Subject: Re: [PATCH v1 3/3] Let host NIC driver to DMA to guest user space.

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 8 Mar 2010 13:18:26 +0200

On Sat, Mar 06, 2010 at 05:38:38PM +0800, [email-masked] wrote:
> From: Xin Xiaohui <[email-masked]>
> 
> The patch let host NIC driver to receive user space skb,
> then the driver has chance to directly DMA to guest user
> space buffers thru single ethX interface.
> 
> Signed-off-by: Xin Xiaohui <[email-masked]>
> Signed-off-by: Zhao Yu <[email-masked]>
> Sigend-off-by: Jeff Dike <[email-masked]>

I have a feeling I commented on some of the below issues already.
Do you plan to send a version with comments addressed?

> ---
>  include/linux/netdevice.h |   76 ++++++++++++++++++++++++++++++++++++++++++-
>  include/linux/skbuff.h    |   30 +++++++++++++++--
>  net/core/dev.c            |   32 ++++++++++++++++++
>  net/core/skbuff.c         |   79 +++++++++++++++++++++++++++++++++++++++++----
>  4 files changed, 205 insertions(+), 12 deletions(-)
> 
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 94958c1..97bf12c 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -485,6 +485,17 @@ struct netdev_queue {
>  	unsigned long		tx_dropped;
>  } ____cacheline_aligned_in_smp;
>  
> +#if defined(CONFIG_VHOST_PASSTHRU) || defined(CONFIG_VHOST_PASSTHRU_MODULE)
> +struct mpassthru_port	{
> +	int		hdr_len;
> +	int		data_len;
> +	int		npages;
> +	unsigned	flags;
> +	struct socket	*sock;
> +	struct skb_user_page	*(*ctor)(struct mpassthru_port *,
> +				struct sk_buff *, int);
> +};
> +#endif
>  
>  /*
>   * This structure defines the management hooks for network devices.
> @@ -636,6 +647,10 @@ struct net_device_ops {
>  	int			(*ndo_fcoe_ddp_done)(struct net_device *dev,
>  						     u16 xid);
>  #endif
> +#if defined(CONFIG_VHOST_PASSTHRU) || defined(CONFIG_VHOST_PASSTHRU_MODULE)
> +	int			(*ndo_mp_port_prep)(struct net_device *dev,
> +						struct mpassthru_port *port);
> +#endif
>  };
>  
>  /*
> @@ -891,7 +906,8 @@ struct net_device
>  	struct macvlan_port	*macvlan_port;
>  	/* GARP */
>  	struct garp_port	*garp_port;
> -
> +	/* mpassthru */
> +	struct mpassthru_port	*mp_port;
>  	/* class/net/name entry */
>  	struct device		dev;
>  	/* space for optional statistics and wireless sysfs groups */
> @@ -2013,6 +2029,62 @@ static inline u32 dev_ethtool_get_flags(struct net_device *dev)
>  		return 0;
>  	return dev->ethtool_ops->get_flags(dev);
>  }
> -#endif /* __KERNEL__ */
>  
> +#if defined(CONFIG_VHOST_PASSTHRU) || defined(CONFIG_VHOST_PASSTHRU_MODULE)
> +static inline int netdev_mp_port_prep(struct net_device *dev,
> +		struct mpassthru_port *port)
> +{

This function lacks documentation.

> +	int rc;
> +	int npages, data_len;
> +	const struct net_device_ops *ops = dev->netdev_ops;
> +
> +	/* needed by packet split */
> +	if (ops->ndo_mp_port_prep) {
> +		rc = ops->ndo_mp_port_prep(dev, port);
> +		if (rc)
> +			return rc;
> +	} else {  /* should be temp */
> +		port->hdr_len = 128;
> +		port->data_len = 2048;
> +		port->npages = 1;

where do the numbers come from?

> +	}
> +
> +	if (port->hdr_len <= 0)
> +		goto err;
> +
> +	npages = port->npages;
> +	data_len = port->data_len;
> +	if (npages <= 0 || npages > MAX_SKB_FRAGS ||
> +			(data_len < PAGE_SIZE * (npages - 1) ||
> +			 data_len > PAGE_SIZE * npages))
> +		goto err;
> +
> +	return 0;
> +err:
> +	dev_warn(&dev->dev, "invalid page constructor parameters\n");
> +
> +	return -EINVAL;
> +}
> +
> +static inline int netdev_mp_port_attach(struct net_device *dev,
> +		struct mpassthru_port *port)
> +{
> +	if (rcu_dereference(dev->mp_port))
> +		return -EBUSY;
> +
> +	rcu_assign_pointer(dev->mp_port, port);
> +
> +	return 0;
> +}
> +
> +static inline void netdev_mp_port_detach(struct net_device *dev)
> +{
> +	if (!rcu_dereference(dev->mp_port))
> +		return;
> +
> +	rcu_assign_pointer(dev->mp_port, NULL);
> +	synchronize_rcu();
> +}

The above looks wrong, rcu_dereference should be called
under rcu read side, rcu_assign_pointer usually should not,
synchronize_rcu definitely should not.

As I suggested already, these functions are better opencoded,
rcu is tricky as is without hiding it in inline helpers.

> +#endif /* CONFIG_VHOST_PASSTHRU */
> +#endif /* __KERNEL__ */
>  #endif	/* _LINUX_NETDEVICE_H */
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index df7b23a..e59fa57 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -209,6 +209,13 @@ struct skb_shared_info {
>  	void *		destructor_arg;
>  };
>  
> +struct skb_user_page {
> +	u8              *start;
> +	int             size;
> +	struct skb_frag_struct *frags;
> +	struct skb_shared_info *ushinfo;
> +	void		(*dtor)(struct skb_user_page *);
> +};
>  /* We divide dataref into two halves.  The higher 16 bits hold references
>   * to the payload part of skb->data.  The lower 16 bits hold references to
>   * the entire skb->data.  A clone of a headerless skb holds the length of
> @@ -441,17 +448,18 @@ extern void kfree_skb(struct sk_buff *skb);
>  extern void consume_skb(struct sk_buff *skb);
>  extern void	       __kfree_skb(struct sk_buff *skb);
>  extern struct sk_buff *__alloc_skb(unsigned int size,
> -				   gfp_t priority, int fclone, int node);
> +				   gfp_t priority, int fclone,
> +				   int node, struct net_device *dev);
>  static inline struct sk_buff *alloc_skb(unsigned int size,
>  					gfp_t priority)
>  {
> -	return __alloc_skb(size, priority, 0, -1);
> +	return __alloc_skb(size, priority, 0, -1, NULL);
>  }
>  
>  static inline struct sk_buff *alloc_skb_fclone(unsigned int size,
>  					       gfp_t priority)
>  {
> -	return __alloc_skb(size, priority, 1, -1);
> +	return __alloc_skb(size, priority, 1, -1, NULL);
>  }
>  
>  extern int skb_recycle_check(struct sk_buff *skb, int skb_size);
> @@ -1509,6 +1517,22 @@ static inline void netdev_free_page(struct net_device *dev, struct page *page)
>  	__free_page(page);
>  }
>  
> +extern struct skb_user_page *netdev_alloc_user_pages(struct net_device *dev,
> +			struct sk_buff *skb, int npages);
> +
> +static inline struct skb_user_page *netdev_alloc_user_page(
> +		struct net_device *dev,
> +		struct sk_buff *skb, unsigned int size)
> +{
> +	struct skb_user_page *user;
> +	int npages = (size < PAGE_SIZE) ? 1 : (size / PAGE_SIZE);

Should round up to full pages?

> +
> +	user = netdev_alloc_user_pages(dev, skb, npages);
> +	if (likely(user))
> +		return user;
> +	return NULL;
> +}
> +
>  /**
>   *	skb_clone_writable - is the header of a clone writable
>   *	@skb: buffer to check
> diff --git a/net/core/dev.c b/net/core/dev.c
> index b8f74cf..ab8b082 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -2265,6 +2265,30 @@ void netif_nit_deliver(struct sk_buff *skb)
>  	rcu_read_unlock();
>  }
>  
> +#if defined(CONFIG_VHOST_PASSTHRU) || defined(CONFIG_VHOST_PASSTHRU_MODULE)
> +static inline struct sk_buff *handle_mpassthru(struct sk_buff *skb,
> +					struct packet_type **pt_prev,
> +					int *ret, struct net_device *orig_dev)

please document

pt_prev, orig_dev and ret seem unused?

> +{
> +	struct mpassthru_port *ctor = NULL;

Why do you call the port "ctor"?

> +	struct sock *sk = NULL;
> +
> +	if (skb->dev)
> +		ctor = skb->dev->mp_port;
> +	if (!ctor)
> +		return skb;
> +
> +	sk = ctor->sock->sk;
> +
> +	skb_queue_tail(&sk->sk_receive_queue, skb);
> +
> +	sk->sk_data_ready(sk, skb->len);
> +	return NULL;
> +}
> +#else
> +#define handle_mpassthru(skb, pt_prev, ret, orig_dev)      (skb)
> +#endif
> +
>  /**
>   *	netif_receive_skb - process receive buffer from network
>   *	@skb: buffer to process
> @@ -2342,6 +2366,9 @@ int netif_receive_skb(struct sk_buff *skb)
>  		goto out;
>  ncls:
>  #endif
> +	skb = handle_mpassthru(skb, &pt_prev, &ret, orig_dev);
> +	if (!skb)
> +		goto out;
>  
>  	skb = handle_bridge(skb, &pt_prev, &ret, orig_dev);
>  	if (!skb)
> @@ -2455,6 +2482,11 @@ int dev_gro_receive(struct napi_struct *napi, struct sk_buff *skb)
>  	if (skb_is_gso(skb) || skb_has_frags(skb))
>  		goto normal;
>  
> +#if defined(CONFIG_VHOST_PASSTHRU) || defined(CONFIG_VHOST_PASSTHRU_MODULE)
> +	if (skb->dev && skb->dev->mp_port)
> +		goto normal;
> +#endif
> +
>  	rcu_read_lock();
>  	list_for_each_entry_rcu(ptype, head, list) {
>  		if (ptype->type != type || ptype->dev || !ptype->gro_receive)
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 80a9616..6510e5b 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -170,13 +170,15 @@ EXPORT_SYMBOL(skb_under_panic);
>   *	%GFP_ATOMIC.
>   */
>  struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
> -			    int fclone, int node)
> +			    int fclone, int node, struct net_device *dev)
>  {
>  	struct kmem_cache *cache;
>  	struct skb_shared_info *shinfo;
>  	struct sk_buff *skb;
>  	u8 *data;
> -
> +#if defined(CONFIG_VHOST_PASSTHRU) || defined(CONFIG_VHOST_PASSTHRU_MODULE)
> +	struct skb_user_page *user = NULL;
> +#endif
>  	cache = fclone ? skbuff_fclone_cache : skbuff_head_cache;
>  
>  	/* Get the HEAD */
> @@ -185,8 +187,26 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
>  		goto out;
>  
>  	size = SKB_DATA_ALIGN(size);
> -	data = kmalloc_node_track_caller(size + sizeof(struct skb_shared_info),
> -			gfp_mask, node);
> +#if defined(CONFIG_VHOST_PASSTHRU) || defined(CONFIG_VHOST_PASSTHRU_MODULE)
> +	if (!dev || !dev->mp_port) { /* Legacy alloc func */
> +#endif
> +		data = kmalloc_node_track_caller(
> +				size + sizeof(struct skb_shared_info),
> +				gfp_mask, node);
> +#if defined(CONFIG_VHOST_PASSTHRU) || defined(CONFIG_VHOST_PASSTHRU_MODULE)
> +	} else { /* Allocation may from page constructor of device */

what does the comment mean?

> +		user = netdev_alloc_user_page(dev, skb, size);
> +		if (!user) {
> +			data = kmalloc_node_track_caller(
> +				size + sizeof(struct skb_shared_info),
> +				gfp_mask, node);
> +			printk(KERN_INFO "can't alloc user buffer.\n");
> +		} else {
> +			data = user->start;
> +			size = SKB_DATA_ALIGN(user->size);
> +		}
> +	}
> +#endif
>  	if (!data)
>  		goto nodata;
>  
> @@ -208,6 +228,11 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
>  	skb->mac_header = ~0U;
>  #endif
>  
> +#if defined(CONFIG_VHOST_PASSTHRU) || defined(CONFIG_VHOST_PASSTHRU_MODULE)
> +	if (user)
> +		memcpy(user->ushinfo, skb_shinfo(skb),
> +				sizeof(struct skb_shared_info));
> +#endif
>  	/* make sure we initialize shinfo sequentially */
>  	shinfo = skb_shinfo(skb);
>  	atomic_set(&shinfo->dataref, 1);
> @@ -231,6 +256,10 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
>  
>  		child->fclone = SKB_FCLONE_UNAVAILABLE;
>  	}
> +#if defined(CONFIG_VHOST_PASSTHRU) || defined(CONFIG_VHOST_PASSTHRU_MODULE)
> +	shinfo->destructor_arg = user;
> +#endif
> +
>  out:
>  	return skb;
>  nodata:
> @@ -259,7 +288,7 @@ struct sk_buff *__netdev_alloc_skb(struct net_device *dev,
>  	int node = dev->dev.parent ? dev_to_node(dev->dev.parent) : -1;
>  	struct sk_buff *skb;
>  
> -	skb = __alloc_skb(length + NET_SKB_PAD, gfp_mask, 0, node);
> +	skb = __alloc_skb(length + NET_SKB_PAD, gfp_mask, 0, node, dev);
>  	if (likely(skb)) {
>  		skb_reserve(skb, NET_SKB_PAD);
>  		skb->dev = dev;
> @@ -278,6 +307,29 @@ struct page *__netdev_alloc_page(struct net_device *dev, gfp_t gfp_mask)
>  }
>  EXPORT_SYMBOL(__netdev_alloc_page);
>  
> +#if defined(CONFIG_VHOST_PASSTHRU) || defined(CONFIG_VHOST_PASSTHRU_MODULE)
> +struct skb_user_page *netdev_alloc_user_pages(struct net_device *dev,
> +			struct sk_buff *skb, int npages)
> +{
> +	struct mpassthru_port *ctor;
> +	struct skb_user_page *user = NULL;
> +
> +	rcu_read_lock();
> +	ctor = rcu_dereference(dev->mp_port);
> +	if (!ctor)
> +		goto out;
> +
> +	BUG_ON(npages > ctor->npages);
> +
> +	user = ctor->ctor(ctor, skb, npages);

With the assumption that "ctor" pins userspace pages,
can't it sleep? If yes you can't call it under rcu read side
critical section.

> +out:
> +	rcu_read_unlock();
> +
> +	return user;
> +}
> +EXPORT_SYMBOL(netdev_alloc_user_pages);
> +#endif
> +
>  void skb_add_rx_frag(struct sk_buff *skb, int i, struct page *page, int off,
>  		int size)
>  {
> @@ -338,6 +390,10 @@ static void skb_clone_fraglist(struct sk_buff *skb)
>  
>  static void skb_release_data(struct sk_buff *skb)
>  {
> +#if defined(CONFIG_VHOST_PASSTHRU) || defined(CONFIG_VHOST_PASSTHRU_MODULE)
> +	struct skb_user_page *user = skb_shinfo(skb)->destructor_arg;
> +#endif
> +
>  	if (!skb->cloned ||
>  	    !atomic_sub_return(skb->nohdr ? (1 << SKB_DATAREF_SHIFT) + 1 : 1,
>  			       &skb_shinfo(skb)->dataref)) {
> @@ -349,7 +405,10 @@ static void skb_release_data(struct sk_buff *skb)
>  
>  		if (skb_has_frags(skb))
>  			skb_drop_fraglist(skb);
> -
> +#if defined(CONFIG_VHOST_PASSTHRU) || defined(CONFIG_VHOST_PASSTHRU_MODULE)
> +		if (skb->dev && skb->dev->mp_port && user && user->dtor)
> +			user->dtor(user);
> +#endif
>  		kfree(skb->head);
>  	}
>  }
> @@ -503,8 +562,14 @@ int skb_recycle_check(struct sk_buff *skb, int skb_size)
>  	if (skb_shared(skb) || skb_cloned(skb))
>  		return 0;
>  
> -	skb_release_head_state(skb);
> +#if defined(CONFIG_VHOST_PASSTHRU) || defined(CONFIG_VHOST_PASSTHRU_MODULE)
> +	if (skb->dev && skb->dev->mp_port)
> +		return 0;
> +#endif
> +
>  	shinfo = skb_shinfo(skb);
> +
> +	skb_release_head_state(skb);
>  	atomic_set(&shinfo->dataref, 1);
>  	shinfo->nr_frags = 0;
>  	shinfo->gso_size = 0;
> -- 
> 1.5.4.4