Re: [Xen-devel] [PATCH v2 11/18] libxl: synchronise configuration when we hotplug a device

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Wed, Aug 27, 2014 at 03:00:39AM +0100, Ian Campbell wrote:
> On Wed, 2014-07-30 at 19:23 +0100, Wei Liu wrote:
> > As those routines are called both during domain creation and device
> > hotplug, we add a flag to indicate whether we need to update JSON
> > config. This flag is only set to true when we hotplug a device. We
> > cannot update JSON config during domain creation as JSON config is
> > committed to disk only when domain creation finishes.
> 
> Rather than carry a flag around did you consider just checking for the
> presence of the file? I think you indicated in an earlier patch that you
> were going to treat lack of the file as meaning creation/destruction was
> happening.
> 

I think a flag is more explict.

These libxl__device_*_add functions are called under two circumstances,
a) domain creation, b) device hotplug.

In b) if the file is not present it could also be in abnormal state --
file is deleted by accident, file system error etc. Carrying a flag can
help up distinguish normal and abnormal state. (Though I can see my code
currently lacks checking this at the moment)

> 
> > + * They take 6 parameters:
> > + *  type:    the type of the device, say nic, vtpm, disk, pci etc
> > + *  ptr:     the pointer to array inside libxl_domain_config
> 
> To the array or to a specific element of the array? I think the latter.

To the array of libxl_device_#type. It doesn't point to specific
element.

> You might also want to indicate that the ptr must be of type
> libxl_device_#type?
> 

Sure.

> > + *  cnt:     the counter of array
> 
> I think you either mean index or length, I suspect the former?
> 
> > + *  domid:   domain id of target domain
> > + *  dev:     the device that is to be added / removed / updated
> > + *  compare: the COMPARE_* macro used to compare @dev's identifier to
> > + *           those in the array pointed to by @ptr
> > + *
> > + * Return 0 if no error occurs, ERROR_* otherwise.
> > + *
> > + * For most device types (nic, vtpm), the array name @ptr and array
> > + * counter @cnt can be derived from @type, pci device being the
> > + * exception, hence we need to have @ptr and @cnt.
> 
> You could get away with a single variable naming the Array ("nics",
> "pcidevs") to which you paste num_ on the front when you need cnt.
> 

OK.

Wei.

> Ian.

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