[Xen-devel] [PATCH v2 09/13] xen/pvcalls: implement recvmsg

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

Implement recvmsg by copying data from the "in" ring. If not enough data
is available and the recvmsg call is blocking, then wait on the
inflight_conn_req waitqueue. Take the active socket in_mutex so that
only one function can access the ring at any given time.

If not enough data is available on the ring, rather than returning
immediately or sleep-waiting, spin for up to 5000 cycles. This small
optimization turns out to improve performance and latency significantly.

Signed-off-by: Stefano Stabellini <stefano@xxxxxxxxxxx>
CC: boris.ostrovsky@xxxxxxxxxx
CC: jgross@xxxxxxxx
---
 drivers/xen/pvcalls-front.c | 106 ++++++++++++++++++++++++++++++++++++++++++++
 drivers/xen/pvcalls-front.h |   4 ++
 2 files changed, 110 insertions(+)

diff --git a/drivers/xen/pvcalls-front.c b/drivers/xen/pvcalls-front.c
index d8ed280..b4ca569 100644
--- a/drivers/xen/pvcalls-front.c
+++ b/drivers/xen/pvcalls-front.c
@@ -96,6 +96,20 @@ static int pvcalls_front_write_todo(struct sock_mapping *map)
        return size - pvcalls_queued(prod, cons, size);
 }
 
+static bool pvcalls_front_read_todo(struct sock_mapping *map)
+{
+       struct pvcalls_data_intf *intf = map->active.ring;
+       RING_IDX cons, prod;
+       int32_t error;
+
+       cons = intf->in_cons;
+       prod = intf->in_prod;
+       error = intf->in_error;
+       return (error != 0 ||
+               pvcalls_queued(prod, cons,
+                              XEN_FLEX_RING_SIZE(intf->ring_order))) != 0;
+}
+
 static irqreturn_t pvcalls_front_event_handler(int irq, void *dev_id)
 {
        struct xenbus_device *dev = dev_id;
@@ -418,6 +432,98 @@ int pvcalls_front_sendmsg(struct socket *sock, struct 
msghdr *msg,
        return tot_sent;
 }
 
+static int __read_ring(struct pvcalls_data_intf *intf,
+                      struct pvcalls_data *data,
+                      struct iov_iter *msg_iter,
+                      size_t len, int flags)
+{
+       RING_IDX cons, prod, size, masked_prod, masked_cons;
+       RING_IDX array_size = XEN_FLEX_RING_SIZE(intf->ring_order);
+       int32_t error;
+
+       cons = intf->in_cons;
+       prod = intf->in_prod;
+       error = intf->in_error;
+       /* get pointers before reading from the ring */
+       virt_rmb();
+       if (error < 0)
+               return error;
+
+       size = pvcalls_queued(prod, cons, array_size);
+       masked_prod = pvcalls_mask(prod, array_size);
+       masked_cons = pvcalls_mask(cons, array_size);
+
+       if (size == 0)
+               return 0;
+
+       if (len > size)
+               len = size;
+
+       if (masked_prod > masked_cons) {
+               copy_to_iter(data->in + masked_cons, len, msg_iter);
+       } else {
+               if (len > (array_size - masked_cons)) {
+                       copy_to_iter(data->in + masked_cons,
+                                    array_size - masked_cons, msg_iter);
+                       copy_to_iter(data->in,
+                                    len - (array_size - masked_cons),
+                                    msg_iter);
+               } else {
+                       copy_to_iter(data->in + masked_cons, len, msg_iter);
+               }
+       }
+       /* read data from the ring before increasing the index */
+       virt_mb();
+       if (!(flags & MSG_PEEK))
+               intf->in_cons += len;
+
+       return len;
+}
+
+int pvcalls_front_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
+                    int flags)
+{
+       struct pvcalls_bedata *bedata;
+       int ret = -EAGAIN;
+       struct sock_mapping *map;
+       int count = 0;
+
+       if (!pvcalls_front_dev)
+               return -ENOTCONN;
+       bedata = dev_get_drvdata(&pvcalls_front_dev->dev);
+
+       map = (struct sock_mapping *) READ_ONCE(sock->sk->sk_send_head);
+       if (!map)
+               return -ENOTSOCK;
+
+       if (flags & (MSG_CMSG_CLOEXEC|MSG_ERRQUEUE|MSG_OOB|MSG_TRUNC))
+               return -EOPNOTSUPP;
+
+       mutex_lock(&map->active.in_mutex);
+       if (len > XEN_FLEX_RING_SIZE(map->active.ring->ring_order))
+               len = XEN_FLEX_RING_SIZE(map->active.ring->ring_order);
+
+       while (!(flags & MSG_DONTWAIT) && !pvcalls_front_read_todo(map)) {
+               if (count < PVCALLS_FRONT_MAX_SPIN)
+                       count++;
+               else
+                       wait_event_interruptible(map->active.inflight_conn_req,
+                                                pvcalls_front_read_todo(map));
+       }
+       ret = __read_ring(map->active.ring, &map->active.data,
+                         &msg->msg_iter, len, flags);
+
+       if (ret > 0)
+               notify_remote_via_irq(map->active.irq);
+       if (ret == 0)
+               ret = -EAGAIN;
+       if (ret == -ENOTCONN)
+               ret = 0;
+
+       mutex_unlock(&map->active.in_mutex);
+       return ret;
+}
+
 int pvcalls_front_bind(struct socket *sock, struct sockaddr *addr, int 
addr_len)
 {
        struct pvcalls_bedata *bedata;
diff --git a/drivers/xen/pvcalls-front.h b/drivers/xen/pvcalls-front.h
index d937c24..de24041 100644
--- a/drivers/xen/pvcalls-front.h
+++ b/drivers/xen/pvcalls-front.h
@@ -16,5 +16,9 @@ int pvcalls_front_accept(struct socket *sock,
 int pvcalls_front_sendmsg(struct socket *sock,
                          struct msghdr *msg,
                          size_t len);
+int pvcalls_front_recvmsg(struct socket *sock,
+                         struct msghdr *msg,
+                         size_t len,
+                         int flags);
 
 #endif
-- 
1.9.1


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