Da Feng's x64 fixes include changes to the JNI Functions FindClass and DefineClass that ensure that a proper classloader is returned to JNI even for threads that were newly attached via JNI. We cannot test this without an implementation of the functions for attaching and detaching threads.
This makes this issue a transitive dependency of Da Feng's fixes for x64 if we don't want to separate the FindClass/DefineClass bugfix from the x64 patch set.
There is an implementation of thread attaching and detaching in MRP-110 but it was not part of Da Feng's x64 patch set for the RVM. Da Feng has a contributor statement for MRP (see MRP-43, readmeAndCLa.tar.gz) so I suppose that it would be legally ok for us to use that code.