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

From: Jason Wang
Date: Mon Jun 04 2012 - 00:46:42 EST

On 05/13/2012 08:34 PM, Michael S. Tsirkin wrote:
Let vhost-net utilize zero copy tx when used with tun.

Signed-off-by: Michael S. Tsirkin<mst@xxxxxxxxxx>
---
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)
{

Looks like V2 uses count as the number of vectors and V3 correct this, so does V3 still have any issue during test?
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;
}

Add a check of UIO_MAXIOV like macvtap? Other looks good to me.
Thanks.
- 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;


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/