On Thu, 19 May 2016, Arnd Bergmann wrote:
2 drivers/xen/balloon.c:154:13: warning: 'release_memory_resource' declared 'static' but never defined [-Wunused-function]
I sent a patch on May 11, subject "xen: remove incorrect forward declaration" and Stefano Stabellini reviewed it. Ross Lagerwall did the same patch a day earlier, but neither of them has made it into linux-next so far. According to Ross, this one should be backported to v4.4.
It's on our radar, the patch hasn't been lost.