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>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 134 insertions, 12 deletions

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index b95a7f44a6d..c62163e272c 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 */
@@ -604,19 +606,100 @@ 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)))
@@ -625,7 +708,7 @@ 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)))
@@ -647,14 +730,46 @@ 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;
@@ -728,12 +843,18 @@ 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,
@@ -748,7 +869,7 @@ 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);
@@ -962,8 +1083,8 @@ 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,
@@ -1133,6 +1254,7 @@ 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;