Date:   Mon, 3 Jul 2017 13:57:47 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Juergen Gross <jgross@...e.com>
cc:     Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xen.org, linux-kernel@...r.kernel.org,
        boris.ostrovsky@...cle.com,
        Stefano Stabellini <stefano@...reto.com>
Subject: Re: [PATCH v5 06/18] xen/pvcalls: handle commands from the
 frontend

On Mon, 3 Jul 2017, Juergen Gross wrote:
> On 22/06/17 21:14, Stefano Stabellini wrote:
> > When the other end notifies us that there are commands to be read
> > (pvcalls_back_event), wake up the backend thread to parse the command.
> > 
> > The command ring works like most other Xen rings, so use the usual
> > ring macros to read and write to it. The functions implementing the
> > commands are empty stubs for now.
> > 
> > Signed-off-by: Stefano Stabellini <stefano@...reto.com>
> > CC: boris.ostrovsky@...cle.com
> > CC: jgross@...e.com
> > ---
> >  drivers/xen/pvcalls-back.c | 119 +++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 119 insertions(+)
> > 
> > diff --git a/drivers/xen/pvcalls-back.c b/drivers/xen/pvcalls-back.c
> > index e4c2e46..437c2ad 100644
> > --- a/drivers/xen/pvcalls-back.c
> > +++ b/drivers/xen/pvcalls-back.c
> > @@ -51,12 +51,131 @@ struct pvcalls_fedata {
> >  	struct work_struct register_work;
> >  };
> >  
> > +static int pvcalls_back_socket(struct xenbus_device *dev,
> > +		struct xen_pvcalls_request *req)
> > +{
> > +	return 0;
> > +}
> > +
> > +static int pvcalls_back_connect(struct xenbus_device *dev,
> > +				struct xen_pvcalls_request *req)
> > +{
> > +	return 0;
> > +}
> > +
> > +static int pvcalls_back_release(struct xenbus_device *dev,
> > +				struct xen_pvcalls_request *req)
> > +{
> > +	return 0;
> > +}
> > +
> > +static int pvcalls_back_bind(struct xenbus_device *dev,
> > +			     struct xen_pvcalls_request *req)
> > +{
> > +	return 0;
> > +}
> > +
> > +static int pvcalls_back_listen(struct xenbus_device *dev,
> > +			       struct xen_pvcalls_request *req)
> > +{
> > +	return 0;
> > +}
> > +
> > +static int pvcalls_back_accept(struct xenbus_device *dev,
> > +			       struct xen_pvcalls_request *req)
> > +{
> > +	return 0;
> > +}
> > +
> > +static int pvcalls_back_poll(struct xenbus_device *dev,
> > +			     struct xen_pvcalls_request *req)
> > +{
> > +	return 0;
> > +}
> > +
> > +static int pvcalls_back_handle_cmd(struct xenbus_device *dev,
> > +				   struct xen_pvcalls_request *req)
> > +{
> > +	int ret = 0;
> > +
> > +	switch (req->cmd) {
> > +	case PVCALLS_SOCKET:
> > +		ret = pvcalls_back_socket(dev, req);
> > +		break;
> > +	case PVCALLS_CONNECT:
> > +		ret = pvcalls_back_connect(dev, req);
> > +		break;
> > +	case PVCALLS_RELEASE:
> > +		ret = pvcalls_back_release(dev, req);
> > +		break;
> > +	case PVCALLS_BIND:
> > +		ret = pvcalls_back_bind(dev, req);
> > +		break;
> > +	case PVCALLS_LISTEN:
> > +		ret = pvcalls_back_listen(dev, req);
> > +		break;
> > +	case PVCALLS_ACCEPT:
> > +		ret = pvcalls_back_accept(dev, req);
> > +		break;
> > +	case PVCALLS_POLL:
> > +		ret = pvcalls_back_poll(dev, req);
> > +		break;
> > +	default:
> > +		ret = -ENOTSUPP;
> > +		break;
> > +	}
> > +	return ret;
> > +}
> > +
> >  static void pvcalls_back_work(struct work_struct *work)
> >  {
> > +	struct pvcalls_fedata *fedata = container_of(work,
> > +		struct pvcalls_fedata, register_work);
> > +	int notify, notify_all = 0, more = 1;
> > +	struct xen_pvcalls_request req;
> > +	struct xenbus_device *dev = fedata->dev;
> > +
> > +	while (more) {
> > +		while (RING_HAS_UNCONSUMED_REQUESTS(&fedata->ring)) {
> > +			RING_COPY_REQUEST(&fedata->ring,
> > +					  fedata->ring.req_cons++,
> > +					  &req);
> > +
> > +			if (!pvcalls_back_handle_cmd(dev, &req)) {
> 
> Hmm, no response in case of not supported command?

Good point, I'll add one.


> > +				RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(
> > +					&fedata->ring, notify);
> > +				notify_all += notify;
> > +			}
> > +		}
> > +
> > +		if (notify_all)
> > +			notify_remote_via_irq(fedata->irq);
> 
> Want to reset notify_all in above if?
> Could have been an "accept" which didn't queues a response.

Yes, I'll do that.


> > +
> > +		RING_FINAL_CHECK_FOR_REQUESTS(&fedata->ring, more);
> > +	}
> >  }
> >  
> >  static irqreturn_t pvcalls_back_event(int irq, void *dev_id)
> >  {
> > +	struct xenbus_device *dev = dev_id;
> > +	struct pvcalls_fedata *fedata = NULL;
> > +
> > +	if (dev == NULL)
> > +		return IRQ_HANDLED;
> > +
> > +	fedata = dev_get_drvdata(&dev->dev);
> > +	if (fedata == NULL)
> > +		return IRQ_HANDLED;
> > +
> > +	/*
> > +	 * TODO: a small theoretical race exists if we try to queue work
> > +	 * after pvcalls_back_work checked for final requests and before
> > +	 * it returns. The queuing will fail, and pvcalls_back_work
> > +	 * won't do the work because it is about to return. In that
> > +	 * case, we lose the notification.
> > +	 */
> > +	queue_work(fedata->wq, &fedata->register_work);
> 
> I know you like workqueues more than IRQ threads. But probably the above
> TODO could be handled via an IRQ thread more easily?
> 
> I think you should either solve above race, or add a comment why it is
> not problematic, or show us why an IRQ thread doesn't solve the problem.

I think actually that an irq thread is exactly what we need to solve
this race. Thanks for the suggestion! I'll change the code to use it.

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