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There are building instructions in the workspace itself: http://hg.openjdk.java.net/mlvm/mlvm/file/tip/README.txt
Make a directory which will contain both sets of repositories (patches and full sources), and pull everything there.
The sources repository based at http://hg.openjdk.java.net/hsx/hotspot-comp (This is as of 9/2011; earlier they were based at jdk7/jdk7 and then at bsd-port/bsd-port.)
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