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

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Date:   Mon, 3 Jul 2017 13:23:33 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xen.org
Cc:     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 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?

> +				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