Mostly is China I believe. About %80-85.
lets first clarify whether you are talking about a hypothetical situation or the reality because in reality the hashrate that is coming from China is nowhere near that percentage. it is probably closer to 20% at this point if not lower. what you may be referring to is the mining pools with high hashrate but they are "pools" not "farms". a pool is where miners from all around the world can connect to (like a server) and share their work. those servers aren't all in China! think of bitcointalk for example of a pool. it was registered by someone in US, but that doesn't mean all the 2.6 million users of bitcointalk are from US. like miners connecting to a pool, the users are connecting to bitcointalk from all around the world.
Thanks a lot for clarifying this issue or concern. Actually, I have been asking and wondering the same question for so long and now I can sense what are the things really going on with bitcoin mining. So even if China will eventually implement a total ban on mining, we will not have a major problem and that things can still be fixed. I am sure that many miners in China right now are already doing something about this situation.