It would just be the taxable amount of SS. But I don’t know if it is excluded from your AGI. There are 2 extra taxes forms 8959 and 8960.
IRS info on Net Investment Income Tax
https://www.irs.gov/newsroom/questions-and-answers-on-the-net-investment-income-tax
IRS Info on the Additional Medicare tax on high earners (for wages)
http://www.irs.gov/Businesses/Small-Businesses-&-Self-Employed/Questions-and-Answers-for-the-Additio...
