re-entry visa

Hi,

I am coming to China from Paris on an F visa valid for 6 months.

I would like to travel to Hong Kong in this time to visit a friend, and am worried about getting another visa while I am there as it will probably be a short weekend trip.

Is there a way to get a re-entry visa before I leave for Hong Kong? I am now staying in Shanghai

What documentation might I need?

Thanks for any help!