linux-kernel - Re: [PATCH v2 04/18] xen/pvcalls: xenbus state handling

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Date:   Thu, 25 May 2017 20:12:04 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xen.org
Cc:     linux-kernel@...r.kernel.org, jgross@...e.com,
        Stefano Stabellini <stefano@...reto.com>
Subject: Re: [PATCH v2 04/18] xen/pvcalls: xenbus state handling

On 05/19/2017 07:22 PM, Stefano Stabellini wrote:
> Introduce the code to handle xenbus state changes.
>
> Implement the probe function for the pvcalls backend. Write the
> supported versions, max-page-order and function-calls nodes to xenstore,
> as required by the protocol.
>
> Introduce stub functions for disconnecting/connecting to a frontend.
>
> Signed-off-by: Stefano Stabellini <stefano@...reto.com>
> CC: boris.ostrovsky@...cle.com
> CC: jgross@...e.com
> ---
>  drivers/xen/pvcalls-back.c | 135 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 135 insertions(+)
>
> diff --git a/drivers/xen/pvcalls-back.c b/drivers/xen/pvcalls-back.c
> index 9044cf2..b4da138 100644
> --- a/drivers/xen/pvcalls-back.c
> +++ b/drivers/xen/pvcalls-back.c
> @@ -25,20 +25,155 @@
>  #include <xen/xenbus.h>
>  #include <xen/interface/io/pvcalls.h>
>  
> +#define PVCALLS_VERSIONS "1"

Shouldn't this be in a header file that will be shared with frontends?

> +#define MAX_RING_ORDER XENBUS_MAX_RING_GRANT_ORDER
> +
>  struct pvcalls_back_global {
>  	struct list_head frontends;
>  	struct semaphore frontends_lock;
>  } pvcalls_back_global;
>  
> +static int backend_connect(struct xenbus_device *dev)
> +{
> +	return 0;
> +}
> +
> +static int backend_disconnect(struct xenbus_device *dev)
> +{
> +	return 0;
> +}
> +
>  static int pvcalls_back_probe(struct xenbus_device *dev,
>  			      const struct xenbus_device_id *id)
>  {
> +	int err;
> +
> +	err = xenbus_printf(XBT_NIL, dev->nodename, "versions", "%s",
> +			    PVCALLS_VERSIONS);
> +	if (err) {
> +		pr_warn("%s write out 'version' failed\n", __func__);
> +		return -EINVAL;

err?

> +	}
> +
> +	err = xenbus_printf(XBT_NIL, dev->nodename, "max-page-order", "%u",
> +			    MAX_RING_ORDER);
> +	if (err) {
> +		pr_warn("%s write out 'max-page-order' failed\n", __func__);
> +		return err;
> +	}
> +
> +	/* "1" means socket, connect, release, bind, listen, accept and poll*/

This comment is obsolete, or at least should refer to
XENBUS_FUNCTIONS_CALLS (and is missing a space at the end).

> +	err = xenbus_printf(XBT_NIL, dev->nodename, "function-calls",
> +			    XENBUS_FUNCTIONS_CALLS);
> +	if (err) {
> +		pr_warn("%s write out 'function-calls' failed\n", __func__);
> +		return err;
> +	}


In case of errors we will end up with all previous entries. I think this
should be done as a transaction which would be aborted in case of an error.

> +
> +	err = xenbus_switch_state(dev, XenbusStateInitWait);
> +	if (err)
> +		return err;


Not sure what to do on an error here (wrt xenstore entries).
xenbus_switch_state() itself uses transactions. Are we allowed to have
nested transactions?

OTOH, xenbus_switch_state() never returns an error, at least now. In
fact, in most cases we ignore return value.


-boris


> +
>  	return 0;
>  }
>  
>