Re: [Xen-devel] [PATCH v3 09/13] xen/pvcalls: implement sendmsg

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 14 Aug 2017, Boris Ostrovsky wrote:
> On 07/31/2017 06:57 PM, Stefano Stabellini wrote:
> > Send data to an active socket by copying data to the "out" ring. Take
> > the active socket out_mutex so that only one function can access the
> > ring at any given time.
> > 
> > If not enough room 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 significantly.
> > 
> > Signed-off-by: Stefano Stabellini <stefano@xxxxxxxxxxx>
> > CC: boris.ostrovsky@xxxxxxxxxx
> > CC: jgross@xxxxxxxx
> > ---
> >   drivers/xen/pvcalls-front.c | 109
> > ++++++++++++++++++++++++++++++++++++++++++++
> >   drivers/xen/pvcalls-front.h |   3 ++
> >   2 files changed, 112 insertions(+)
> > 
> > diff --git a/drivers/xen/pvcalls-front.c b/drivers/xen/pvcalls-front.c
> > index f83b910..369acde 100644
> > --- a/drivers/xen/pvcalls-front.c
> > +++ b/drivers/xen/pvcalls-front.c
> > @@ -29,6 +29,7 @@
> >   #define PVCALLS_INVALID_ID UINT_MAX
> >   #define PVCALLS_RING_ORDER XENBUS_MAX_RING_GRANT_ORDER
> >   #define PVCALLS_NR_REQ_PER_RING __CONST_RING_SIZE(xen_pvcalls,
> > XEN_PAGE_SIZE)
> > +#define PVCALLS_FRONT_MAX_SPIN 5000
> >     struct pvcalls_bedata {
> >     struct xen_pvcalls_front_ring ring;
> > @@ -88,6 +89,22 @@ static inline int get_request(struct pvcalls_bedata
> > *bedata, int *req_id)
> >     return 0;
> >   }
> >   +static int pvcalls_front_write_todo(struct sock_mapping *map)
> > +{
> > +   struct pvcalls_data_intf *intf = map->active.ring;
> > +   RING_IDX cons, prod, size = XEN_FLEX_RING_SIZE(intf->ring_order);
> > +   int32_t error;
> > +
> > +   cons = intf->out_cons;
> > +   prod = intf->out_prod;
> > +   error = intf->out_error;
> > +   if (error == -ENOTCONN)
> > +           return 0;
> > +   if (error != 0)
> > +           return error;
> > +   return size - pvcalls_queued(prod, cons, size);
> 
> Do you ever look at actual return value except whether it's zero or not? Both
> here and in the poll patch you look for !=0 and never check for an error.

No, I can turn the return value into bool.


> > +}
> > +
> >   static irqreturn_t pvcalls_front_event_handler(int irq, void *dev_id)
> >   {
> >     struct xenbus_device *dev = dev_id;
> > @@ -325,6 +342,98 @@ int pvcalls_front_connect(struct socket *sock, struct
> > sockaddr *addr,
> >     return ret;
> >   }
> >   +static int __write_ring(struct pvcalls_data_intf *intf,
> > +                   struct pvcalls_data *data,
> > +                   struct iov_iter *msg_iter,
> > +                   size_t len)
> > +{
> > +   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->out_cons;
> > +   prod = intf->out_prod;
> > +   error = intf->out_error;
> > +   /* read indexes before continuing */
> > +   virt_mb();
> > +
> > +   if (error < 0)
> > +           return error;
> 
> This can be done before the barrier. (In fact, this can be done first thing,
> before cons and prod are read).

Good point.


> > +
> > +   size = pvcalls_queued(prod, cons, array_size);
> > +   if (size >= array_size)
> > +           return 0;
> > +   if (len > array_size - size)
> > +           len = array_size - size;
> > +
> > +   masked_prod = pvcalls_mask(prod, array_size);
> > +   masked_cons = pvcalls_mask(cons, array_size);
> > +
> > +   if (masked_prod < masked_cons) {
> > +           copy_from_iter(data->out + masked_prod, len, msg_iter);
> > +   } else {
> > +           if (len > array_size - masked_prod) {
> > +                   copy_from_iter(data->out + masked_prod,
> > +                                  array_size - masked_prod, msg_iter);
> > +                   copy_from_iter(data->out,
> > +                                  len - (array_size - masked_prod),
> > +                                  msg_iter);
> > +           } else {
> > +                   copy_from_iter(data->out + masked_prod, len,
> > msg_iter);
> > +           }
> > +   }
> > +   /* write to ring before updating pointer */
> > +   virt_wmb();
> > +   intf->out_prod += len;
> > +
> > +   return len;
> 
> You are returning size_t. I actually was going to ask in one of the previous
> patches whether using int for sizes was correct. Unfortunately I can't
> remember which struct/function I was looking at ;-(
> 
> Of course, you are also possibly returning a (negative) error here.

You are right, len should not be size_t but ssize_t or int. Especially
given that we cannot write more than array_size bytes to the ring.

For simplicity, I'll change the type of len to int in __write_ring and
add a check on len >= INT_MAX in the caller.


> > +}
> > +
> > +int pvcalls_front_sendmsg(struct socket *sock, struct msghdr *msg,
> > +                     size_t len)
> > +{
> > +   struct pvcalls_bedata *bedata;
> > +   struct sock_mapping *map;
> > +   int sent = 0, tot_sent = 0;
> 
> 'sent' doesn't need to be initialized.

OK


> > +   int count = 0, flags;
> > +
> > +   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;
> 
> IIRC the error value for sk_send_head being zero is inconsistent across
> patches.

I'll fix it.


> > +
> > +   flags = msg->msg_flags;
> > +   if (flags & (MSG_CONFIRM|MSG_DONTROUTE|MSG_EOR|MSG_OOB))
> > +           return -EOPNOTSUPP;
> > +
> > +   mutex_lock(&map->active.out_mutex);
> > +   if ((flags & MSG_DONTWAIT) && !pvcalls_front_write_todo(map)) {
> > +           mutex_unlock(&map->active.out_mutex);
> > +           return -EAGAIN;
> > +   }
> > +
> > +again:
> > +   count++;
> > +   sent = __write_ring(map->active.ring,
> > +                       &map->active.data, &msg->msg_iter,
> > +                       len);
> > +   if (sent > 0) {
> > +           len -= sent;
> > +           tot_sent += sent;
> > +           notify_remote_via_irq(map->active.irq);
> > +   }
> > +   if (sent >= 0 && len > 0 && count < PVCALLS_FRONT_MAX_SPIN)
> > +           goto again;
> > +   if (sent < 0)
> > +           tot_sent = sent;
> 
> What does it mean when an error is detected on the interface? Does it need to
> be somehow propagated to the caller?

The error could be a genuine socket error set by the backend, for
example ECONNRESET (Connection reset by peer). The error will be
returned by __write_ring. By setting tot_sent to sent, we return it to
the caller of pvcalls_front_sendmsg. Eventually the userspace
application will see an errno = ECONNRESET.

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