[xwiki-users] Retrieving main wiki name

Hello, I'm moving toward Workspace enabled wiki, and trying to retrieve a global user Document from a subwiki. I'm using LDAP, but creating users only on global wiki. My problem, is that from various APIs (workspace, wikimanager, xwiki...), I was not able to find an easy to retrieve the name of the main wiki - which in my case is not "xwiki". I need it to provide it to my xwiki.getDocument(username) to retrieve correct doc. Or maybe there's an API to fallback the getDocument() on the global wiki, if doc is not found in local subwiki ? Sorry if it's obvious, I'm quite new to the virtual mode ;-) Thanks, Jeremie