Subject: Re: [PATCH v6] Add mergeable rx buffer support to vhost_net

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 27 Apr 2010 00:42:54 +0300

On Mon, Apr 26, 2010 at 02:20:52PM -0700, David L Stevens wrote:
> This patch adds mergeable receive buffer support to vhost_net.
> 
> 					+-DLS
> 
> Signed-off-by: David L Stevens <[email-masked]>

OK, looks good. I still think iovec handling is a bit off,
as commented below.

> diff -ruNp net-next-v0/drivers/vhost/net.c net-next-v6/drivers/vhost/net.c
> --- net-next-v0/drivers/vhost/net.c	2010-04-24 21:36:54.000000000 -0700
> +++ net-next-v6/drivers/vhost/net.c	2010-04-26 01:13:04.000000000 -0700
> @@ -109,7 +109,7 @@ static void handle_tx(struct vhost_net *
>  	};
>  	size_t len, total_len = 0;
>  	int err, wmem;
> -	size_t hdr_size;
> +	size_t vhost_hlen;
>  	struct socket *sock = rcu_dereference(vq->private_data);
>  	if (!sock)
>  		return;
> @@ -128,13 +128,13 @@ static void handle_tx(struct vhost_net *
>  
>  	if (wmem < sock->sk->sk_sndbuf / 2)
>  		tx_poll_stop(net);
> -	hdr_size = vq->hdr_size;
> +	vhost_hlen = vq->vhost_hlen;
>  
>  	for (;;) {
> -		head = vhost_get_vq_desc(&net->dev, vq, vq->iov,
> -					 ARRAY_SIZE(vq->iov),
> -					 &out, &in,
> -					 NULL, NULL);
> +		head = vhost_get_desc(&net->dev, vq, vq->iov,
> +				      ARRAY_SIZE(vq->iov),
> +				      &out, &in,
> +				      NULL, NULL);
>  		/* Nothing new?  Wait for eventfd to tell us they refilled. */
>  		if (head == vq->num) {
>  			wmem = atomic_read(&sock->sk->sk_wmem_alloc);
> @@ -155,20 +155,20 @@ static void handle_tx(struct vhost_net *
>  			break;
>  		}
>  		/* Skip header. TODO: support TSO. */
> -		s = move_iovec_hdr(vq->iov, vq->hdr, hdr_size, out);
> +		s = move_iovec_hdr(vq->iov, vq->hdr, vhost_hlen, out);
>  		msg.msg_iovlen = out;
>  		len = iov_length(vq->iov, out);
>  		/* Sanity check */
>  		if (!len) {
>  			vq_err(vq, "Unexpected header len for TX: "
>  			       "%zd expected %zd\n",
> -			       iov_length(vq->hdr, s), hdr_size);
> +			       iov_length(vq->hdr, s), vhost_hlen);
>  			break;
>  		}
>  		/* 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);
> +			vhost_discard_desc(vq, 1);
>  			tx_poll_start(net, sock);
>  			break;
>  		}
> @@ -187,12 +187,25 @@ static void handle_tx(struct vhost_net *
>  	unuse_mm(net->dev.mm);
>  }
>  
> +static int vhost_head_len(struct vhost_virtqueue *vq, struct sock *sk)
> +{
> +	struct sk_buff *head;
> +	int len = 0;
> +
> +	lock_sock(sk);
> +	head = skb_peek(&sk->sk_receive_queue);
> +	if (head)
> +		len = head->len + vq->sock_hlen;
> +	release_sock(sk);
> +	return len;
> +}
> +
>  /* Expects to be always run from workqueue - which acts as
>   * read-size critical section for our kind of RCU. */
>  static void handle_rx(struct vhost_net *net)
>  {
>  	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_RX];
> -	unsigned head, out, in, log, s;
> +	unsigned in, log, s;
>  	struct vhost_log *vq_log;
>  	struct msghdr msg = {
>  		.msg_name = NULL,
> @@ -203,14 +216,14 @@ static void handle_rx(struct vhost_net *
>  		.msg_flags = MSG_DONTWAIT,
>  	};
>  
> -	struct virtio_net_hdr hdr = {
> -		.flags = 0,
> -		.gso_type = VIRTIO_NET_HDR_GSO_NONE
> +	struct virtio_net_hdr_mrg_rxbuf hdr = {
> +		.hdr.flags = 0,
> +		.hdr.gso_type = VIRTIO_NET_HDR_GSO_NONE
>  	};
>  
>  	size_t len, total_len = 0;
> -	int err;
> -	size_t hdr_size;
> +	int err, headcount, datalen;
> +	size_t vhost_hlen;
>  	struct socket *sock = rcu_dereference(vq->private_data);
>  	if (!sock || skb_queue_empty(&sock->sk->sk_receive_queue))
>  		return;
> @@ -218,18 +231,19 @@ static void handle_rx(struct vhost_net *
>  	use_mm(net->dev.mm);
>  	mutex_lock(&vq->mutex);
>  	vhost_disable_notify(vq);
> -	hdr_size = vq->hdr_size;
> +	vhost_hlen = vq->vhost_hlen;
>  
>  	vq_log = unlikely(vhost_has_feature(&net->dev, VHOST_F_LOG_ALL)) ?
>  		vq->log : NULL;
>  
> -	for (;;) {
> -		head = vhost_get_vq_desc(&net->dev, vq, vq->iov,
> -					 ARRAY_SIZE(vq->iov),
> -					 &out, &in,
> -					 vq_log, &log);
> +	while ((datalen = vhost_head_len(vq, sock->sk))) {
> +		headcount = vhost_get_desc_n(vq, vq->heads,
> +					     datalen + vhost_hlen,
> +					     &in, vq_log, &log);
> +		if (headcount < 0)
> +			break;
>  		/* OK, now we need to know about added descriptors. */
> -		if (head == vq->num) {
> +		if (!headcount) {
>  			if (unlikely(vhost_enable_notify(vq))) {
>  				/* They have slipped one in as we were
>  				 * doing that: check again. */
> @@ -241,46 +255,53 @@ static void handle_rx(struct vhost_net *
>  			break;
>  		}
>  		/* We don't need to be notified again. */
> -		if (out) {
> -			vq_err(vq, "Unexpected descriptor format for RX: "
> -			       "out %d, int %d\n",
> -			       out, in);
> -			break;
> -		}
> -		/* Skip header. TODO: support TSO/mergeable rx buffers. */
> -		s = move_iovec_hdr(vq->iov, vq->hdr, hdr_size, in);
> +		/* Skip header. TODO: support TSO. */
> +		s = move_iovec_hdr(vq->iov, vq->hdr, vhost_hlen, in);
>  		msg.msg_iovlen = in;
>  		len = iov_length(vq->iov, in);
>  		/* Sanity check */
>  		if (!len) {
>  			vq_err(vq, "Unexpected header len for RX: "
>  			       "%zd expected %zd\n",
> -			       iov_length(vq->hdr, s), hdr_size);
> +			       iov_length(vq->hdr, s), vhost_hlen);
>  			break;
>  		}
>  		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);
> +			vhost_discard_desc(vq, headcount);
>  			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);
> +		if (err != datalen) {
> +			pr_err("Discarded rx packet: "
> +			       " len %d, expected %zd\n", err, datalen);
> +			vhost_discard_desc(vq, headcount);
>  			continue;
>  		}
>  		len = err;
> -		err = memcpy_toiovec(vq->hdr, (unsigned char *)&hdr, hdr_size);
> +		err = memcpy_toiovec(vq->hdr, (unsigned char *)&hdr,
> +				     vhost_hlen);
>  		if (err) {
>  			vq_err(vq, "Unable to write vnet_hdr at addr %p: %d\n",
>  			       vq->iov->iov_base, err);
>  			break;
>  		}
> -		len += hdr_size;
> -		vhost_add_used_and_signal(&net->dev, vq, head, len);
> +		/* TODO: Should check and handle checksum. */
> +		if (vhost_has_feature(&net->dev, VIRTIO_NET_F_MRG_RXBUF)) {
> +			struct iovec *iov = vhost_hlen ? vq->hdr : vq->iov;
> +

Hmm, this looks a bit wrong:
	- memcpy_toiovec above can modify vq->hdr
	- recvmsg can modify vq->iov
it works for you because in practice the address isn't modified
(only length) and in practice the header fits in the first iovec
entry.

It is probably easiest to add another copy of the header,
call it, say, hdr_copy, and a variant of move_iovec_hdr
called, say, copy_iovec_hdr, which does not modify the
origin, just copies part of iovec, call copy_iovec_hdr before
move_iovec_hdr.
Oh and we'll need vq->hdr_size which is max of vq->vhost_hlen
and vq->sock_hlen to know how much to copy.

In practice we'll always copy 1 entry, so no performance issue.

> +			if (memcpy_toiovecend(iov, (unsigned char *)&headcount,
> +				      offsetof(typeof(hdr), num_buffers),
> +				      sizeof(hdr.num_buffers))) {
> +				vq_err(vq, "Failed num_buffers write");
> +				vhost_discard_desc(vq, headcount);
> +				break;
> +			}
> +		}
> +		len += vhost_hlen;
> +		vhost_add_used_and_signal_n(&net->dev, vq, vq->heads,
> +					    headcount);
>  		if (unlikely(vq_log))
>  			vhost_log_write(vq, vq_log, log, len);
>  		total_len += len;
> @@ -561,9 +582,24 @@ done:
>  
>  static int vhost_net_set_features(struct vhost_net *n, u64 features)
>  {
> -	size_t hdr_size = features & (1 << VHOST_NET_F_VIRTIO_NET_HDR) ?
> -		sizeof(struct virtio_net_hdr) : 0;
> +	size_t vhost_hlen;
> +	size_t sock_hlen;
>  	int i;
> +
> +	if (features & (1 << VHOST_NET_F_VIRTIO_NET_HDR)) {
> +		/* vhost provides vnet_hdr */
> +		vhost_hlen = sizeof(struct virtio_net_hdr);
> +		if (features & (1 << VIRTIO_NET_F_MRG_RXBUF))
> +			vhost_hlen = sizeof(struct virtio_net_hdr_mrg_rxbuf);
> +		sock_hlen = 0;
> +	} else {
> +		/* socket provides vnet_hdr */
> +		vhost_hlen = 0;
> +		if (features & (1 << VIRTIO_NET_F_MRG_RXBUF))
> +			sock_hlen = sizeof(struct virtio_net_hdr_mrg_rxbuf);
> +		else
> +			sock_hlen = sizeof(struct virtio_net_hdr);
> +	}

logic is the same above, right?
So

	size_t hdr_len;
	if (features & (1 << VIRTIO_NET_F_MRG_RXBUF))
		hdr_len = sizeof(struct virtio_net_hdr_mrg_rxbuf);
	else
		hdr_len = sizeof(struct virtio_net_hdr);

	if (features & (1 << VHOST_NET_F_VIRTIO_NET_HDR)) {
		vhost_hlen = hdr_len;
		sock_hlen = 0;
	} else {
		sock_hlen = hdr_len;
		vhost_hlen = 0;
	}

looks a bit better, right?

> +	if (features & (1 << VHOST_NET_F_VIRTIO_NET_HDR)) {
> +		/* vhost provides vnet_hdr */
> +		vhost_hlen = sizeof(struct virtio_net_hdr);
> +		if (features & (1 << VIRTIO_NET_F_MRG_RXBUF))
> +			vhost_hlen = sizeof(struct virtio_net_hdr_mrg_rxbuf);
> +		sock_hlen = 0;
> +	} else {
> +		/* socket provides vnet_hdr */
> +		vhost_hlen = 0;
> +		if (features & (1 << VIRTIO_NET_F_MRG_RXBUF))
> +			sock_hlen = sizeof(struct virtio_net_hdr_mrg_rxbuf);
> +		else
> +			sock_hlen = sizeof(struct virtio_net_hdr);
> +	}
>  	mutex_lock(&n->dev.mutex);
>  	if ((features & (1 << VHOST_F_LOG_ALL)) &&
>  	    !vhost_log_access_ok(&n->dev)) {
> @@ -574,7 +610,8 @@ static int vhost_net_set_features(struct
>  	smp_wmb();
>  	for (i = 0; i < VHOST_NET_VQ_MAX; ++i) {
>  		mutex_lock(&n->vqs[i].mutex);
> -		n->vqs[i].hdr_size = hdr_size;
> +		n->vqs[i].vhost_hlen = vhost_hlen;
> +		n->vqs[i].sock_hlen = sock_hlen;
>  		mutex_unlock(&n->vqs[i].mutex);
>  	}
>  	vhost_net_flush(n);
> diff -ruNp net-next-v0/drivers/vhost/vhost.c net-next-v6/drivers/vhost/vhost.c
> --- net-next-v0/drivers/vhost/vhost.c	2010-04-22 11:31:57.000000000 -0700
> +++ net-next-v6/drivers/vhost/vhost.c	2010-04-26 11:18:58.000000000 -0700
> @@ -114,7 +114,8 @@ static void vhost_vq_reset(struct vhost_
>  	vq->used_flags = 0;
>  	vq->log_used = false;
>  	vq->log_addr = -1ull;
> -	vq->hdr_size = 0;
> +	vq->vhost_hlen = 0;
> +	vq->sock_hlen = 0;
>  	vq->private_data = NULL;
>  	vq->log_base = NULL;
>  	vq->error_ctx = NULL;
> @@ -861,6 +862,53 @@ static unsigned get_indirect(struct vhos
>  	return 0;
>  }
>  
> +/* This is a multi-buffer version of vhost_get_desc
> + * @vq		- the relevant virtqueue
> + * datalen	- data length we'll be reading
> + * @iovcount	- returned count of io vectors we fill
> + * @log		- vhost log
> + * @log_num	- log offset
> + *	returns number of buffer heads allocated, negative on error
> + */
> +int vhost_get_desc_n(struct vhost_virtqueue *vq, struct vring_used_elem *heads,
> +		     int datalen, unsigned *iovcount, struct vhost_log *log,
> +		     unsigned int *log_num)
> +{
> +	unsigned int out, in;
> +	int seg = 0;
> +	int headcount = 0;
> +	int r;
> +
> +	while (datalen > 0) {
> +		if (headcount >= VHOST_NET_MAX_SG) {
> +			r = -ENOBUFS;
> +			goto err;
> +		}
> +		heads[headcount].id = vhost_get_desc(vq->dev, vq, vq->iov + seg,
> +					      ARRAY_SIZE(vq->iov) - seg, &out,
> +					      &in, log, log_num);
> +		if (heads[headcount].id == vq->num) {
> +			r = 0;
> +			goto err;
> +		}
> +		if (out || in <= 0) {
> +			vq_err(vq, "unexpected descriptor format for RX: "
> +				"out %d, in %d\n", out, in);
> +			r = -EINVAL;
> +			goto err;
> +		}
> +		heads[headcount].len = iov_length(vq->iov + seg, in);
> +		datalen -= heads[headcount].len;
> +		++headcount;
> +		seg += in;
> +	}
> +	*iovcount = seg;
> +	return headcount;
> +err:
> +	vhost_discard_desc(vq, headcount);
> +	return r;
> +}
> +
>  /* 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
> @@ -868,7 +916,7 @@ static unsigned get_indirect(struct vhos
>   *
>   * 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,
> +unsigned vhost_get_desc(struct vhost_dev *dev, struct vhost_virtqueue *vq,
>  			   struct iovec iov[], unsigned int iov_size,
>  			   unsigned int *out_num, unsigned int *in_num,
>  			   struct vhost_log *log, unsigned int *log_num)
> @@ -986,9 +1034,9 @@ unsigned vhost_get_vq_desc(struct vhost_
>  }
>  
>  /* Reverse the effect of vhost_get_vq_desc. Useful for error handling. */
> -void vhost_discard_vq_desc(struct vhost_virtqueue *vq)
> +void vhost_discard_desc(struct vhost_virtqueue *vq, int n)
>  {
> -	vq->last_avail_idx--;
> +	vq->last_avail_idx -= n;
>  }
>  
>  /* After we've used one of their buffers, we tell them about it.  We'll then
> @@ -1033,6 +1081,68 @@ int vhost_add_used(struct vhost_virtqueu
>  	return 0;
>  }
>  
> +static void vhost_log_used(struct vhost_virtqueue *vq,
> +			   struct vring_used_elem __user *used)
> +{
> +	/* Make sure data is seen before log. */
> +	smp_wmb();
> +	/* Log used ring entry write. */
> +	log_write(vq->log_base,
> +		  vq->log_addr +
> +		   ((void __user *)used - (void __user *)vq->used),
> +		  sizeof *used);
> +	/* Log used index update. */
> +	log_write(vq->log_base,
> +		  vq->log_addr + offsetof(struct vring_used, idx),
> +		  sizeof vq->used->idx);
> +	if (vq->log_ctx)
> +		eventfd_signal(vq->log_ctx, 1);
> +}
> +
> +static int __vhost_add_used_n(struct vhost_virtqueue *vq,
> +			    struct vring_used_elem *heads,
> +			    unsigned count)
> +{
> +	struct vring_used_elem __user *used;
> +	int start;
> +
> +	start = vq->last_used_idx % vq->num;
> +	used = vq->used->ring + start;
> +	if (copy_to_user(used, heads, count * sizeof *used)) {
> +		vq_err(vq, "Failed to write used");
> +		return -EFAULT;
> +	}
> +	/* Make sure buffer is written before we update index. */
> +	smp_wmb();
> +	if (put_user(vq->last_used_idx + count, &vq->used->idx)) {
> +		vq_err(vq, "Failed to increment used idx");
> +		return -EFAULT;
> +	}
> +	if (unlikely(vq->log_used))
> +		vhost_log_used(vq, used);
> +	vq->last_used_idx += count;
> +	return 0;
> +}
> +
> +/* After we've used one of their buffers, we tell them about it.  We'll then
> + * want to notify the guest, using eventfd. */
> +int vhost_add_used_n(struct vhost_virtqueue *vq, struct vring_used_elem *heads,
> +		     unsigned count)
> +{
> +	int start, n, r;
> +
> +	start = vq->last_used_idx % vq->num;
> +	n = min(vq->num - start, count);

This can be just n = vq->num - start now.

> +	if (n < count) {
> +		r = __vhost_add_used_n(vq, heads, n);
> +		if (r < 0)
> +			return r;
> +		heads += n;
> +		count -= n;
> +	}
> +	return __vhost_add_used_n(vq, heads, count);
> +}
> +
>  /* This actually signals the guest, using eventfd. */
>  void vhost_signal(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>  {
> @@ -1062,6 +1172,15 @@ void vhost_add_used_and_signal(struct vh
>  	vhost_signal(dev, vq);
>  }
>  
> +/* multi-buffer version of vhost_add_used_and_signal */
> +void vhost_add_used_and_signal_n(struct vhost_dev *dev,
> +				 struct vhost_virtqueue *vq,
> +				 struct vring_used_elem *heads, unsigned count)
> +{
> +	vhost_add_used_n(vq, heads, count);
> +	vhost_signal(dev, vq);
> +}
> +
>  /* OK, now we need to know about added descriptors. */
>  bool vhost_enable_notify(struct vhost_virtqueue *vq)
>  {
> @@ -1086,7 +1205,7 @@ bool vhost_enable_notify(struct vhost_vi
>  		return false;
>  	}
>  
> -	return avail_idx != vq->last_avail_idx;
> +	return avail_idx != vq->avail_idx;
>  }
>  
>  /* We don't need to be notified again. */
> diff -ruNp net-next-v0/drivers/vhost/vhost.h net-next-v6/drivers/vhost/vhost.h
> --- net-next-v0/drivers/vhost/vhost.h	2010-04-24 21:37:41.000000000 -0700
> +++ net-next-v6/drivers/vhost/vhost.h	2010-04-26 10:35:25.000000000 -0700
> @@ -84,7 +84,9 @@ struct vhost_virtqueue {
>  	struct iovec indirect[VHOST_NET_MAX_SG];
>  	struct iovec iov[VHOST_NET_MAX_SG];
>  	struct iovec hdr[VHOST_NET_MAX_SG];
> -	size_t hdr_size;
> +	size_t vhost_hlen;
> +	size_t sock_hlen;
> +	struct vring_used_elem heads[VHOST_NET_MAX_SG];
>  	/* We use a kind of RCU to access private pointer.
>  	 * All readers access it from workqueue, which makes it possible to
>  	 * flush the workqueue instead of synchronize_rcu. Therefore readers do
> @@ -120,16 +122,23 @@ long vhost_dev_ioctl(struct vhost_dev *,
>  int vhost_vq_access_ok(struct vhost_virtqueue *vq);
>  int vhost_log_access_ok(struct vhost_dev *);
>  
> -unsigned vhost_get_vq_desc(struct vhost_dev *, struct vhost_virtqueue *,
> +int vhost_get_desc_n(struct vhost_virtqueue *, struct vring_used_elem *heads,
> +		     int datalen, unsigned int *iovcount, struct vhost_log *log,
> +		     unsigned int *log_num);
> +unsigned vhost_get_desc(struct vhost_dev *, struct vhost_virtqueue *,
>  			   struct iovec iov[], unsigned int iov_count,
>  			   unsigned int *out_num, unsigned int *in_num,
>  			   struct vhost_log *log, unsigned int *log_num);
> -void vhost_discard_vq_desc(struct vhost_virtqueue *);
> +void vhost_discard_desc(struct vhost_virtqueue *, int);
>  
>  int vhost_add_used(struct vhost_virtqueue *, unsigned int head, int len);
> -void vhost_signal(struct vhost_dev *, struct vhost_virtqueue *);
> +int vhost_add_used_n(struct vhost_virtqueue *, struct vring_used_elem *heads,
> +		     unsigned count);
>  void vhost_add_used_and_signal(struct vhost_dev *, struct vhost_virtqueue *,
> -			       unsigned int head, int len);
> +			       unsigned int id, int len);
> +void vhost_add_used_and_signal_n(struct vhost_dev *, struct vhost_virtqueue *,
> +			       struct vring_used_elem *heads, unsigned count);
> +void vhost_signal(struct vhost_dev *, struct vhost_virtqueue *);
>  void vhost_disable_notify(struct vhost_virtqueue *);
>  bool vhost_enable_notify(struct vhost_virtqueue *);
>  
> @@ -149,7 +158,8 @@ enum {
>  	VHOST_FEATURES = (1 << VIRTIO_F_NOTIFY_ON_EMPTY) |
>  			 (1 << VIRTIO_RING_F_INDIRECT_DESC) |
>  			 (1 << VHOST_F_LOG_ALL) |
> -			 (1 << VHOST_NET_F_VIRTIO_NET_HDR),
> +			 (1 << VHOST_NET_F_VIRTIO_NET_HDR) |
> +			 (1 << VIRTIO_NET_F_MRG_RXBUF),
>  };
>  
>  static inline int vhost_has_feature(struct vhost_dev *dev, int bit)
>