Re: [PATCH v3 10/13] xen/pvcalls: implement recvmsg

[Stefano Stabellini]

On Mon, 14 Aug 2017, Boris Ostrovsky wrote:
> On 07/31/2017 06:57 PM, Stefano Stabellini wrote:
> > 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 no 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 | 102
> > ++++++++++++++++++++++++++++++++++++++++++++
> >   drivers/xen/pvcalls-front.h |   4 ++
> >   2 files changed, 106 insertions(+)
> > 
> > diff --git a/drivers/xen/pvcalls-front.c b/drivers/xen/pvcalls-front.c
> > index 369acde..635a83a 100644
> > --- a/drivers/xen/pvcalls-front.c
> > +++ b/drivers/xen/pvcalls-front.c
> > @@ -105,6 +105,20 @@ 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;
> > @@ -434,6 +448,94 @@ 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;
> 
> Not necessary to initialize.

Yes, you are right.


> > +	struct sock_mapping *map;
> > +
> > +	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)) {
> > +		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;
> 
> Are errors converted by the caller? (I am asking because recvmsg can only
> return -1 for errors)

Yes, the caller will set errno appropriately.


> > +}
> > +
> >   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
> > 
>