[Xen-devel] Xenstore watch interface in the kernel

[Juergen Gross <jgross@xxxxxxxx>]

While working on the Linux xenbus kernel driver I stumbled over a rather
strange interface: a Xenstore watch event is delivered via a callback
defined as:

void (*callback)(struct xenbus_watch *,
                         const char **vec, unsigned int len);

vec is an array of strings and len the number of strings in that
array.

Looking at the Xenstore interface I don't see how there could ever be
an array with another len than 2 be presented (the first string being
the modified path, the second the token specified when registering
the watch).

I'd like to modify the callback's prototype to:

void (*callback)(struct xenbus_watch *,
                         const char *path, const char *token);

This would make it easier to avoid some memory allocation when
reading a Xenstore reply, as in case of a watch event I'd know
how many strings are expected.

Is there any reason not to change the interface in the kernel?

BTW: The handling of more than 2 strings as watch event parameters is
even repeated in the interface to libxenstore. I looked through the Xen
sources and could find no use of the number of strings returned in case
of a watch event. While we can't change the interface of libxenstore
I don't think we have to be prepared for an arbitrary number of strings
for a watch event at the kernel interface.

Juergen

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