Re: [Xen-devel] [PATCH v5 06/18] xen/pvcalls: handle commands from the frontend

[Juergen Gross <jgross@xxxxxxxx>]

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@xxxxxxxxxxx>
> CC: boris.ostrovsky@xxxxxxxxxx
> CC: jgross@xxxxxxxx
> ---
>  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?

> +                             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.

> +
> +             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.


Juergen

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