Thanks for letting us know of any issues you might have.
GDS support on GCN is slightly different than GDS support on EG/NI and it has not been fully verified to the level where I can say it will always work. While we do have some tests that show it does work for values outside of what you are having a problem with, we have not yet been able to fully verify it at the same level as using other memory features on SI. Also, the timer also not working on SI is a known issue and something we are working to fix.
Thanks for the very clear answers. Even 8 words is still useful for synchronizing CUs, which is
mostly what it is used for.