Re: [PATCHv3 6/6] tun: experimental zero copy tx support

Let vhost-net utilize zero copy tx when used with tun.

Signed-off-by: Michael S. Tsirkin<mst@redhat.com>
---
  drivers/net/tun.c | 146 +++++++++++++++++++++++++++++++++++++++++++++++++-----
  1 file changed, 134 insertions(+), 12 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index fe5cd2f3..74d7e5e 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c

@@ -100,6 +100,8 @@ do {								\
  } while (0)
  #endif

+#define GOODCOPY_LEN 128
+
  #define FLT_EXACT_COUNT 8
  struct tap_filter {
  	unsigned int    count;    /* Number of addrs. Zero means disabled */

@@ -602,19 +604,100 @@ static struct sk_buff *tun_alloc_skb(struct tun_struct *tun,
  	return skb;
  }

+/* set skb frags from iovec, this can move to core network code for reuse */
+static int zerocopy_sg_from_iovec(struct sk_buff *skb, const struct iovec *from,
+				  int offset, size_t count)
+{
+	int len = iov_length(from, count) - offset;
+	int copy = skb_headlen(skb);
+	int size, offset1 = 0;
+	int i = 0;
+
+	/* Skip over from offset */
+	while (count&&  (offset>= from->iov_len)) {
+		offset -= from->iov_len;
+		++from;
+		--count;
+	}
+
+	/* copy up to skb headlen */
+	while (count&&  (copy>  0)) {
+		size = min_t(unsigned int, copy, from->iov_len - offset);
+		if (copy_from_user(skb->data + offset1, from->iov_base + offset,
+				   size))
+			return -EFAULT;
+		if (copy>  size) {
+			++from;
+			--count;
+			offset = 0;
+		} else
+			offset += size;
+		copy -= size;
+		offset1 += size;
+	}
+
+	if (len == offset1)
+		return 0;
+
+	while (count--) {
+		struct page *page[MAX_SKB_FRAGS];
+		int num_pages;
+		unsigned long base;
+		unsigned long truesize;
+
+		len = from->iov_len - offset;
+		if (!len) {
+			offset = 0;
+			++from;
+			continue;
+		}
+		base = (unsigned long)from->iov_base + offset;
+		size = ((base&  ~PAGE_MASK) + len + ~PAGE_MASK)>>  PAGE_SHIFT;
+		if (i + size>  MAX_SKB_FRAGS)
+			return -EMSGSIZE;
+		num_pages = get_user_pages_fast(base, size, 0,&page[i]);
+		if (num_pages != size) {
+			for (i = 0; i<  num_pages; i++)
+				put_page(page[i]);
+			return -EFAULT;
+		}
+		truesize = size * PAGE_SIZE;
+		skb->data_len += len;
+		skb->len += len;
+		skb->truesize += truesize;
+		atomic_add(truesize,&skb->sk->sk_wmem_alloc);
+		while (len) {
+			int off = base&  ~PAGE_MASK;
+			int size = min_t(int, len, PAGE_SIZE - off);
+			__skb_fill_page_desc(skb, i, page[i], off, size);
+			skb_shinfo(skb)->nr_frags++;
+			/* increase sk_wmem_alloc */
+			base += size;
+			len -= size;
+			i++;
+		}
+		offset = 0;
+		++from;
+	}
+	return 0;
+}
+
  /* Get packet from user space buffer */
-static ssize_t tun_get_user(struct tun_struct *tun,
-			    const struct iovec *iv, size_t count,
-			    int noblock)
+static ssize_t tun_get_user(struct tun_struct *tun, void *msg_control,
+			    const struct iovec *iv, size_t total_len,
+			    size_t count, int noblock)
  {

  	struct tun_pi pi = { 0, cpu_to_be16(ETH_P_IP) };
  	struct sk_buff *skb;
-	size_t len = count, align = NET_SKB_PAD;
+	size_t len = total_len, align = NET_SKB_PAD;
  	struct virtio_net_hdr gso = { 0 };
  	int offset = 0;
+	int copylen;
+	bool zerocopy = false;
+	int err;

  	if (!(tun->flags&  TUN_NO_PI)) {
-		if ((len -= sizeof(pi))>  count)
+		if ((len -= sizeof(pi))>  total_len)
  			return -EINVAL;

  		if (memcpy_fromiovecend((void *)&pi, iv, 0, sizeof(pi)))

@@ -623,7 +706,7 @@ static ssize_t tun_get_user(struct tun_struct *tun,
  	}

  	if (tun->flags&  TUN_VNET_HDR) {
-		if ((len -= tun->vnet_hdr_sz)>  count)
+		if ((len -= tun->vnet_hdr_sz)>  total_len)
  			return -EINVAL;

  		if (memcpy_fromiovecend((void *)&gso, iv, offset, sizeof(gso)))

@@ -645,14 +728,46 @@ static ssize_t tun_get_user(struct tun_struct *tun,
  			return -EINVAL;
  	}
  

-	skb = tun_alloc_skb(tun, align, len, gso.hdr_len, noblock);
+	if (msg_control)
+		zerocopy = true;
+
+	if (zerocopy) {
+		/* Userspace may produce vectors with count greater than
+		 * MAX_SKB_FRAGS, so we need to linearize parts of the skb
+		 * to let the rest of data to be fit in the frags.
+		 */
+		if (count>  MAX_SKB_FRAGS) {
+			copylen = iov_length(iv, count - MAX_SKB_FRAGS);
+			if (copylen<  offset)
+				copylen = 0;
+			else
+				copylen -= offset;
+		} else
+				copylen = 0;
+		/* There are 256 bytes to be copied in skb, so there is enough
+		 * room for skb expand head in case it is used.
+		 * The rest of the buffer is mapped from userspace.
+		 */
+		if (copylen<  gso.hdr_len)
+			copylen = gso.hdr_len;
+		if (!copylen)
+			copylen = GOODCOPY_LEN;
+	} else
+		copylen = len;
+
+	skb = tun_alloc_skb(tun, align, copylen, gso.hdr_len, noblock);
  	if (IS_ERR(skb)) {
  		if (PTR_ERR(skb) != -EAGAIN)
  			tun->dev->stats.rx_dropped++;
  		return PTR_ERR(skb);
  	}

-	if (skb_copy_datagram_from_iovec(skb, 0, iv, offset, len)) {
+	if (zerocopy)
+		err = zerocopy_sg_from_iovec(skb, iv, offset, count);
+	else
+		err = skb_copy_datagram_from_iovec(skb, 0, iv, offset, len);
+
+	if (err) {
  		tun->dev->stats.rx_dropped++;
  		kfree_skb(skb);
  		return -EFAULT;

@@ -726,12 +841,18 @@ static ssize_t tun_get_user(struct tun_struct *tun,
  		skb_shinfo(skb)->gso_segs = 0;
  	}

+	/* copy skb_ubuf_info for callback when skb has no error */
+	if (zerocopy) {
+		skb_shinfo(skb)->destructor_arg = msg_control;
+		skb_shinfo(skb)->tx_flags |= SKBTX_DEV_ZEROCOPY;
+	}
+
  	netif_rx_ni(skb);

  	tun->dev->stats.rx_packets++;
  	tun->dev->stats.rx_bytes += len;

-	return count;
+	return total_len;
  }

  static ssize_t tun_chr_aio_write(struct kiocb *iocb, const struct iovec *iv,

@@ -746,7 +867,7 @@ static ssize_t tun_chr_aio_write(struct kiocb *iocb, const struct iovec *iv,

  	tun_debug(KERN_INFO, tun, "tun_chr_write %ld\n", count);

-	result = tun_get_user(tun, iv, iov_length(iv, count),
+	result = tun_get_user(tun, NULL, iv, iov_length(iv, count), count,
  			      file->f_flags&  O_NONBLOCK);

  	tun_put(tun);

@@ -960,8 +1081,8 @@ static int tun_sendmsg(struct kiocb *iocb, struct socket *sock,
  		       struct msghdr *m, size_t total_len)
  {
  	struct tun_struct *tun = container_of(sock, struct tun_struct, socket);
-	return tun_get_user(tun, m->msg_iov, total_len,
-			    m->msg_flags&  MSG_DONTWAIT);
+	return tun_get_user(tun, m->msg_control, m->msg_iov, total_len,
+			    m->msg_iovlen, m->msg_flags&  MSG_DONTWAIT);
  }

  static int tun_recvmsg(struct kiocb *iocb, struct socket *sock,

@@ -1130,6 +1251,7 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
  		sock_init_data(&tun->socket, sk);
  		sk->sk_write_space = tun_sock_write_space;
  		sk->sk_sndbuf = INT_MAX;
+		sock_set_flag(sk, SOCK_ZEROCOPY);

  		tun_sk(sk)->tun = tun;