Subject: Re: [PATCH v2 3/5] VSOCK: support receive mergeable rx buffer in guest

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 12 Dec 2018 10:31:16 -0500

On Wed, Dec 12, 2018 at 05:31:39PM +0800, jiangyiwen wrote:
> Guest receive mergeable rx buffer, it can merge
> scatter rx buffer into a big buffer and then copy
> to user space.
> 
> In addition, it also use iovec to replace buf in struct
> virtio_vsock_pkt, keep tx and rx consistency. The only
> difference is now tx still uses a segment of continuous
> physical memory to implement.
> 
> Signed-off-by: Yiwen Jiang <[email-masked]>
> ---
>  drivers/vhost/vsock.c                   |  31 +++++++---
>  include/linux/virtio_vsock.h            |   6 +-
>  net/vmw_vsock/virtio_transport.c        | 105 ++++++++++++++++++++++++++++----
>  net/vmw_vsock/virtio_transport_common.c |  59 ++++++++++++++----
>  4 files changed, 166 insertions(+), 35 deletions(-)

This was supposed to be a guest patch, why is vhost changed here?

> diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
> index dc52b0f..c7ab0dd 100644
> --- a/drivers/vhost/vsock.c
> +++ b/drivers/vhost/vsock.c
> @@ -179,6 +179,8 @@ static int get_rx_bufs(struct vhost_virtqueue *vq,
>  		size_t nbytes;
>  		size_t len;
>  		s16 headcount;
> +		size_t remain_len;
> +		int i;
> 
>  		spin_lock_bh(&vsock->send_pkt_list_lock);
>  		if (list_empty(&vsock->send_pkt_list)) {
> @@ -221,11 +223,19 @@ static int get_rx_bufs(struct vhost_virtqueue *vq,
>  			break;
>  		}
> 
> -		nbytes = copy_to_iter(pkt->buf, pkt->len, &iov_iter);
> -		if (nbytes != pkt->len) {
> -			virtio_transport_free_pkt(pkt);
> -			vq_err(vq, "Faulted on copying pkt buf\n");
> -			break;
> +		remain_len = pkt->len;
> +		for (i = 0; i < pkt->nr_vecs; i++) {
> +			int tmp_len;
> +
> +			tmp_len = min(remain_len, pkt->vec[i].iov_len);
> +			nbytes = copy_to_iter(pkt->vec[i].iov_base, tmp_len, &iov_iter);
> +			if (nbytes != tmp_len) {
> +				virtio_transport_free_pkt(pkt);
> +				vq_err(vq, "Faulted on copying pkt buf\n");
> +				break;
> +			}
> +
> +			remain_len -= tmp_len;
>  		}
> 
>  		vhost_add_used_n(vq, vq->heads, headcount);
> @@ -341,6 +351,7 @@ static void vhost_transport_send_pkt_work(struct vhost_work *work)
>  	struct iov_iter iov_iter;
>  	size_t nbytes;
>  	size_t len;
> +	void *buf;
> 
>  	if (in != 0) {
>  		vq_err(vq, "Expected 0 input buffers, got %u\n", in);
> @@ -375,13 +386,17 @@ static void vhost_transport_send_pkt_work(struct vhost_work *work)
>  		return NULL;
>  	}
> 
> -	pkt->buf = kmalloc(pkt->len, GFP_KERNEL);
> -	if (!pkt->buf) {
> +	buf = kmalloc(pkt->len, GFP_KERNEL);
> +	if (!buf) {
>  		kfree(pkt);
>  		return NULL;
>  	}
> 
> -	nbytes = copy_from_iter(pkt->buf, pkt->len, &iov_iter);
> +	pkt->vec[0].iov_base = buf;
> +	pkt->vec[0].iov_len = pkt->len;
> +	pkt->nr_vecs = 1;
> +
> +	nbytes = copy_from_iter(buf, pkt->len, &iov_iter);
>  	if (nbytes != pkt->len) {
>  		vq_err(vq, "Expected %u byte payload, got %zu bytes\n",
>  		       pkt->len, nbytes);
> diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
> index da9e1fe..734eeed 100644
> --- a/include/linux/virtio_vsock.h
> +++ b/include/linux/virtio_vsock.h
> @@ -13,6 +13,8 @@
>  #define VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE	(1024 * 4)
>  #define VIRTIO_VSOCK_MAX_BUF_SIZE		0xFFFFFFFFUL
>  #define VIRTIO_VSOCK_MAX_PKT_BUF_SIZE		(1024 * 64)
> +/* virtio_vsock_pkt + max_pkt_len(default MAX_PKT_BUF_SIZE) */
> +#define VIRTIO_VSOCK_MAX_VEC_NUM ((VIRTIO_VSOCK_MAX_PKT_BUF_SIZE / PAGE_SIZE) + 1)
> 
>  /* Virtio-vsock feature */
>  #define VIRTIO_VSOCK_F_MRG_RXBUF 0 /* Host can merge receive buffers. */
> @@ -55,10 +57,12 @@ struct virtio_vsock_pkt {
>  	struct list_head list;
>  	/* socket refcnt not held, only use for cancellation */
>  	struct vsock_sock *vsk;
> -	void *buf;
> +	struct kvec vec[VIRTIO_VSOCK_MAX_VEC_NUM];
> +	int nr_vecs;
>  	u32 len;
>  	u32 off;
>  	bool reply;
> +	bool mergeable;
>  };
> 
>  struct virtio_vsock_pkt_info {
> diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
> index c4a465c..148b58a 100644
> --- a/net/vmw_vsock/virtio_transport.c
> +++ b/net/vmw_vsock/virtio_transport.c
> @@ -155,8 +155,10 @@ static int virtio_transport_send_pkt_loopback(struct virtio_vsock *vsock,
> 
>  		sg_init_one(&hdr, &pkt->hdr, sizeof(pkt->hdr));
>  		sgs[out_sg++] = &hdr;
> -		if (pkt->buf) {
> -			sg_init_one(&buf, pkt->buf, pkt->len);
> +		if (pkt->len) {
> +			/* Currently only support a segment of memory in tx */
> +			BUG_ON(pkt->vec[0].iov_len != pkt->len);
> +			sg_init_one(&buf, pkt->vec[0].iov_base, pkt->vec[0].iov_len);
>  			sgs[out_sg++] = &buf;
>  		}
> 
> @@ -304,23 +306,28 @@ static int fill_old_rx_buff(struct virtqueue *vq)
>  	struct virtio_vsock_pkt *pkt;
>  	struct scatterlist hdr, buf, *sgs[2];
>  	int ret;
> +	void *pkt_buf;
> 
>  	pkt = kzalloc(sizeof(*pkt), GFP_KERNEL);
>  	if (!pkt)
>  		return -ENOMEM;
> 
> -	pkt->buf = kmalloc(buf_len, GFP_KERNEL);
> -	if (!pkt->buf) {
> +	pkt_buf = kmalloc(buf_len, GFP_KERNEL);
> +	if (!pkt_buf) {
>  		virtio_transport_free_pkt(pkt);
>  		return -ENOMEM;
>  	}
> 
> +	pkt->vec[0].iov_base = pkt_buf;
> +	pkt->vec[0].iov_len = buf_len;
> +	pkt->nr_vecs = 1;
> +
>  	pkt->len = buf_len;
> 
>  	sg_init_one(&hdr, &pkt->hdr, sizeof(pkt->hdr));
>  	sgs[0] = &hdr;
> 
> -	sg_init_one(&buf, pkt->buf, buf_len);
> +	sg_init_one(&buf, pkt->vec[0].iov_base, buf_len);
>  	sgs[1] = &buf;
>  	ret = virtqueue_add_sgs(vq, sgs, 0, 2, pkt, GFP_KERNEL);
>  	if (ret)
> @@ -388,11 +395,78 @@ static bool virtio_transport_more_replies(struct virtio_vsock *vsock)
>  	return val < virtqueue_get_vring_size(vq);
>  }
> 
> +static struct virtio_vsock_pkt *receive_mergeable(struct virtqueue *vq,
> +		struct virtio_vsock *vsock, unsigned int *total_len)
> +{
> +	struct virtio_vsock_pkt *pkt;
> +	u16 num_buf;
> +	void *buf;
> +	unsigned int len;
> +	size_t vsock_hlen = sizeof(struct virtio_vsock_pkt);
> +
> +	buf = virtqueue_get_buf(vq, &len);
> +	if (!buf)
> +		return NULL;
> +
> +	*total_len = len;
> +	vsock->rx_buf_nr--;
> +
> +	if (unlikely(len < vsock_hlen)) {
> +		put_page(virt_to_head_page(buf));
> +		return NULL;
> +	}
> +
> +	pkt = buf;
> +	num_buf = le16_to_cpu(pkt->mrg_rxbuf_hdr.num_buffers);
> +	if (!num_buf || num_buf > VIRTIO_VSOCK_MAX_VEC_NUM) {
> +		put_page(virt_to_head_page(buf));
> +		return NULL;
> +	}

So everything just stops going, and host and user don't even
know what the reason is. And not only that - the next
packet will be corrupted because we skipped the first one.

> +
> +	/* Initialize pkt residual structure */
> +	memset(&pkt->work, 0, vsock_hlen - sizeof(struct virtio_vsock_hdr) -
> +			sizeof(struct virtio_vsock_mrg_rxbuf_hdr));
> +
> +	pkt->mergeable = true;
> +	pkt->len = le32_to_cpu(pkt->hdr.len);
> +	if (!pkt->len)
> +		return pkt;
> +
> +	len -= vsock_hlen;
> +	if (len) {
> +		pkt->vec[pkt->nr_vecs].iov_base = buf + vsock_hlen;
> +		pkt->vec[pkt->nr_vecs].iov_len = len;
> +		/* Shared page with pkt, so get page in advance */
> +		get_page(virt_to_head_page(buf));
> +		pkt->nr_vecs++;
> +	}
> +
> +	while (--num_buf) {
> +		buf = virtqueue_get_buf(vq, &len);
> +		if (!buf)
> +			goto err;
> +
> +		*total_len += len;
> +		vsock->rx_buf_nr--;
> +
> +		pkt->vec[pkt->nr_vecs].iov_base = buf;
> +		pkt->vec[pkt->nr_vecs].iov_len = len;
> +		pkt->nr_vecs++;
> +	}
> +
> +	return pkt;
> +err:
> +	virtio_transport_free_pkt(pkt);
> +	return NULL;
> +}
> +
>  static void virtio_transport_rx_work(struct work_struct *work)
>  {
>  	struct virtio_vsock *vsock =
>  		container_of(work, struct virtio_vsock, rx_work);
>  	struct virtqueue *vq;
> +	size_t vsock_hlen = vsock->mergeable ? sizeof(struct virtio_vsock_pkt) :
> +			sizeof(struct virtio_vsock_hdr);
> 
>  	vq = vsock->vqs[VSOCK_VQ_RX];
> 
> @@ -412,21 +486,26 @@ static void virtio_transport_rx_work(struct work_struct *work)
>  				goto out;
>  			}
> 
> -			pkt = virtqueue_get_buf(vq, &len);
> -			if (!pkt) {
> -				break;
> -			}
> +			if (likely(vsock->mergeable)) {
> +				pkt = receive_mergeable(vq, vsock, &len);
> +				if (!pkt)
> +					break;
> +			} else {
> +				pkt = virtqueue_get_buf(vq, &len);
> +				if (!pkt)
> +					break;
> 

So looking at it, this seems to be the main source of the gain.
But why does this require host/guest changes?

The way I see it:
	- get a buffer and create an skb
	- get the next one, check header matches, if yes
	  tack it on the skb as a fragment. If not then
	  don't, deliver previous one and queue the new one.

> -			vsock->rx_buf_nr--;
> +				vsock->rx_buf_nr--;
> +			}
> 
>  			/* Drop short/long packets */
> -			if (unlikely(len < sizeof(pkt->hdr) ||
> -				     len > sizeof(pkt->hdr) + pkt->len)) {
> +			if (unlikely(len < vsock_hlen ||
> +				     len > vsock_hlen + pkt->len)) {
>  				virtio_transport_free_pkt(pkt);
>  				continue;
>  			}
> 
> -			pkt->len = len - sizeof(pkt->hdr);
> +			pkt->len = len - vsock_hlen;
>  			virtio_transport_deliver_tap_pkt(pkt);
>  			virtio_transport_recv_pkt(pkt);
>  		}
> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> index 3ae3a33..123a8b6 100644
> --- a/net/vmw_vsock/virtio_transport_common.c
> +++ b/net/vmw_vsock/virtio_transport_common.c
> @@ -44,6 +44,7 @@ static const struct virtio_transport *virtio_transport_get_ops(void)
>  {
>  	struct virtio_vsock_pkt *pkt;
>  	int err;
> +	void *buf = NULL;
> 
>  	pkt = kzalloc(sizeof(*pkt), GFP_KERNEL);
>  	if (!pkt)
> @@ -62,12 +63,16 @@ static const struct virtio_transport *virtio_transport_get_ops(void)
>  	pkt->vsk		= info->vsk;
> 
>  	if (info->msg && len > 0) {
> -		pkt->buf = kmalloc(len, GFP_KERNEL);
> -		if (!pkt->buf)
> +		buf = kmalloc(len, GFP_KERNEL);
> +		if (!buf)
>  			goto out_pkt;
> -		err = memcpy_from_msg(pkt->buf, info->msg, len);
> +		err = memcpy_from_msg(buf, info->msg, len);
>  		if (err)
>  			goto out;
> +
> +		pkt->vec[0].iov_base = buf;
> +		pkt->vec[0].iov_len = len;
> +		pkt->nr_vecs = 1;
>  	}
> 
>  	trace_virtio_transport_alloc_pkt(src_cid, src_port,
> @@ -80,7 +85,7 @@ static const struct virtio_transport *virtio_transport_get_ops(void)
>  	return pkt;
> 
>  out:
> -	kfree(pkt->buf);
> +	kfree(buf);
>  out_pkt:
>  	kfree(pkt);
>  	return NULL;
> @@ -92,6 +97,7 @@ static struct sk_buff *virtio_transport_build_skb(void *opaque)
>  	struct virtio_vsock_pkt *pkt = opaque;
>  	struct af_vsockmon_hdr *hdr;
>  	struct sk_buff *skb;
> +	int i;
> 
>  	skb = alloc_skb(sizeof(*hdr) + sizeof(pkt->hdr) + pkt->len,
>  			GFP_ATOMIC);
> @@ -134,7 +140,8 @@ static struct sk_buff *virtio_transport_build_skb(void *opaque)
>  	skb_put_data(skb, &pkt->hdr, sizeof(pkt->hdr));
> 
>  	if (pkt->len) {
> -		skb_put_data(skb, pkt->buf, pkt->len);
> +		for (i = 0; i < pkt->nr_vecs; i++)
> +			skb_put_data(skb, pkt->vec[i].iov_base, pkt->vec[i].iov_len);
>  	}
> 
>  	return skb;
> @@ -260,6 +267,9 @@ static int virtio_transport_send_credit_update(struct vsock_sock *vsk,
> 
>  	spin_lock_bh(&vvs->rx_lock);
>  	while (total < len && !list_empty(&vvs->rx_queue)) {
> +		size_t copy_bytes, last_vec_total = 0, vec_off;
> +		int i;
> +
>  		pkt = list_first_entry(&vvs->rx_queue,
>  				       struct virtio_vsock_pkt, list);
> 
> @@ -272,14 +282,28 @@ static int virtio_transport_send_credit_update(struct vsock_sock *vsk,
>  		 */
>  		spin_unlock_bh(&vvs->rx_lock);
> 
> -		err = memcpy_to_msg(msg, pkt->buf + pkt->off, bytes);
> -		if (err)
> -			goto out;
> +		for (i = 0; i < pkt->nr_vecs; i++) {
> +			if (pkt->off > last_vec_total + pkt->vec[i].iov_len) {
> +				last_vec_total += pkt->vec[i].iov_len;
> +				continue;
> +			}
> +
> +			vec_off = pkt->off - last_vec_total;
> +			copy_bytes = min(pkt->vec[i].iov_len - vec_off, bytes);
> +			err = memcpy_to_msg(msg, pkt->vec[i].iov_base + vec_off,
> +					copy_bytes);
> +			if (err)
> +				goto out;
> +
> +			bytes -= copy_bytes;
> +			pkt->off += copy_bytes;
> +			total += copy_bytes;
> +			last_vec_total += pkt->vec[i].iov_len;
> +			if (!bytes)
> +				break;
> +		}
> 
>  		spin_lock_bh(&vvs->rx_lock);
> -
> -		total += bytes;
> -		pkt->off += bytes;
>  		if (pkt->off == pkt->len) {
>  			virtio_transport_dec_rx_pkt(vvs, pkt);
>  			list_del(&pkt->list);
> @@ -1050,8 +1074,17 @@ void virtio_transport_recv_pkt(struct virtio_vsock_pkt *pkt)
> 
>  void virtio_transport_free_pkt(struct virtio_vsock_pkt *pkt)
>  {
> -	kfree(pkt->buf);
> -	kfree(pkt);
> +	int i;
> +
> +	if (pkt->mergeable) {
> +		for (i = 0; i < pkt->nr_vecs; i++)
> +			put_page(virt_to_head_page(pkt->vec[i].iov_base));
> +		put_page(virt_to_head_page((void *)pkt));
> +	} else {
> +		for (i = 0; i < pkt->nr_vecs; i++)
> +			kfree(pkt->vec[i].iov_base);
> +		kfree(pkt);
> +	}
>  }
>  EXPORT_SYMBOL_GPL(virtio_transport_free_pkt);
> 
> -- 
> 1.8.3.1
>