Subject: Re: [PATCH V4 4/8]vhost: vhost TX zero-copy support

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 4 May 2011 12:56:50 +0300

On Wed, May 04, 2011 at 01:11:24AM -0700, Shirley Ma wrote:
> This patch maintains the outstanding userspace buffers in the 
> sequence it is delivered to vhost. The outstanding userspace buffers 
> will be marked as done once the lower device buffers DMA has finished. 
> This is monitored through last reference of kfree_skb callback. Two
> buffer index are used for this purpose.
> 
> The vhost passes the userspace buffers info to lower device skb 
> through message control. Since there will be some done DMAs when
> entering vhost handle_tx. The worse case is all buffers in the vq are
> in pending/done status, so we need to notify guest to release DMA done 
> buffers first before get any new buffers from the vq.
> 
> Signed-off-by: Shirley <[email-masked]>

Looks good overall. Some nits to iron out below.

> ---
> 
>  drivers/vhost/net.c   |   30 +++++++++++++++++++++++++++-
>  drivers/vhost/vhost.c |   50
> ++++++++++++++++++++++++++++++++++++++++++++++++-
>  drivers/vhost/vhost.h |   10 +++++++++
>  3 files changed, 87 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 2f7c76a..c403afb 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -32,6 +32,8 @@
>   * Using this limit prevents one virtqueue from starving others. */
>  #define VHOST_NET_WEIGHT 0x80000
>  
> +#define MAX_ZEROCOPY_PEND 64
> +

Pls document what the above is. Also scope with VHOST_

>  enum {
>  	VHOST_NET_VQ_RX = 0,
>  	VHOST_NET_VQ_TX = 1,
> @@ -129,6 +131,7 @@ static void handle_tx(struct vhost_net *net)
>  	int err, wmem;
>  	size_t hdr_size;
>  	struct socket *sock;
> +	struct skb_ubuf_info pend;
>  
>  	/* TODO: check that we are running from vhost_worker? */
>  	sock = rcu_dereference_check(vq->private_data, 1);
> @@ -151,6 +154,10 @@ static void handle_tx(struct vhost_net *net)
>  	hdr_size = vq->vhost_hlen;
>  
>  	for (;;) {
> +		/* Release DMAs done buffers first */
> +		if (sock_flag(sock->sk, SOCK_ZEROCOPY))
> +			vhost_zerocopy_signal_used(vq);
> +
>  		head = vhost_get_vq_desc(&net->dev, vq, vq->iov,
>  					 ARRAY_SIZE(vq->iov),
>  					 &out, &in,
> @@ -166,6 +173,12 @@ static void handle_tx(struct vhost_net *net)
>  				set_bit(SOCK_ASYNC_NOSPACE, &sock->flags);
>  				break;
>  			}
> +			/* If more outstanding DMAs, queue the work */
> +			if (sock_flag(sock->sk, SOCK_ZEROCOPY) &&
> +			    (atomic_read(&vq->refcnt) > MAX_ZEROCOPY_PEND)) {
> +				vhost_poll_queue(&vq->poll);

Well, this just keeps polling, doesn't it?
If you want to wait until # of DMAs is below MAX_ZEROCOPY_PEND,
you'll need to do the queueing from some callback.

Something like this: when refcnt is above 2 * MAX_ZEROCOPY_PEND,
stop submitting and wait until some are freed.

BTW, can the socket poll wakeup do the job?

> +				break;
> +			}
>  			if (unlikely(vhost_enable_notify(vq))) {
>  				vhost_disable_notify(vq);
>  				continue;
> @@ -188,17 +201,30 @@ static void handle_tx(struct vhost_net *net)
>  			       iov_length(vq->hdr, s), hdr_size);
>  			break;
>  		}
> +		/* use msg_control to pass vhost zerocopy ubuf info to skb */
> +		if (sock_flag(sock->sk, SOCK_ZEROCOPY)) {
> +			pend.callback = vhost_zerocopy_callback;
> +			pend.arg = vq;
> +			pend.desc = vq->upend_idx;
> +			msg.msg_control = &pend;
> +			msg.msg_controllen = sizeof(pend);
> +			vq->heads[vq->upend_idx].id = head;
> +			vq->upend_idx = (vq->upend_idx + 1) % UIO_MAXIOV;
> +			atomic_inc(&vq->refcnt);
> +		}
>  		/* TODO: Check specific error and bomb out unless ENOBUFS? */
>  		err = sock->ops->sendmsg(NULL, sock, &msg, len);
>  		if (unlikely(err < 0)) {
> -			vhost_discard_vq_desc(vq, 1);
> +			if (!sock_flag(sock->sk, SOCK_ZEROCOPY))
> +				vhost_discard_vq_desc(vq, 1);
>  			tx_poll_start(net, sock);
>  			break;
>  		}
>  		if (err != len)
>  			pr_debug("Truncated TX packet: "
>  				 " len %d != %zd\n", err, len);
> -		vhost_add_used_and_signal(&net->dev, vq, head, 0);
> +		if (!sock_flag(sock->sk, SOCK_ZEROCOPY))
> +			vhost_add_used_and_signal(&net->dev, vq, head, 0);
>  		total_len += len;
>  		if (unlikely(total_len >= VHOST_NET_WEIGHT)) {
>  			vhost_poll_queue(&vq->poll);
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 2ab2912..3048953 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -174,6 +174,9 @@ static void vhost_vq_reset(struct vhost_dev *dev,
>  	vq->call_ctx = NULL;
>  	vq->call = NULL;
>  	vq->log_ctx = NULL;
> +	vq->upend_idx = 0;
> +	vq->done_idx = 0;
> +	atomic_set(&vq->refcnt, 0);
>  }
>  
>  static int vhost_worker(void *data)
> @@ -230,7 +233,7 @@ static long vhost_dev_alloc_iovecs(struct vhost_dev
> *dev)
>  					       UIO_MAXIOV, GFP_KERNEL);
>  		dev->vqs[i].log = kmalloc(sizeof *dev->vqs[i].log * UIO_MAXIOV,
>  					  GFP_KERNEL);
> -		dev->vqs[i].heads = kmalloc(sizeof *dev->vqs[i].heads *
> +		dev->vqs[i].heads = kzalloc(sizeof *dev->vqs[i].heads *
>  					    UIO_MAXIOV, GFP_KERNEL);

Do we really need to zero it all out? We generally tried to only
init what is necessary ...

>  		if (!dev->vqs[i].indirect || !dev->vqs[i].log ||
> @@ -385,10 +388,41 @@ long vhost_dev_reset_owner(struct vhost_dev *dev)
>  	return 0;
>  }
>  

Pls document what the below does.

> +void vhost_zerocopy_signal_used(struct vhost_virtqueue *vq)
> +{
> +	int i, j = 0;
> +
> +	i = vq->done_idx;
> +	while (i != vq->upend_idx) {
> +		/* len = 1 means DMA done */

Hmm. Guests aren't likely to use len 1 in practice,
but I think it's better to support this.

I'd suggest sticking extra stuff in id, IIRC only values
< vq size are legal there, anything else we can use.
Also, pls add some defines for special values, better than
a comment:
	if (len == VHOST_DMA_DONE_LEN)

> +		if (vq->heads[i].len == 1) {
> +			/* reset len = 0 */
> +			vq->heads[i].len = 0;
> +			i = (i + 1) % UIO_MAXIOV;
> +			++j;
> +		} else
> +			break;
> +	}
> +	if (j) {

Pls add some comments to explain the logic here.

> +		if (i > vq->done_idx)
> +			vhost_add_used_n(vq, &vq->heads[vq->done_idx], j);
> +		else {
> +			vhost_add_used_n(vq, &vq->heads[vq->done_idx],
> +					 UIO_MAXIOV - vq->done_idx);
> +			vhost_add_used_n(vq, vq->heads, i);
> +		}
> +		vq->done_idx = i;
> +		vhost_signal(vq->dev, vq);
> +		atomic_sub(j, &vq->refcnt);
> +	}
> +}
> +
>  /* Caller should have device mutex */
>  void vhost_dev_cleanup(struct vhost_dev *dev)
>  {
>  	int i;
> +	unsigned long begin = jiffies;
> +
>  
>  	for (i = 0; i < dev->nvqs; ++i) {
>  		if (dev->vqs[i].kick && dev->vqs[i].handle_kick) {
> @@ -405,6 +439,11 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
>  			eventfd_ctx_put(dev->vqs[i].call_ctx);
>  		if (dev->vqs[i].call)
>  			fput(dev->vqs[i].call);
> +		/* wait for all lower device DMAs done, then notify guest */
> +		while (atomic_read(&dev->vqs[i].refcnt)) {
> +			if (time_after(jiffies, begin + 5 * HZ))

Hmm, does this actually busy-wait?  Let's at least sleep here.
Or maybe some wakeup scheme can be cooked up.
For example, have a kref with release function that signals some
completion.

> +				vhost_zerocopy_signal_used(&dev->vqs[i]);
> +		}
>  		vhost_vq_reset(dev, dev->vqs + i);
>  	}
>  	vhost_dev_free_iovecs(dev);
> @@ -1416,3 +1455,12 @@ void vhost_disable_notify(struct vhost_virtqueue
> *vq)
>  		vq_err(vq, "Failed to enable notification at %p: %d\n",
>  		       &vq->used->flags, r);
>  }
> +
> +void vhost_zerocopy_callback(struct sk_buff *skb)
> +{
> +	int idx = skb_shinfo(skb)->ubuf.desc;
> +	struct vhost_virtqueue *vq = skb_shinfo(skb)->ubuf.arg;
> +
> +	/* set len = 1 to mark this desc buffers done DMA */
> +	vq->heads[idx].len = 1;
> +}

So any kind of callback like that, that goes into the skb,
will be racy wrt module unloading because module can go away
after you mark dma done and before this function returns.
Solution is to have a core function that does the
final signalling (e.g. sock_wfree is in core).
Would be nice to fix, even though this race is
completely theoretical, I don't believe it will
trigger in practice.

> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index b3363ae..ec032a0 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -108,6 +108,14 @@ struct vhost_virtqueue {
>  	/* Log write descriptors */
>  	void __user *log_base;
>  	struct vhost_log *log;
> +	/* vhost zerocopy support */
> +	atomic_t refcnt; /* num of outstanding zerocopy DMAs */
> +	/* index of zerocopy pending DMA buffers */
> +	int upend_idx;
> +	/* index of zerocopy done DMA buffers, but not notify guest yet */
> +	int done_idx;

Pls try to find more descriptive names for the above,
and clarify the comments: I could not tell what do the
comments mean.

upend_idx seems to be a copy of avail idx?
done_idx is ... ?

> +	/* notify vhost zerocopy DMA buffers has done in lower device */

Do you mean 'notify vhost that zerocopy DMA is complete'?

> +	void (*callback)(struct sk_buff *);

Is this actually used?
If yes rename it zerocopy_dma_done or something like this?

>  };
>  
>  struct vhost_dev {
> @@ -154,6 +162,8 @@ bool vhost_enable_notify(struct vhost_virtqueue *);
>  
>  int vhost_log_write(struct vhost_virtqueue *vq, struct vhost_log *log,
>  		    unsigned int log_num, u64 len);
> +void vhost_zerocopy_callback(struct sk_buff *skb);
> +void vhost_zerocopy_signal_used(struct vhost_virtqueue *vq);
>  
>  #define vq_err(vq, fmt, ...) do {                                  \
>  		pr_debug(pr_fmt(fmt), ##__VA_ARGS__);       \
>