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Date:	Wed, 17 Mar 2010 12:27:10 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	"Xin, Xiaohui" <xiaohui.xin@...el.com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"jdike@...toit.com" <jdike@...toit.com>
Subject: Re: [PATCH v1 2/3] Provides multiple submits and asynchronous
	notifications.

On Wed, Mar 17, 2010 at 05:48:10PM +0800, Xin, Xiaohui wrote:
> >> Michael,
> >> I don't use the kiocb comes from the sendmsg/recvmsg,
> > >since I have embeded the kiocb in page_info structure,
> > >and allocate it when page_info allocated.
> 
> >So what I suggested was that vhost allocates and tracks the iocbs, and
> >passes them to your device with sendmsg/ recvmsg calls. This way your
> >device won't need to share structures and locking strategy with vhost:
> >you get an iocb, handle it, invoke a callback to notify vhost about
> >completion.
> 
> >This also gets rid of the 'receiver' callback
> 
> I'm not sure receiver callback can be removed here:
> The patch describes a work flow like this:
> netif_receive_skb() gets the packet, it does nothing but just queue the skb
> and wakeup the handle_rx() of vhost. handle_rx() then calls the receiver callback
> to deal with skb and and get the necessary notify info into a list, vhost owns the 
> list and in the same handle_rx() context use it to complete.
> 
> We use "receiver" callback here is because only handle_rx() is waked up from
> netif_receive_skb(), and we need mp device context to deal with the skb and
> notify info attached to it. We also have some lock in the callback function.
> 
> If I remove the receiver callback, I can only deal with the skb and notify
> info in netif_receive_skb(), but this function is in an interrupt context,
> which I think lock is not allowed there. But I cannot remove the lock there.
> 

The basic idea is that vhost passes iocb to recvmsg and backend
completes the iocb to signal that data is ready. That completion could
be in interrupt context and so we need to switch to workqueue to handle
the event, it is true, but the code to do this would live in vhost.c or
net.c.

With this structure your device won't depend on
vhost, and can go under drivers/net/, opening up possibility
to use it for zero copy without vhost in the future.



> >> Please have a review and thanks for the instruction
> >> for replying email which helps me a lot.
> >> 
> > >Thanks,
> > >Xiaohui
> > >
> > > drivers/vhost/net.c   |  159 +++++++++++++++++++++++++++++++++++++++++++++++--
> >>  drivers/vhost/vhost.h |   12 ++++
> >>  2 files changed, 166 insertions(+), 5 deletions(-)
> >> 
> >> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > >index 22d5fef..5483848 100644
> > >--- a/drivers/vhost/net.c
> > >+++ b/drivers/vhost/net.c
> > >@@ -17,11 +17,13 @@
> > > #include <linux/workqueue.h>
> > > #include <linux/rcupdate.h>
> > > #include <linux/file.h>
> > >+#include <linux/aio.h>
> > > 
> > > #include <linux/net.h>
> > > #include <linux/if_packet.h>
> > > #include <linux/if_arp.h>
> > > #include <linux/if_tun.h>
> > >+#include <linux/mpassthru.h>
> > > 
> > > #include <net/sock.h>
> > > 
> > >@@ -91,6 +93,12 @@ static void tx_poll_start(struct vhost_net *net, struct socket *sock)
> > > 	net->tx_poll_state = VHOST_NET_POLL_STARTED;
> > > }
> > > 
> > >+static void handle_async_rx_events_notify(struct vhost_net *net,
> > >+					struct vhost_virtqueue *vq);
> > >+
> > >+static void handle_async_tx_events_notify(struct vhost_net *net,
> > >+					struct vhost_virtqueue *vq);
> > >+
> 
> >A couple of style comments:
> >
> >- It's better to arrange functions in such order that forward declarations
> >aren't necessary.  Since we don't have recursion, this should always be
> >possible.
> 
> >- continuation lines should be idented at least at the position of '('
> >on the previous line.
> 
> Thanks. I'd correct that.
> 
> >>  /* Expects to be always run from workqueue - which acts as
> >>   * read-size critical section for our kind of RCU. */
> >>  static void handle_tx(struct vhost_net *net)
> >> @@ -124,6 +132,8 @@ static void handle_tx(struct vhost_net *net)
> >>  		tx_poll_stop(net);
> >>  	hdr_size = vq->hdr_size;
> >>  
> >> +	handle_async_tx_events_notify(net, vq);
> > >+
> >>  	for (;;) {
> >>  		head = vhost_get_vq_desc(&net->dev, vq, vq->iov,
> >>  					 ARRAY_SIZE(vq->iov),
> > >@@ -151,6 +161,12 @@ static void handle_tx(struct vhost_net *net)
> >>  		/* Skip header. TODO: support TSO. */
> >>  		s = move_iovec_hdr(vq->iov, vq->hdr, hdr_size, out);
> >>  		msg.msg_iovlen = out;
> > >+
> > >+		if (vq->link_state == VHOST_VQ_LINK_ASYNC) {
> > >+			vq->head = head;
> > >+			msg.msg_control = (void *)vq;
> 
> >So here a device gets a pointer to vhost_virtqueue structure. If it gets
> >an iocb and invokes a callback, it would not care about vhost internals.
> 
> >> +		}
> >> +
> >> 		len = iov_length(vq->iov, out);
> >>  		/* Sanity check */
> >>  		if (!len) {
> >> @@ -166,6 +182,10 @@ static void handle_tx(struct vhost_net *net)
> >>  			tx_poll_start(net, sock);
> >>  			break;
> >>  		}
> >> +
> >> +		if (vq->link_state == VHOST_VQ_LINK_ASYNC)
> >> +			continue;
> >>+
> >>  		if (err != len)
> >>  			pr_err("Truncated TX packet: "
> >>  			       " len %d != %zd\n", err, len);
> >> @@ -177,6 +197,8 @@ static void handle_tx(struct vhost_net *net)
> >>  		}
> >>  	}
> >>  
> >> +	handle_async_tx_events_notify(net, vq);
> >> +
> >>  	mutex_unlock(&vq->mutex);
> >>  	unuse_mm(net->dev.mm);
> >>  }
> >>@@ -206,7 +228,8 @@ static void handle_rx(struct vhost_net *net)
> >>  	int err;
> >>  	size_t hdr_size;
> >>  	struct socket *sock = rcu_dereference(vq->private_data);
> >> -	if (!sock || skb_queue_empty(&sock->sk->sk_receive_queue))
> >> +	if (!sock || (skb_queue_empty(&sock->sk->sk_receive_queue) &&
> >> +			vq->link_state == VHOST_VQ_LINK_SYNC))
> >>  		return;
> >>  
> >>  	use_mm(net->dev.mm);
> >> @@ -214,9 +237,18 @@ static void handle_rx(struct vhost_net *net)
> >>  	vhost_disable_notify(vq);
> >>  	hdr_size = vq->hdr_size;
> >>  
> >> -	vq_log = unlikely(vhost_has_feature(&net->dev, VHOST_F_LOG_ALL)) ?
> >> +	/* In async cases, for write logging, the simple way is to get
> >> +	 * the log info always, and really logging is decided later.
> >>+	 * Thus, when logging enabled, we can get log, and when logging
> >> +	 * disabled, we can get log disabled accordingly.
> >> +	 */
> >> +
> 
> 
> >This adds overhead and might be one of the reasons
> >your patch does not perform that well. A better way
> >would be to flush outstanding requests or reread the vq
> >when logging is enabled.
> 
> Since the guest may submit a lot of buffers and h/w have already used them
> to allocate host skb, it's difficult to know how many and which one is the
> outstanding request, it's not just only inside in notifier list or sk->receive_queue.

Well, that was just a thought.  I guess there needs to be some way to
recover outstanding requests at least for cleanup when device is closed?
Maybe we could put in a special request marked "flush" and wait until it
completes?

> But what does reread mean? 

If we want to know the physical address of the iovec, we can look in the
virtqueue to find it. I do this in one go when building up the iovec
now, but logged mode is not a common case so it is not a must.

> > +	vq_log = unlikely(vhost_has_feature(&net->dev, VHOST_F_LOG_ALL)) |
> > +		(vq->link_state == VHOST_VQ_LINK_ASYNC) ?
> >  		vq->log : NULL;
> >  
> > +	handle_async_rx_events_notify(net, vq);
> > +
> >  	for (;;) {
> >  		head = vhost_get_vq_desc(&net->dev, vq, vq->iov,
> >  					 ARRAY_SIZE(vq->iov),
> > @@ -245,6 +277,11 @@ static void handle_rx(struct vhost_net *net)
> >  		s = move_iovec_hdr(vq->iov, vq->hdr, hdr_size, in);
> >  		msg.msg_iovlen = in;
> >  		len = iov_length(vq->iov, in);
> > +		if (vq->link_state == VHOST_VQ_LINK_ASYNC) {
> > +			vq->head = head;
> > +			vq->_log = log;
> > +			msg.msg_control = (void *)vq;
> > +		}
> >  		/* Sanity check */
> >  		if (!len) {
> >  			vq_err(vq, "Unexpected header len for RX: "
> > @@ -259,6 +296,10 @@ static void handle_rx(struct vhost_net *net)
> >  			vhost_discard_vq_desc(vq);
> >  			break;
> >  		}
> > +
> > +		if (vq->link_state == VHOST_VQ_LINK_ASYNC)
> > +			continue;
> > +
> >  		/* TODO: Should check and handle checksum. */
> >  		if (err > len) {
> >  			pr_err("Discarded truncated rx packet: "
> > @@ -284,10 +325,85 @@ static void handle_rx(struct vhost_net *net)
> >  		}
> >  	}
> >  
> > +	handle_async_rx_events_notify(net, vq);
> > +
> >  	mutex_unlock(&vq->mutex);
> >  	unuse_mm(net->dev.mm);
> >  }
> >  
> > +struct kiocb *notify_dequeue(struct vhost_virtqueue *vq)
> > +{
> > +	struct kiocb *iocb = NULL;
> > +	unsigned long flags;
> > +
> > +	spin_lock_irqsave(&vq->notify_lock, flags);
> > +	if (!list_empty(&vq->notifier)) {
> > +		iocb = list_first_entry(&vq->notifier,
> > +				struct kiocb, ki_list);
> > +		list_del(&iocb->ki_list);
> > +	}
> > +	spin_unlock_irqrestore(&vq->notify_lock, flags);
> > +	return iocb;
> > +}
> > +
> > +static void handle_async_rx_events_notify(struct vhost_net *net,
> > +				struct vhost_virtqueue *vq)
> > +{
> > +	struct kiocb *iocb = NULL;
> > +	struct vhost_log *vq_log = NULL;
> > +	int rx_total_len = 0;
> > +	int log, size;
> > +
> > +	if (vq->link_state != VHOST_VQ_LINK_ASYNC)
> > +		return;
> > +	if (vq != &net->dev.vqs[VHOST_NET_VQ_RX])
> > +		return;
> > +
> > +	if (vq->receiver)
> > +		vq->receiver(vq);
> > +	vq_log = unlikely(vhost_has_feature(
> > +				&net->dev, VHOST_F_LOG_ALL)) ? vq->log : NULL;
> > +	while ((iocb = notify_dequeue(vq)) != NULL) {
> > +		vhost_add_used_and_signal(&net->dev, vq,
> > +				iocb->ki_pos, iocb->ki_nbytes);
> > +		log = (int)iocb->ki_user_data;
> > +		size = iocb->ki_nbytes;
> > +		rx_total_len += iocb->ki_nbytes;
> > +		if (iocb->ki_dtor)
> > +			iocb->ki_dtor(iocb);
> > +		if (unlikely(vq_log))
> > +			vhost_log_write(vq, vq_log, log, size);
> > +		if (unlikely(rx_total_len >= VHOST_NET_WEIGHT)) {
> > +			vhost_poll_queue(&vq->poll);
> > +			break;
> > +		}
> > +	}
> > +}
> > +
> > +static void handle_async_tx_events_notify(struct vhost_net *net,
> > +		struct vhost_virtqueue *vq)
> > +{
> > +	struct kiocb *iocb = NULL;
> > +	int tx_total_len = 0;
> > +
> > +	if (vq->link_state != VHOST_VQ_LINK_ASYNC)
> > +		return;
> > +	if (vq != &net->dev.vqs[VHOST_NET_VQ_TX])
> > +		return;
> > +
> 
> Hard to see why the second check would be necessary
> 
> > +	while ((iocb = notify_dequeue(vq)) != NULL) {
> > +		vhost_add_used_and_signal(&net->dev, vq,
> > +				iocb->ki_pos, 0);
> > +		tx_total_len += iocb->ki_nbytes;
> > +		if (iocb->ki_dtor)
> > +			iocb->ki_dtor(iocb);
> > +		if (unlikely(tx_total_len >= VHOST_NET_WEIGHT)) {
> > +			vhost_poll_queue(&vq->poll);
> > +			break;
> > +		}
> > +	}
> > +}
> > +
> >  static void handle_tx_kick(struct work_struct *work)
> >  {
> >  	struct vhost_virtqueue *vq;
> > @@ -462,7 +578,19 @@ static struct socket *get_tun_socket(int fd)
> >  	return sock;
> >  }
> >  
> > -static struct socket *get_socket(int fd)
> > +static struct socket *get_mp_socket(int fd)
> > +{
> > +	struct file *file = fget(fd);
> > +	struct socket *sock;
> > +	if (!file)
> > +		return ERR_PTR(-EBADF);
> > +	sock = mp_get_socket(file);
> > +	if (IS_ERR(sock))
> > +		fput(file);
> > +	return sock;
> > +}
> > +
> > +static struct socket *get_socket(struct vhost_virtqueue *vq, int fd)
> >  {
> >  	struct socket *sock;
> >  	if (fd == -1)
> > @@ -473,9 +601,26 @@ static struct socket *get_socket(int fd)
> >  	sock = get_tun_socket(fd);
> >  	if (!IS_ERR(sock))
> >  		return sock;
> > +	sock = get_mp_socket(fd);
> > +	if (!IS_ERR(sock)) {
> > +		vq->link_state = VHOST_VQ_LINK_ASYNC;
> > +		return sock;
> > +	}
> >  	return ERR_PTR(-ENOTSOCK);
> >  }
> >  
> > +static void vhost_init_link_state(struct vhost_net *n, int index)
> > +{
> > +	struct vhost_virtqueue *vq = n->vqs + index;
> > +
> > +	WARN_ON(!mutex_is_locked(&vq->mutex));
> > +	if (vq->link_state == VHOST_VQ_LINK_ASYNC) {
> > +		vq->receiver = NULL;
> > +		INIT_LIST_HEAD(&vq->notifier);
> > +		spin_lock_init(&vq->notify_lock);
> > +	}
> > +}
> > +
> >  static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
> >  {
> >  	struct socket *sock, *oldsock;
> > @@ -493,12 +638,15 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
> >  	}
> >  	vq = n->vqs + index;
> >  	mutex_lock(&vq->mutex);
> > -	sock = get_socket(fd);
> > +	vq->link_state = VHOST_VQ_LINK_SYNC;
> > +	sock = get_socket(vq, fd);
> >  	if (IS_ERR(sock)) {
> >  		r = PTR_ERR(sock);
> >  		goto err;
> >  	}
> >  
> > +	vhost_init_link_state(n, index);
> > +
> >  	/* start polling new socket */
> >  	oldsock = vq->private_data;
> >  	if (sock == oldsock)
> > @@ -507,8 +655,8 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
> >  	vhost_net_disable_vq(n, vq);
> >  	rcu_assign_pointer(vq->private_data, sock);
> >  	vhost_net_enable_vq(n, vq);
> > -	mutex_unlock(&vq->mutex);
> >  done:
> > +	mutex_unlock(&vq->mutex);
> >  	mutex_unlock(&n->dev.mutex);
> >  	if (oldsock) {
> >  		vhost_net_flush_vq(n, index);
> > @@ -516,6 +664,7 @@ done:
> >  	}
> >  	return r;
> >  err:
> > +	mutex_unlock(&vq->mutex);
> >  	mutex_unlock(&n->dev.mutex);
> >  	return r;
> >  }
> > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> > index d1f0453..297af1c 100644
> > --- a/drivers/vhost/vhost.h
> > +++ b/drivers/vhost/vhost.h
> > @@ -43,6 +43,11 @@ struct vhost_log {
> >  	u64 len;
> >  };
> >  
> > +enum vhost_vq_link_state {
> > +	VHOST_VQ_LINK_SYNC = 	0,
> > +	VHOST_VQ_LINK_ASYNC = 	1,
> > +};
> > +
> >  /* The virtqueue structure describes a queue attached to a device. */
> >  struct vhost_virtqueue {
> >  	struct vhost_dev *dev;
> > @@ -96,6 +101,13 @@ struct vhost_virtqueue {
> >  	/* Log write descriptors */
> >  	void __user *log_base;
> >  	struct vhost_log log[VHOST_NET_MAX_SG];
> > +	/*Differiate async socket for 0-copy from normal*/
> > +	enum vhost_vq_link_state link_state;
> > +	int head;
> > +	int _log;
> > +	struct list_head notifier;
> > +	spinlock_t notify_lock;
> > +	void (*receiver)(struct vhost_virtqueue *);
> >  };
> >  
> >  struct vhost_dev {
> > -- 
> > 1.5.4.4
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