Re: [PATCH v4 09/13] xen/pvcalls: implement sendmsg

[Stefano Stabellini]

On Fri, 22 Sep 2017, Boris Ostrovsky wrote:
> > +static bool 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(PVCALLS_RING_ORDER);
> > +	int32_t error;
> > +
> > +	cons = intf->out_cons;
> > +	prod = intf->out_prod;
> > +	error = intf->out_error;
> > +	if (error == -ENOTCONN)
> > +		return false;
> > +	if (error != 0)
> > +		return true;
> 
> Just like below, error processing can be moved up.

OK


> > +	return !!(size - pvcalls_queued(prod, cons, size));
> > +}
> > +
> >  static irqreturn_t pvcalls_front_event_handler(int irq, void *dev_id)
> >  {
> >  	struct xenbus_device *dev = dev_id;
> > @@ -363,6 +380,108 @@ 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,
> > +			int len)
> > +{
> > +	RING_IDX cons, prod, size, masked_prod, masked_cons;
> > +	RING_IDX array_size = XEN_FLEX_RING_SIZE(PVCALLS_RING_ORDER);
> > +	int32_t error;
> > +
> > +	error = intf->out_error;
> > +	if (error < 0)
> > +		return error;
> > +	cons = intf->out_cons;
> > +	prod = intf->out_prod;
> > +	/* read indexes before continuing */
> > +	virt_mb();
> > +
> > +	size = pvcalls_queued(prod, cons, array_size);
> > +	if (size >= array_size)
> > +		return 0;
> 
> 
> Is it possible to have size > array_size?

Yes, if somebody makes a mistake in writing to prod. Of course, it is
not valid. I guess I could return error instead of 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;
> 
> 
> I know that you said you'd be changing len's type to int but now that I
> am looking at it I wonder whether you could pass len as a 'size_t *' and
> have this routine return error code (i.e. <=0).
> 
> OTOH, we'd be mixing up types again since RING_IDX is an unsigned int.
> 
> So I'll leave it to you (or anyone else reviewing this) to decide which
> way is better.

see below

> > +}
> > +
> > +int pvcalls_front_sendmsg(struct socket *sock, struct msghdr *msg,
> > +			  size_t len)
> 
> Also, the signature here looks suspicious --- you are trying to send
> 'size_t len' bytes but returning an int, which is how many bytes you've
> actually sent. Right?

Yes, but it is OK because it is limited by the size of the array which
is far smaller than INT_MAX (the array size is 262144). This is also why
I would just keep len as int.