Re: [PATCH net-next 08/11] tun: switch to new type of msg_control

[Jason Wang]

On 2018年09月07日 00:54, Michael S. Tsirkin wrote:

On Thu, Sep 06, 2018 at 12:05:23PM +0800, Jason Wang wrote:

This patch introduces to a new tun/tap specific msg_control:

#define TUN_MSG_UBUF 1
#define TUN_MSG_PTR  2
struct tun_msg_ctl {
        int type;
        void *ptr;
};

This allows us to pass different kinds of msg_control through
sendmsg(). The first supported type is ubuf (TUN_MSG_UBUF) which will
be used by the existed vhost_net zerocopy code. The second is XDP
buff, which allows vhost_net to pass XDP buff to TUN. This could be
used to implement accepting an array of XDP buffs from vhost_net in
the following patches.

Signed-off-by: Jason Wang <jasow...@redhat.com>

At this point, do we want to just add a new sock opt for tap's
benefit? Seems cleaner than (ab)using sendmsg.

I think it won't be much difference, we still need a void pointer.

---
  drivers/net/tap.c      | 18 ++++++++++++------
  drivers/net/tun.c      |  6 +++++-
  drivers/vhost/net.c    |  7 +++++--
  include/linux/if_tun.h |  7 +++++++
  4 files changed, 29 insertions(+), 9 deletions(-)

diff --git a/drivers/net/tap.c b/drivers/net/tap.c
index f0f7cd977667..7996ed7cbf18 100644
--- a/drivers/net/tap.c
+++ b/drivers/net/tap.c
@@ -619,7 +619,7 @@ static inline struct sk_buff *tap_alloc_skb(struct sock 
*sk, size_t prepad,
  #define TAP_RESERVE HH_DATA_OFF(ETH_HLEN)

/* Get packet from user space buffer */

-static ssize_t tap_get_user(struct tap_queue *q, struct msghdr *m,
+static ssize_t tap_get_user(struct tap_queue *q, void *msg_control,
                            struct iov_iter *from, int noblock)
  {
        int good_linear = SKB_MAX_HEAD(TAP_RESERVE);
@@ -663,7 +663,7 @@ static ssize_t tap_get_user(struct tap_queue *q, struct 
msghdr *m,
        if (unlikely(len < ETH_HLEN))
                goto err;

- if (m && m->msg_control && sock_flag(&q->sk, SOCK_ZEROCOPY)) {

+       if (msg_control && sock_flag(&q->sk, SOCK_ZEROCOPY)) {
                struct iov_iter i;

copylen = vnet_hdr.hdr_len ?

@@ -724,11 +724,11 @@ static ssize_t tap_get_user(struct tap_queue *q, struct 
msghdr *m,
        tap = rcu_dereference(q->tap);
        /* copy skb_ubuf_info for callback when skb has no error */
        if (zerocopy) {
-               skb_shinfo(skb)->destructor_arg = m->msg_control;
+               skb_shinfo(skb)->destructor_arg = msg_control;
                skb_shinfo(skb)->tx_flags |= SKBTX_DEV_ZEROCOPY;
                skb_shinfo(skb)->tx_flags |= SKBTX_SHARED_FRAG;
-       } else if (m && m->msg_control) {
-               struct ubuf_info *uarg = m->msg_control;
+       } else if (msg_control) {
+               struct ubuf_info *uarg = msg_control;
                uarg->callback(uarg, false);
        }

@@ -1150,7 +1150,13 @@ static int tap_sendmsg(struct socket *sock, struct msghdr *m,

                       size_t total_len)
  {
        struct tap_queue *q = container_of(sock, struct tap_queue, sock);
-       return tap_get_user(q, m, &m->msg_iter, m->msg_flags & MSG_DONTWAIT);
+       struct tun_msg_ctl *ctl = m->msg_control;
+
+       if (ctl && ctl->type != TUN_MSG_UBUF)
+               return -EINVAL;
+
+       return tap_get_user(q, ctl ? ctl->ptr : NULL, &m->msg_iter,
+                           m->msg_flags & MSG_DONTWAIT);
  }

static int tap_recvmsg(struct socket *sock, struct msghdr *m,

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index ff1cbf3ebd50..c839a4bdcbd9 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -2429,11 +2429,15 @@ static int tun_sendmsg(struct socket *sock, struct 
msghdr *m, size_t total_len)
        int ret;
        struct tun_file *tfile = container_of(sock, struct tun_file, socket);
        struct tun_struct *tun = tun_get(tfile);
+       struct tun_msg_ctl *ctl = m->msg_control;

if (!tun)

                return -EBADFD;

- ret = tun_get_user(tun, tfile, m->msg_control, &m->msg_iter,

+       if (ctl && ctl->type != TUN_MSG_UBUF)
+               return -EINVAL;
+
+       ret = tun_get_user(tun, tfile, ctl ? ctl->ptr : NULL, &m->msg_iter,
                           m->msg_flags & MSG_DONTWAIT,
                           m->msg_flags & MSG_MORE);
        tun_put(tun);
diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 4e656f89cb22..fb01ce6d981c 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -620,6 +620,7 @@ static void handle_tx_zerocopy(struct vhost_net *net, 
struct socket *sock)
                .msg_controllen = 0,
                .msg_flags = MSG_DONTWAIT,
        };
+       struct tun_msg_ctl ctl;
        size_t len, total_len = 0;
        int err;
        struct vhost_net_ubuf_ref *uninitialized_var(ubufs);
@@ -664,8 +665,10 @@ static void handle_tx_zerocopy(struct vhost_net *net, 
struct socket *sock)
                        ubuf->ctx = nvq->ubufs;
                        ubuf->desc = nvq->upend_idx;
                        refcount_set(&ubuf->refcnt, 1);
-                       msg.msg_control = ubuf;
-                       msg.msg_controllen = sizeof(ubuf);
+                       msg.msg_control = &ctl;
+                       ctl.type = TUN_MSG_UBUF;
+                       ctl.ptr = ubuf;
+                       msg.msg_controllen = sizeof(ctl);
                        ubufs = nvq->ubufs;
                        atomic_inc(&ubufs->refcount);
                        nvq->upend_idx = (nvq->upend_idx + 1) % UIO_MAXIOV;
diff --git a/include/linux/if_tun.h b/include/linux/if_tun.h
index 3d2996dc7d85..ba46dced1f38 100644
--- a/include/linux/if_tun.h
+++ b/include/linux/if_tun.h
@@ -19,6 +19,13 @@

#define TUN_XDP_FLAG 0x1UL +#define TUN_MSG_UBUF 1

+#define TUN_MSG_PTR  2

Looks like TUN_MSG_PTR should be pushed out to a follow-up patch?

Ok.

+struct tun_msg_ctl {
+       int type;
+       void *ptr;
+};
+

type actually includes a size. Why not two short fields then?

Yes, this sounds better.

Thanks

  #if defined(CONFIG_TUN) || defined(CONFIG_TUN_MODULE)
  struct socket *tun_get_socket(struct file *);
  struct ptr_ring *tun_get_tx_ring(struct file *file);
--
2.17.1

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