[Xen-devel] libxl: Is the nic param to libxl_network_device_add an (in)_out parameter?

2014-11-18 Thread Euan Harris 
Hi,

If I call libxl_device_nic_add and pass in a mostly-default
libxl_device_nic structure, the function fills in the unspecified default
config fields with data for the NIC which it has just created:

libxl_device_nic nic;
libxl_device_nic_init(nic);
/* 
   nic.devid == -1 
   nic.mac == 00:00:00:00:00:00 
   nic.model == null
   etc.
 */

libxl_device_nic_add(ctx, domid, nic, NULL);
/* 
   nic.devid == 3 
   nic.mac == 00:16:3e:1b:7b:12
   nic.model == rtl8139
   etc.
 */

Is this behaviour an intentional part of the API which I can rely on,
or just an artefact of the current implementation?  In other words, is
nic meant to be an (in)_out parameter?  If it's not, what is the correct
way to find out the device ID which was allocated for the new device,
for example?

Thanks,
Euan

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Re: [Xen-devel] libxl: Is the nic param to libxl_network_device_add an (in)_out parameter?

2014-11-18 Thread Ian Campbell 
On Tue, 2014-11-18 at 15:28 +, Euan Harris wrote:
 Hi,
 
 If I call libxl_device_nic_add and pass in a mostly-default
 libxl_device_nic structure, the function fills in the unspecified default
 config fields with data for the NIC which it has just created:
 
   libxl_device_nic nic;
   libxl_device_nic_init(nic);
   /* 
  nic.devid == -1 
  nic.mac == 00:00:00:00:00:00 
nic.model == null
  etc.
*/
 
   libxl_device_nic_add(ctx, domid, nic, NULL);
   /* 
  nic.devid == 3 
  nic.mac == 00:16:3e:1b:7b:12
nic.model == rtl8139
  etc.
*/
 
 Is this behaviour an intentional part of the API which I can rely on,
 or just an artefact of the current implementation?  In other words, is
 nic meant to be an (in)_out parameter?

I believe so, yes. The comment under Devices in libxl.h probably ought
to be adjusted to say so explicitly.

Ian (J) -- do you agree?

   If it's not, what is the correct
 way to find out the device ID which was allocated for the new device,
 for example?

You would have to libxl_device_type_list and look for it, which is
clearly suboptimal.

Ian.


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