[RFC PATCH 2/2] macvtap: TX zero copy between guest and host kernel

Shirley Ma
2010-09-13T20:48:10Z

Add zero copy feature between userspace and kernel in macvtap when lower device supports
high memory DMA.

Signed-off-by: Shirley Ma <xma@us.ibm.com>
---

drivers/net/macvtap.c | 136 +++++++++++++++++++++++++++++++++++++++++++++----
1 files changed, 126 insertions(+), 10 deletions(-)

diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
index 3b1c54a..186cde1 100644
--- a/drivers/net/macvtap.c
+++ b/drivers/net/macvtap.c
@@ -274,6 +274,7 @@ static int macvtap_open(struct inode *inode, struct file *file)
struct net *net = current->nsproxy->net_ns;
struct net_device *dev = dev_get_by_index(net, iminor(inode));
struct macvtap_queue *q;
+ struct macvlan_dev *vlan = netdev_priv(dev);
int err;

err = -ENODEV;
@@ -302,6 +303,17 @@ static int macvtap_open(struct inode *inode, struct file *file)
q->flags = IFF_VNET_HDR | IFF_NO_PI | IFF_TAP;
q->vnet_hdr_sz = sizeof(struct virtio_net_hdr);

+ /*
+ * so far only VM uses macvtap, enable zero copy between guest
+ * kernel and host kernel when lower device supports high memory
+ * DMA
+ */
+ if (vlan) {
+ if ((vlan->lowerdev->features & NETIF_F_HIGHDMA) &&
+ (vlan->lowerdev->features & NETIF_F_SG))
+ sock_set_flag(&q->sk, SOCK_ZEROCOPY);
+ }
+
err = macvtap_set_queue(dev, file, q);
if (err)
sock_put(&q->sk);
@@ -343,6 +355,24 @@ out:
return mask;
}

+#define GOODCOPY_LEN (L1_CACHE_BYTES < 64 ? 64 : L1_CACHE_BYTES)
+
+static inline struct sk_buff *macvtap_alloc_skb_goodcopy(struct sock *sk,
+ size_t prepad, size_t copy,
+ int noblock, int *err)
+{
+ struct sk_buff *skb;
+
+ skb = sock_alloc_send_pskb(sk, prepad + copy, 0, noblock, err);
+ if (!skb)
+ return NULL;
+ skb_reserve(skb, prepad);
+ skb_put(skb, copy);
+
+ return skb;
+
+}
+
static inline struct sk_buff *macvtap_alloc_skb(struct sock *sk, size_t prepad,
size_t len, size_t linear,
int noblock, int *err)
@@ -447,15 +477,91 @@ static int macvtap_skb_to_vnet_hdr(const struct sk_buff *skb,
return 0;
}

+/* set skb frags from iovec, this can move to core network code for reuse */
+static int set_sg_from_iovec_zerocopy(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;
+ skb_frag_t *f;
+
+ /* Skip over from offset */
+ while (offset >= from->iov_len) {
+ offset -= from->iov_len;
+ ++from;
+ --count;
+ }
+
+ /* copy up to skb headlen */
+ while (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;
+ }
+ copy -= size;
+ offset1 += size;
+ offset = 0;
+ }
+
+ if (len == offset1)
+ return 0;
+
+ while (count--) {
+ struct page *page[MAX_SKB_FRAGS];
+ int num_pages;
+ unsigned long base;
+
+ len = from->iov_len - offset1;
+ if (!len) {
+ offset1 = 0;
+ ++from;
+ continue;
+ }
+ base = (unsigned long)from->iov_base + offset1;
+ size = ((base & ~PAGE_MASK) + len + ~PAGE_MASK) >> PAGE_SHIFT;
+ num_pages = get_user_pages_fast(base, size, 0, &page[i]);
+ if ((num_pages != size) ||
+ (num_pages > MAX_SKB_FRAGS - skb_shinfo(skb)->nr_frags))
+ /* put_page is in skb free */
+ return -EFAULT;
+ while (len) {
+ f = &skb_shinfo(skb)->frags[i];
+ f->page = page[i];
+ f->page_offset = base & ~PAGE_MASK;
+ f->size = min_t(int, len, PAGE_SIZE - f->page_offset);
+ skb->data_len += f->size;
+ skb->len += f->size;
+ skb->truesize += f->size;
+ skb_shinfo(skb)->nr_frags++;
+ /* increase sk_wmem_alloc */
+ if (skb->sk && skb->destructor == sock_wfree)
+ atomic_add(f->size, &skb->sk->sk_wmem_alloc);
+ base += f->size;
+ len -= f->size;
+ i++;
+ }
+ offset1 = 0;
+ ++from;
+ }
+ return 0;
+}

/* Get packet from user space buffer */
static ssize_t macvtap_get_user(struct macvtap_queue *q,
- const struct iovec *iv, size_t count,
- int noblock)
+ const struct iovec *iv,
+ unsigned long total_len,
+ unsigned long count, int noblock)
{
struct sk_buff *skb;
struct macvlan_dev *vlan;
- size_t len = count;
+ unsigned len = total_len;
int err;
struct virtio_net_hdr vnet_hdr = { 0 };
int vnet_hdr_len = 0;
@@ -485,12 +591,22 @@ static ssize_t macvtap_get_user(struct macvtap_queue *q,
if (unlikely(len < ETH_HLEN))
goto err;

- skb = macvtap_alloc_skb(&q->sk, NET_IP_ALIGN, len, vnet_hdr.hdr_len,
- noblock, &err);
+ if (sock_flag(&q->sk, SOCK_ZEROCOPY)) {
+ int copy = len > 2 * GOODCOPY_LEN ? GOODCOPY_LEN : len;
+ skb = macvtap_alloc_skb_goodcopy(&q->sk, NET_IP_ALIGN, copy,
+ noblock, &err);
+ } else
+ skb = macvtap_alloc_skb(&q->sk, NET_IP_ALIGN, len,
+ vnet_hdr.hdr_len, noblock, &err);
if (!skb)
goto err;

- err = skb_copy_datagram_from_iovec(skb, 0, iv, vnet_hdr_len, len);
+ if (sock_flag(&q->sk, SOCK_ZEROCOPY))
+ err = set_sg_from_iovec_zerocopy(skb, iv, vnet_hdr_len, count);
+ else
+ err = skb_copy_datagram_from_iovec(skb, 0, iv, vnet_hdr_len,
+ len);
+
if (err)
goto err_kfree;

@@ -512,7 +628,7 @@ static ssize_t macvtap_get_user(struct macvtap_queue *q,
kfree_skb(skb);
rcu_read_unlock_bh();

- return count;
+ return total_len;

err_kfree:
kfree_skb(skb);
@@ -534,8 +650,8 @@ static ssize_t macvtap_aio_write(struct kiocb *iocb, const struct iovec *iv,
ssize_t result = -ENOLINK;
struct macvtap_queue *q = file->private_data;

- result = macvtap_get_user(q, iv, iov_length(iv, count),
- file->f_flags & O_NONBLOCK);
+ result = macvtap_get_user(q, iv, iov_length(iv, count), count,
+ file->f_flags & O_NONBLOCK);
return result;
}

@@ -748,7 +864,7 @@ static int macvtap_sendmsg(struct kiocb *iocb, struct socket *sock,
struct msghdr *m, size_t total_len)
{
struct macvtap_queue *q = container_of(sock, struct macvtap_queue, sock);
- return macvtap_get_user(q, m->msg_iov, total_len,
+ return macvtap_get_user(q, m->msg_iov, total_len, m->msg_iovlen,
m->msg_flags & MSG_DONTWAIT);
}