +	int         		total;
> +	int         		offset;
> +	struct page     	*pages[MAX_SKB_FRAGS+1];
> +	struct skb_frag_struct 	frag[MAX_SKB_FRAGS+1];
> +	struct sk_buff      	*skb;
> +	struct page_ctor   	*ctor;
> +
> +	/* The pointer relayed to skb, to indicate
> +	 * it's a user space allocated skb or kernel
> +	 */
> +	struct skb_user_page    user;
> +	struct skb_shared_info	ushinfo;
> +
> +#define INFO_READ      		0
> +#define INFO_WRITE     		1
> +	unsigned        	flags;
> +	unsigned        	pnum;
> +
> +	/* It's meaningful for receive, means
> +	 * the max length allowed
> +	 */
> +	size_t          	len;
> +
> +	/* The fields after that is for backend
> +	 * driver, now for vhost-net.
> +	 */
> +	struct vhost_notifier	notifier;
> +	unsigned int    	desc_pos;
> +	unsigned int 		log;
> +	struct iovec 		hdr[VHOST_NET_MAX_SG];
> +	struct iovec 		iov[VHOST_NET_MAX_SG];
> +	void 			*ctl;
> +};
> +
> +struct mp_struct {
> +	struct mp_file   	*mfile;
> +	struct net_device       *dev;
> +	struct page_ctor	*ctor;
> +	struct socket           socket;
> +
> +#ifdef MPASSTHRU_DEBUG
> +	int debug;
> +#endif
> +};
> +
> +struct mp_file {
> +	atomic_t count;
> +	struct mp_struct *mp;
> +	struct net *net;
> +};
> +
> +struct mp_sock {
> +	struct sock            	sk;
> +	struct mp_struct       	*mp;
> +};
> +
> +static int mp_dev_change_flags(struct net_device *dev, unsigned flags)
> +{
> +	int ret = 0;
> +
> +	rtnl_lock();
> +	ret = dev_change_flags(dev, flags);
> +	rtnl_unlock();
> +
> +	if (ret < 0)
> +		printk(KERN_ERR "failed to change dev state of %s", dev->name);
> +
> +	return ret;
> +}
> +
> +/* The main function to allocate user space buffers */
> +static struct skb_user_page *page_ctor(struct mpassthru_port *port,
> +		struct sk_buff *skb, int npages)
> +{
> +	int i;
> +	unsigned long flags;
> +	struct page_ctor *ctor;
> +	struct page_info *info = NULL;
> +
> +	ctor = container_of(port, struct page_ctor, port);
> +
> +	spin_lock_irqsave(&ctor->read_lock, flags);
> +	if (!list_empty(&ctor->readq)) {
> +		info = list_first_entry(&ctor->readq, struct page_info, list);
> +		list_del(&info->list);
> +	}
> +	spin_unlock_irqrestore(&ctor->read_lock, flags);
> +	if (!info)
> +		return NULL;
> +
> +	for (i = 0; i < info->pnum; i++) {
> +		get_page(info->pages[i]);
> +		info->frag[i].page = info->pages[i];
> +		info->frag[i].page_offset = i ? 0 : info->offset;
> +		info->frag[i].size = port->npages > 1 ? PAGE_SIZE :
> +			port->data_len;
> +	}
> +	info->skb = skb;
> +	info->user.frags = info->frag;
> +	info->user.ushinfo = &info->ushinfo;
> +	return &info->user;
> +}
> +
> +static struct vhost_notifier *create_vhost_notifier(struct vhost_virtqueue *vq,
> +			struct page_info *info, int size);
> +
> +static void mp_vhost_notifier_dtor(struct vhost_notifier *vnotify)
> +{
> +	struct page_info *info = (struct page_info *)(vnotify->ctrl);
> +	int i;
> +
> +	for (i = 0; i < info->pnum; i++) {
> +		if (info->pages[i])
> +			put_page(info->pages[i]);
> +	}
> +
> +	if (info->flags == INFO_READ) {
> +		skb_shinfo(info->skb)->destructor_arg = &info->user;
> +		info->skb->destructor = NULL;
> +		kfree(info->skb);
> +	}
> +
> +	kmem_cache_free(info->ctor->cache, info);
> +
> +	return;
> +}
> +
> +/* A helper to clean the skb before the kfree_skb() */
> +
> +static void page_dtor_prepare(struct page_info *info)
> +{
> +	if (info->flags == INFO_READ)
> +		if (info->skb)
> +			info->skb->head = NULL;
> +}
> +
> +/* The callback to destruct the user space buffers or skb */
> +static void page_dtor(struct skb_user_page *user)
> +{
> +	struct page_info *info;
> +	struct page_ctor *ctor;
> +	struct sock *sk;
> +	struct sk_buff *skb;
> +	struct vhost_notifier *vnotify;
> +	struct vhost_virtqueue *vq = NULL;
> +	unsigned long flags;
> +	int i;
> +
> +	if (!user)
> +		return;
> +	info = container_of(user, struct page_info, user);
> +	if (!info)
> +		return;
> +	ctor = info->ctor;
> +	skb = info->skb;
> +
> +	page_dtor_prepare(info);
> +
> +	/* If the info->total is 0, make it to be reused */
> +	if (!info->total) {
> +		spin_lock_irqsave(&ctor->read_lock, flags);
> +		list_add(&info->list, &ctor->readq);
> +		spin_unlock_irqrestore(&ctor->read_lock, flags);
> +		return;
> +	}
> +
> +	/* Receive buffers, should be destructed */
> +	if (info->flags == INFO_READ) {
> +		for (i = 0; info->pages[i]; i++)
> +			put_page(info->pages[i]);
> +		info->skb = NULL;
> +		return;
> +	}
> +
> +	/* For transmit, we should wait for the DMA finish by hardware.
> +	 * Queue the notifier to wake up the backend driver
> +	 */
> +	vq = (struct vhost_virtqueue *)info->ctl;
> +	vnotify = create_vhost_notifier(vq, info, info->total);
> +
> +	spin_lock_irqsave(&vq->notify_lock, flags);
> +	list_add_tail(&vnotify->list, &vq->notifier);
> +	spin_unlock_irqrestore(&vq->notify_lock, flags);
> +
> +	sk = ctor->port.sock->sk;
> +	sk->sk_write_space(sk);
> +
> +	return;
> +}
> +
> +static int page_ctor_attach(struct mp_struct *mp)
> +{
> +	int rc;
> +	struct page_ctor *ctor;
> +	struct net_device *dev = mp->dev;
> +
> +	/* locked by mp_mutex */
> +	if (rcu_dereference(mp->ctor))
> +		return -EBUSY;
> +
> +	ctor = kzalloc(sizeof(*ctor), GFP_KERNEL);
> +	if (!ctor)
> +		return -ENOMEM;
> +	rc = netdev_mp_port_prep(dev, &ctor->port);
> +	if (rc)
> +		goto fail;
> +
> +	ctor->cache = kmem_cache_create("skb_page_info",
> +			sizeof(struct page_info), 0,
> +			SLAB_HWCACHE_ALIGN, NULL);
> +
> +	if (!ctor->cache)
> +		goto cache_fail;
> +
> +	INIT_LIST_HEAD(&ctor->readq);
> +	spin_lock_init(&ctor->read_lock);
> +
> +	ctor->w_len = 0;
> +	ctor->r_len = 0;
> +
> +	dev_hold(dev);
> +	ctor->dev = dev;
> +	ctor->port.ctor = page_ctor;
> +	ctor->port.sock = &mp->socket;
> +	atomic_set(&ctor->refcnt, 1);
> +
> +	rc = netdev_mp_port_attach(dev, &ctor->port);
> +	if (rc)
> +		goto fail;
> +
> +	/* locked by mp_mutex */
> +	rcu_assign_pointer(mp->ctor, ctor);
> +
> +	/* XXX:Need we do set_offload here ? */
> +
> +	return 0;
> +
> +fail:
> +	kmem_cache_destroy(ctor->cache);
> +cache_fail:
> +	kfree(ctor);
> +	dev_put(dev);
> +
> +	return rc;
> +}
> +
> +
> +static inline void get_page_ctor(struct page_ctor *ctor)
> +{
> +       atomic_inc(&ctor->refcnt);
> +}
> +
> +static inline void put_page_ctor(struct page_ctor *ctor)
> +{
> +	if (atomic_dec_and_test(&ctor->refcnt))
> +		kfree(ctor);
> +}
> +
> +struct page_info *info_dequeue(struct page_ctor *ctor)
> +{
> +	unsigned long flags;
> +	struct page_info *info = NULL;
> +	spin_lock_irqsave(&ctor->read_lock, flags);
> +	if (!list_empty(&ctor->readq)) {
> +		info = list_first_entry(&ctor->readq,
> +				struct page_info, list);
> +		list_del(&info->list);
> +	}
> +	spin_unlock_irqrestore(&ctor->read_lock, flags);
> +	return info;
> +}
> +
> +static int page_ctor_detach(struct mp_struct *mp)
> +{
> +	struct page_ctor *ctor;
> +	struct page_info *info;
> +	int i;
> +
> +	ctor = rcu_dereference(mp->ctor);
> +	if (!ctor)
> +		return -ENODEV;
> +
> +	while ((info = info_dequeue(ctor))) {
> +		for (i = 0; i < info->pnum; i++)
> +			if (info->pages[i])
> +				put_page(info->pages[i]);
> +		kmem_cache_free(ctor->cache, info);
> +	}
> +	kmem_cache_destroy(ctor->cache);
> +	netdev_mp_port_detach(ctor->dev);
> +	dev_put(ctor->dev);
> +
> +	/* locked by mp_mutex */
> +	rcu_assign_pointer(mp->ctor, NULL);
> +	synchronize_rcu();
> +
> +	put_page_ctor(ctor);
> +
> +	return 0;
> +}
> +
> +/* For small user space buffers transmit, we don't need to call
> + * get_user_pages().
> + */
> +static struct page_info *alloc_small_page_info(struct page_ctor *ctor,
> +		int total)
> +{
> +	struct page_info *info = kmem_cache_zalloc(ctor->cache, GFP_KERNEL);
> +
> +	if (!info)
> +		return NULL;
> +	info->total = total;
> +	info->user.dtor = page_dtor;
> +	info->ctor = ctor;
> +	info->flags = INFO_WRITE;
> +	return info;
> +}
> +
> +/* The main function to transform the guest user space address
> + * to host kernel address via get_user_pages(). Thus the hardware
> + * can do DMA directly to the user space address.
> + */
> +static struct page_info *alloc_page_info(struct page_ctor *ctor,
> +			struct iovec *iov, int count, struct frag *frags,
> +			int npages, int total)
> +{
> +	int rc;
> +	int i, j, n = 0;
> +	int len;
> +	unsigned long base;
> +	struct page_info *info = kmem_cache_zalloc(ctor->cache, GFP_KERNEL);
> +
> +	if (!info)
> +		return NULL;
> +
> +	down_read(&current->mm->mmap_sem);
> +	for (i = j = 0; i < count; i++) {
> +		base = (unsigned long)iov[i].iov_base;
> +		len = iov[i].iov_len;
> +
> +		if (!len)
> +			continue;
> +		n = ((base & ~PAGE_MASK) + len + ~PAGE_MASK) >> PAGE_SHIFT;
> +
> +		rc = get_user_pages(current, current->mm, base, n,
> +				npages ? 1 : 0, 0, &info->pages[j], NULL);

Try switching to get_user_pages_fast.

We need some limit on the number of pages this can lock,
otherwise it's an obvious DOS.

> +		if (rc != n) {
> +			up_read(&current->mm->mmap_sem);
> +			goto failed;
> +		}
> +
> +		while (n--) {
> +			frags[j].offset = base & ~PAGE_MASK;
> +			frags[j].size = min_t(int, len,
> +					PAGE_SIZE - frags[j].offset);
> +			len -= frags[j].size;
> +			base += frags[j].size;
> +			j++;
> +		}
> +	}
> +	up_read(&current->mm->mmap_sem);
> +
> +#ifdef CONFIG_HIGHMEM
> +	if (npages && !(dev->features & NETIF_F_HIGHDMA)) {
> +		for (i = 0; i < j; i++) {
> +			if (PageHighMem(info->pages[i]))
> +				goto failed;
> +		}
> +	}
> +#endif
> +
> +	info->total = total;
> +	info->user.dtor = page_dtor;
> +	info->ctor = ctor;
> +	info->pnum = j;
> +
> +	if (!npages)
> +		info->flags = INFO_WRITE;
> +	if (info->flags == INFO_READ) {
> +		info->user.start = (u8 *)(((unsigned long)
> +				(pfn_to_kaddr(page_to_pfn(info->pages[0]))) +
> +				frags[0].offset) - NET_IP_ALIGN - NET_SKB_PAD);
> +		info->user.size = iov[0].iov_len + NET_IP_ALIGN + NET_SKB_PAD;
> +	}
> +	return info;
> +
> +failed:
> +	for (i = 0; i < j; i++)
> +		put_page(info->pages[i]);

For reads, I think you must also mark the page dirty.

> +
> +	kmem_cache_free(ctor->cache, info);
> +
> +	return NULL;
> +}
> +
> +struct page_ctor *mp_rcu_get_ctor(struct page_ctor *ctor)
> +{
> +	struct page_ctor *_ctor = NULL;
> +
> +	rcu_read_lock();
> +	_ctor = rcu_dereference(ctor);
> +
> +	if (!_ctor) {
> +		DBG(KERN_INFO "Device %s cannot do mediate passthru.\n",
> +				ctor->dev->name);
> +		rcu_read_unlock();
> +		return NULL;
> +	}
> +	get_page_ctor(_ctor);
> +	rcu_read_unlock();
> +	return _ctor;

So you take a reference to ctor under rcu, but then you keep it
after going outside rcu read side critical section.
This does not seem right.

> +}
> +
> +static int mp_sendmsg(struct kiocb *iocb, struct socket *sock,
> +		struct msghdr *m, size_t total_len)
> +{
> +	struct mp_struct *mp = container_of(sock->sk, struct mp_sock, sk)->mp;
> +	struct page_ctor *ctor;
> +	struct vhost_virtqueue *vq = (struct vhost_virtqueue *)(m->msg_control);
> +	struct iovec *iov = m->msg_iov;
> +	struct page_info *info = NULL;
> +	struct frag frags[MAX_SKB_FRAGS];
> +	struct sk_buff *skb;
> +	int count = m->msg_iovlen;
> +	int total = 0, header, n, i, len, rc;
> +	unsigned long base;
> +
> +	ctor = mp_rcu_get_ctor(mp->ctor);
> +	if (!ctor)
> +		return -ENODEV;
> +
> +	ctor->sendctrl = vq;
> +
> +	total = iov_length(iov, count);
> +
> +	if (total < ETH_HLEN) {
> +		put_page_ctor(ctor);
> +		return -EINVAL;
> +	}
> +
> +	if (total <= COPY_THRESHOLD)
> +		goto copy;
> +
> +	n = 0;
> +	for (i = 0; i < count; i++) {
> +		base = (unsigned long)iov[i].iov_base;
> +		len = iov[i].iov_len;
> +		if (!len)
> +			continue;
> +		n += ((base & ~PAGE_MASK) + len + ~PAGE_MASK) >> PAGE_SHIFT;
> +		if (n > MAX_SKB_FRAGS) {
> +			put_page_ctor(ctor);
> +			return -EINVAL;
> +		}
> +	}
> +
> +copy:
> +	header = total > COPY_THRESHOLD ? COPY_HDR_LEN : total;
> +
> +	skb = alloc_skb(header + NET_IP_ALIGN, GFP_ATOMIC);
> +	if (!skb)
> +		goto drop;
> +
> +	skb_reserve(skb, NET_IP_ALIGN);
> +
> +	skb_set_network_header(skb, ETH_HLEN);
> +
> +	memcpy_fromiovec(skb->data, iov, header);
> +	skb_put(skb, header);
> +	skb->protocol = *((__be16 *)(skb->data) + ETH_ALEN);
> +
> +	if (header == total) {
> +		rc = total;
> +		info = alloc_small_page_info(ctor, total);
> +	} else {
> +		info = alloc_page_info(ctor, iov, count, frags, 0, total);
> +		if (info)
> +			for (i = 0; info->pages[i]; i++) {
> +				skb_add_rx_frag(skb, i, info->pages[i],
> +						frags[i].offset, frags[i].size);
> +				info->pages[i] = NULL;
> +			}
> +	}
> +	if (info != NULL) {
> +		info->desc_pos = vq->head;
> +		info->ctl = vq;
> +		info->total = total;
> +		info->skb = skb;
> +		skb_shinfo(skb)->destructor_arg = &info->user;
> +		skb->dev = mp->dev;
> +		dev_queue_xmit(skb);
> +		mp->dev->stats.tx_packets++;
> +		mp->dev->stats.tx_bytes += total;
> +		put_page_ctor(ctor);
> +		return 0;
> +	}
> +drop:
> +	kfree(skb);
> +	if (info) {
> +		for (i = 0; info->pages[i]; i++)
> +			put_page(info->pages[i]);
> +		kmem_cache_free(info->ctor->cache, info);
> +	}
> +	mp->dev->stats.tx_dropped++;
> +	put_page_ctor(ctor);
> +	return -ENOMEM;
> +}
> +
> +
> +static struct vhost_notifier *create_vhost_notifier(struct vhost_virtqueue *vq,
> +			struct page_info *info, int size)
> +{
> +	struct vhost_notifier *vnotify = NULL;
> +
> +	vnotify = &info->notifier;
> +	memset(vnotify, 0, sizeof(struct vhost_notifier));
> +	vnotify->vq = vq;
> +	vnotify->head = info->desc_pos;
> +	vnotify->size = size;
> +	vnotify->log = info->log;
> +	vnotify->ctrl = (void *)info;
> +	vnotify->dtor = mp_vhost_notifier_dtor;
> +	return vnotify;
> +}
> +
> +static void mp_recvmsg_notify(struct vhost_virtqueue *vq)
> +{
> +	struct socket *sock = vq->private_data;
> +	struct mp_struct *mp = container_of(sock->sk, struct mp_sock, sk)->mp;
> +	struct page_ctor *ctor = NULL;
> +	struct sk_buff *skb = NULL;
> +	struct page_info *info = NULL;
> +	struct ethhdr *eth;
> +	struct vhost_notifier *vnotify = NULL;
> +	int len, i;
> +	unsigned long flags;
> +
> +	struct virtio_net_hdr hdr = {
> +		.flags = 0,
> +		.gso_type = VIRTIO_NET_HDR_GSO_NONE
> +	};
> +
> +	ctor = mp_rcu_get_ctor(mp->ctor);
> +	if (!ctor)
> +		return;
> +
> +	while ((skb = skb_dequeue(&sock->sk->sk_receive_queue)) != NULL) {
> +		if (skb_shinfo(skb)->destructor_arg) {
> +			info = container_of(skb_shinfo(skb)->destructor_arg,
> +					struct page_info, user);
> +			info->skb = skb;
> +			if (skb->len > info->len) {
> +				mp->dev->stats.rx_dropped++;
> +				DBG(KERN_INFO "Discarded truncated rx packet: "
> +					" len %d > %zd\n", skb->len, info->len);
> +				info->total = skb->len;
> +				goto clean;
> +			} else {
> +				int i;
> +				struct skb_shared_info *gshinfo =
> +				(struct skb_shared_info *)(&info->ushinfo);
> +				struct skb_shared_info *hshinfo =
> +						skb_shinfo(skb);
> +
> +				if (gshinfo->nr_frags < hshinfo->nr_frags)
> +					goto clean;
> +				eth = eth_hdr(skb);
> +				skb_push(skb, ETH_HLEN);
> +
> +				hdr.hdr_len = skb_headlen(skb);
> +				info->total = skb->len;
> +
> +				for (i = 0; i < gshinfo->nr_frags; i++)
> +					gshinfo->frags[i].size = 0;
> +				for (i = 0; i < hshinfo->nr_frags; i++)
> +					gshinfo->frags[i].size =
> +						hshinfo->frags[i].size;
> +				memcpy(skb_shinfo(skb), &info->ushinfo,
> +						sizeof(struct skb_shared_info));
> +			}
> +		} else {
> +			/* The skb composed with kernel buffers
> +			 * in case user space buffers are not sufficent.
> +			 * The case should be rare.
> +			 */
> +			unsigned long flags;
> +			int i;
> +			struct skb_shared_info *gshinfo = NULL;
> +
> +			info = NULL;
> +
> +			spin_lock_irqsave(&ctor->read_lock, flags);
> +			if (!list_empty(&ctor->readq)) {
> +				info = list_first_entry(&ctor->readq,
> +						struct page_info, list);
> +				list_del(&info->list);
> +			}
> +			spin_unlock_irqrestore(&ctor->read_lock, flags);
> +			if (!info) {
> +				DBG(KERN_INFO "No user buffer avaliable %p\n",
> +									skb);
> +				skb_queue_head(&sock->sk->sk_receive_queue,
> +									skb);
> +				break;
> +			}
> +			info->skb = skb;
> +			/* compute the guest skb frags info */
> +			gshinfo = (struct skb_shared_info *)(info->user.start +
> +					SKB_DATA_ALIGN(info->user.size));
> +
> +			if (gshinfo->nr_frags < skb_shinfo(skb)->nr_frags)
> +				goto clean;
> +
> +			eth = eth_hdr(skb);
> +			skb_push(skb, ETH_HLEN);
> +			info->total = skb->len;
> +
> +			for (i = 0; i < gshinfo->nr_frags; i++)
> +				gshinfo->frags[i].size = 0;
> +			for (i = 0; i < skb_shinfo(skb)->nr_frags; i++)
> +				gshinfo->frags[i].size =
> +					skb_shinfo(skb)->frags[i].size;
> +			hdr.hdr_len = min_t(int, skb->len,
> +						info->iov[1].iov_len);
> +			skb_copy_datagram_iovec(skb, 0, info->iov, skb->len);
> +		}
> +
> +		len = memcpy_toiovec(info->hdr, (unsigned char *)&hdr,
> +								 sizeof hdr);
> +		if (len) {
> +			DBG(KERN_INFO
> +				"Unable to write vnet_hdr at addr %p: %d\n",
> +				info->hdr->iov_base, len);
> +			goto clean;
> +		}
> +		vnotify = create_vhost_notifier(vq, info,
> +				skb->len + sizeof(hdr));
> +
> +		spin_lock_irqsave(&vq->notify_lock, flags);
> +		list_add_tail(&vnotify->list, &vq->notifier);
> +		spin_unlock_irqrestore(&vq->notify_lock, flags);
> +		continue;
> +
> +clean:
> +		kfree_skb(skb);
> +		for (i = 0; info->pages[i]; i++)
> +			put_page(info->pages[i]);
> +		kmem_cache_free(ctor->cache, info);
> +	}
> +	put_page_ctor(ctor);
> +	return;
> +}
> +
> +static int mp_recvmsg(struct kiocb *iocb, struct socket *sock,
> +		struct msghdr *m, size_t total_len,
> +		int flags)
> +{
> +	struct mp_struct *mp = container_of(sock->sk, struct mp_sock, sk)->mp;
> +	struct page_ctor *ctor;
> +	struct vhost_virtqueue *vq = (struct vhost_virtqueue *)(m->msg_control);
> +	struct iovec *iov = m->msg_iov;
> +	int count = m->msg_iovlen;
> +	int npages, payload;
> +	struct page_info *info;
> +	struct frag frags[MAX_SKB_FRAGS];
> +	unsigned long base;
> +	int i, len;
> +	unsigned long flag;
> +
> +	if (!(flags & MSG_DONTWAIT))
> +		return -EINVAL;
> +
> +	ctor = mp_rcu_get_ctor(mp->ctor);
> +	if (!ctor)
> +		return -EINVAL;
> +
> +	ctor->recvctrl = vq;
> +
> +	/* Error detections in case invalid user space buffer */
> +	if (count > 2 && iov[1].iov_len < ctor->port.hdr_len &&
> +			mp->dev->features & NETIF_F_SG) {
> +		put_page_ctor(ctor);
> +		return -EINVAL;
> +	}
> +
> +	npages = ctor->port.npages;
> +	payload = ctor->port.data_len;
> +
> +	/* If KVM guest virtio-net FE driver use SG feature */
> +	if (count > 2) {
> +		for (i = 2; i < count; i++) {
> +			base = (unsigned long)iov[i].iov_base & ~PAGE_MASK;
> +			len = iov[i].iov_len;
> +			if (npages == 1)
> +				len = min_t(int, len, PAGE_SIZE - base);
> +			else if (base)
> +				break;
> +			payload -= len;
> +			if (payload <= 0)
> +				goto proceed;
> +			if (npages == 1 || (len & ~PAGE_MASK))
> +				break;
> +		}
> +	}
> +
> +	if ((((unsigned long)iov[1].iov_base & ~PAGE_MASK)
> +				- NET_SKB_PAD - NET_IP_ALIGN) >= 0)
> +		goto proceed;
> +
> +	put_page_ctor(ctor);
> +	return -EINVAL;
> +
> +proceed:
> +	/* skip the virtnet head */
> +	iov++;
> +	count--;
> +
> +	/* Translate address to kernel */
> +	info = alloc_page_info(ctor, iov, count, frags, npages, 0);
> +	if (!info) {
> +		put_page_ctor(ctor);
> +		return -ENOMEM;
> +	}
> +
> +	info->len = total_len;
> +	info->hdr[0].iov_base = vq->hdr[0].iov_base;
> +	info->hdr[0].iov_len = vq->hdr[0].iov_len;
> +	info->offset = frags[0].offset;
> +	info->desc_pos = vq->head;
> +	info->log = vq->_log;
> +	info->ctl = NULL;
> +
> +	iov--;
> +	count++;
> +
> +	memcpy(info->iov, vq->iov, sizeof(struct iovec) * count);
> +
> +	spin_lock_irqsave(&ctor->read_lock, flag);
> +	list_add_tail(&info->list, &ctor->readq);
> +	spin_unlock_irqrestore(&ctor->read_lock, flag);
> +
> +	if (!vq->receiver)
> +		vq->receiver = mp_recvmsg_notify;
> +
> +	put_page_ctor(ctor);
> +	return 0;
> +}
> +
> +static void mp_put(struct mp_file *mfile);
> +
> +static int mp_release(struct socket *sock)
> +{
> +	struct mp_struct *mp = container_of(sock->sk, struct mp_sock, sk)->mp;
> +	struct mp_file *mfile = mp->mfile;
> +
> +	mp_put(mfile);
> +	sock_put(mp->socket.sk);
> +	put_net(mfile->net);
> +
> +	return 0;
> +}
> +
> +/* Ops structure to mimic raw sockets with mp device */
> +static const struct proto_ops mp_socket_ops = {
> +	.sendmsg = mp_sendmsg,
> +	.recvmsg = mp_recvmsg,
> +	.release = mp_release,
> +};
> +
> +static struct proto mp_proto = {
> +	.name           = "mp",
> +	.owner          = THIS_MODULE,
> +	.obj_size       = sizeof(struct mp_sock),
> +};
> +
> +static int mp_chr_open(struct inode *inode, struct file * file)
> +{
> +	struct mp_file *mfile;
> +	cycle_kernel_lock();
> +	DBG1(KERN_INFO "mp: mp_chr_open\n");
> +
> +	mfile = kzalloc(sizeof(*mfile), GFP_KERNEL);
> +	if (!mfile)
> +		return -ENOMEM;
> +	atomic_set(&mfile->count, 0);
> +	mfile->mp = NULL;
> +	mfile->net = get_net(current->nsproxy->net_ns);
> +	file->private_data = mfile;
> +	return 0;
> +}
> +
> +static void __mp_detach(struct mp_struct *mp)
> +{
> +	mp->mfile = NULL;
> +
> +	mp_dev_change_flags(mp->dev, mp->dev->flags & ~IFF_UP);
> +	page_ctor_detach(mp);
> +	mp_dev_change_flags(mp->dev, mp->dev->flags | IFF_UP);
> +
> +	/* Drop the extra count on the net device */
> +	dev_put(mp->dev);
> +}
> +
> +static DEFINE_MUTEX(mp_mutex);
> +
> +static void mp_detach(struct mp_struct *mp)
> +{
> +	mutex_lock(&mp_mutex);
> +	__mp_detach(mp);
> +	mutex_unlock(&mp_mutex);
> +}
> +
> +static struct mp_struct *mp_get(struct mp_file *mfile)
> +{
> +	struct mp_struct *mp = NULL;
> +	if (atomic_inc_not_zero(&mfile->count))
> +		mp = mfile->mp;
> +
> +	return mp;
> +}
> +
> +static void mp_put(struct mp_file *mfile)
> +{
> +	if (atomic_dec_and_test(&mfile->count))
> +		mp_detach(mfile->mp);
> +}
> +
> +static int mp_attach(struct mp_struct *mp, struct file *file)
> +{
> +	struct mp_file *mfile = file->private_data;
> +	int err;
> +
> +	netif_tx_lock_bh(mp->dev);
> +
> +	err = -EINVAL;
> +
> +	if (mfile->mp)
> +		goto out;
> +
> +	err = -EBUSY;
> +	if (mp->mfile)
> +		goto out;
> +
> +	err = 0;
> +	mfile->mp = mp;
> +	mp->mfile = mfile;
> +	mp->socket.file = file;
> +	dev_hold(mp->dev);
> +	sock_hold(mp->socket.sk);
> +	atomic_inc(&mfile->count);
> +
> +out:
> +	netif_tx_unlock_bh(mp->dev);
> +	return err;
> +}
> +
> +static void mp_sock_destruct(struct sock *sk)
> +{
> +	struct mp_struct *mp = container_of(sk, struct mp_sock, sk)->mp;
> +	kfree(mp);
> +}
> +
> +static int do_unbind(struct mp_file *mfile)
> +{
> +	struct mp_struct *mp = mp_get(mfile);
> +
> +	if (!mp)
> +		return -EINVAL;
> +
> +	mp_detach(mp);
> +	sock_put(mp->socket.sk);
> +	mp_put(mfile);
> +	return 0;
> +}
> +
> +static void mp_sock_data_ready(struct sock *sk, int len)
> +{
> +	if (sk_has_sleeper(sk))
> +		wake_up_interruptible_sync_poll(sk->sk_sleep, POLLIN);
> +}
> +
> +static void mp_sock_write_space(struct sock *sk)
> +{
> +	if (sk_has_sleeper(sk))
> +		wake_up_interruptible_sync_poll(sk->sk_sleep, POLLOUT);
> +}
> +
> +static long mp_chr_ioctl(struct file *file, unsigned int cmd,
> +		unsigned long arg)
> +{
> +	struct mp_file *mfile = file->private_data;
> +	struct mp_struct *mp;
> +	struct net_device *dev;
> +	void __user* argp = (void __user *)arg;
> +	struct ifreq ifr;
> +	struct sock *sk;
> +	int ret;
> +
> +	ret = -EINVAL;
> +
> +	switch (cmd) {
> +	case MPASSTHRU_BINDDEV:
> +		ret = -EFAULT;
> +		if (copy_from_user(&ifr, argp, sizeof ifr))
> +			break;
> +
> +		ifr.ifr_name[IFNAMSIZ-1] = '\0';
> +
> +		ret = -EBUSY;
> +
> +		if (ifr.ifr_flags & IFF_MPASSTHRU_EXCL)
> +			break;
> +
> +		ret = -ENODEV;
> +		dev = dev_get_by_name(mfile->net, ifr.ifr_name);
> +		if (!dev)
> +			break;
> +
> +		mutex_lock(&mp_mutex);
> +
> +		ret = -EBUSY;
> +		mp = mfile->mp;
> +		if (mp)
> +			goto err_dev_put;
> +
> +		mp = kzalloc(sizeof(*mp), GFP_KERNEL);
> +		if (!mp) {
> +			ret = -ENOMEM;
> +			goto err_dev_put;
> +		}
> +		mp->dev = dev;
> +		ret = -ENOMEM;
> +
> +		sk = sk_alloc(mfile->net, AF_UNSPEC, GFP_KERNEL, &mp_proto);
> +		if (!sk)
> +			goto err_free_mp;
> +
> +		init_waitqueue_head(&mp->socket.wait);
> +		mp->socket.ops = &mp_socket_ops;
> +		sock_init_data(&mp->socket, sk);
> +		sk->sk_sndbuf = INT_MAX;
> +		container_of(sk, struct mp_sock, sk)->mp = mp;
> +
> +		sk->sk_destruct = mp_sock_destruct;
> +		sk->sk_data_ready = mp_sock_data_ready;
> +		sk->sk_write_space = mp_sock_write_space;
> +
> +		ret = mp_attach(mp, file);
> +		if (ret < 0)
> +			goto err_free_sk;
> +
> +		ret = page_ctor_attach(mp);
> +		if (ret < 0)
> +			goto err_free_sk;
> +
> +		ifr.ifr_flags |= IFF_MPASSTHRU_EXCL;
> +		mp_dev_change_flags(mp->dev, mp->dev->flags | IFF_UP);
> +out:
> +		mutex_unlock(&mp_mutex);
> +		break;
> +err_free_sk:
> +		sk_free(sk);
> +err_free_mp:
> +		kfree(mp);
> +err_dev_put:
> +		dev_put(dev);
> +		goto out;
> +
> +	case MPASSTHRU_UNBINDDEV:
> +		ret = do_unbind(mfile);
> +		break;
> +
> +	default:
> +		break;
> +	}
> +	return ret;
> +}
> +
> +static unsigned int mp_chr_poll(struct file *file, poll_table * wait)
> +{
> +	struct mp_file *mfile = file->private_data;
> +	struct mp_struct *mp = mp_get(mfile);
> +	struct sock *sk;
> +	unsigned int mask = 0;
> +
> +	if (!mp)
> +		return POLLERR;
> +
> +	sk = mp->socket.sk;
> +
> +	poll_wait(file, &mp->socket.wait, wait);
> +
> +	if (!skb_queue_empty(&sk->sk_receive_queue))
> +		mask |= POLLIN | POLLRDNORM;
> +
> +	if (sock_writeable(sk) ||
> +		(!test_and_set_bit(SOCK_ASYNC_NOSPACE, &sk->sk_socket->flags) &&
> +			 sock_writeable(sk)))
> +		mask |= POLLOUT | POLLWRNORM;
> +
> +	if (mp->dev->reg_state != NETREG_REGISTERED)
> +		mask = POLLERR;
> +
> +	mp_put(mfile);
> +	return mask;
> +}
> +
> +static int mp_chr_close(struct inode *inode, struct file *file)
> +{
> +	struct mp_file *mfile = file->private_data;
> +
> +	/*
> +	 * Ignore return value since an error only means there was nothing to
> +	 * do
> +	 */
> +	do_unbind(mfile);
> +
> +	put_net(mfile->net);
> +	kfree(mfile);
> +
> +	return 0;
> +}
> +
> +static const struct file_operations mp_fops = {
> +	.owner  = THIS_MODULE,
> +	.llseek = no_llseek,
> +	.poll   = mp_chr_poll,
> +	.unlocked_ioctl = mp_chr_ioctl,
> +	.open   = mp_chr_open,
> +	.release = mp_chr_close,

qemu will need a way to send packets from userspace
(not from guest) as well. So I think you will need
a write as well.

> +};
> +
> +static struct miscdevice mp_miscdev = {
> +	.minor = MISC_DYNAMIC_MINOR,
> +	.name = "mp",
> +	.nodename = "net/mp",
> +	.fops = &mp_fops,
> +};
> +
> +static int mp_device_event(struct notifier_block *unused,
> +		unsigned long event, void *ptr)
> +{
> +	struct net_device *dev = ptr;
> +	struct mpassthru_port *port;
> +	struct mp_struct *mp = NULL;
> +	struct socket *sock = NULL;
> +
> +	port = dev->mp_port;
> +	if (port == NULL)
> +		return NOTIFY_DONE;
> +
> +	switch (event) {
> +	case NETDEV_UNREGISTER:
> +			sock = dev->mp_port->sock;
> +			mp = container_of(sock->sk, struct mp_sock, sk)->mp;
> +			do_unbind(mp->mfile);
> +			break;
> +	}
> +	return NOTIFY_DONE;
> +}
> +
> +static struct notifier_block mp_notifier_block __read_mostly = {
> +	.notifier_call  = mp_device_event,
> +};
> +
> +static int mp_init(void)
> +{
> +	int ret = 0;
> +
> +	ret = misc_register(&mp_miscdev);
> +	if (ret)
> +		printk(KERN_ERR "mp: Can't register misc device\n");
> +	else {
> +		printk(KERN_INFO "Registering mp misc device - minor = %d\n",
> +			mp_miscdev.minor);
> +		register_netdevice_notifier(&mp_notifier_block);
> +	}
> +	return ret;
> +}
> +
> +void mp_cleanup(void)
> +{
> +	unregister_netdevice_notifier(&mp_notifier_block);
> +	misc_deregister(&mp_miscdev);
> +}
> +
> +/* Get an underlying socket object from mp file.  Returns error unless file is
> + * attached to a device.  The returned object works like a packet socket, it
> + * can be used for sock_sendmsg/sock_recvmsg.  The caller is responsible for
> + * holding a reference to the file for as long as the socket is in use. */
> +struct socket *mp_get_socket(struct file *file)
> +{
> +	struct mp_file *mfile = file->private_data;
> +	struct mp_struct *mp;
> +
> +	if (file->f_op != &mp_fops)
> +		return ERR_PTR(-EINVAL);
> +	mp = mp_get(mfile);
> +	if (!mp)
> +		return ERR_PTR(-EBADFD);
> +	mp_put(mfile);
> +	return &mp->socket;
> +}
> +EXPORT_SYMBOL_GPL(mp_get_socket);
> +
> +module_init(mp_init);
> +module_exit(mp_cleanup);
> +MODULE_AUTHOR(DRV_COPYRIGHT);
> +MODULE_DESCRIPTION(DRV_DESCRIPTION);
> +MODULE_LICENSE("GPL v2");
> diff --git a/include/linux/mpassthru.h b/include/linux/mpassthru.h
> new file mode 100644
> index 0000000..2be21c5
> --- /dev/null
> +++ b/include/linux/mpassthru.h
> @@ -0,0 +1,29 @@
> +#ifndef __MPASSTHRU_H
> +#define __MPASSTHRU_H
> +
> +#include <linux/types.h>
> +#include <linux/if_ether.h>
> +
> +/* ioctl defines */
> +#define MPASSTHRU_BINDDEV      _IOW('M', 213, int)
> +#define MPASSTHRU_UNBINDDEV    _IOW('M', 214, int)
> +
> +/* MPASSTHRU ifc flags */
> +#define IFF_MPASSTHRU		0x0001
> +#define IFF_MPASSTHRU_EXCL	0x0002
> +
> +#ifdef __KERNEL__
> +#if defined(CONFIG_VHOST_PASSTHRU) || defined(CONFIG_VHOST_PASSTHRU_MODULE)
> +struct socket *mp_get_socket(struct file *);
> +#else
> +#include <linux/err.h>
> +#include <linux/errno.h>
> +struct file;
> +struct socket;
> +static inline struct socket *mp_get_socket(struct file *f)
> +{
> +	return ERR_PTR(-EINVAL);
> +}
> +#endif /* CONFIG_VHOST_PASSTHRU */
> +#endif /* __KERNEL__ */
> +#endif /* __MPASSTHRU_H */
> -- 
> 1.5.4.4