Can I use crossfire with one pcie 3.0 and one pce 2.0?

I want to use crossfire with dual 7870's, but my motherboard only has one pcie 3.0, but it also has one pcie 2.0. Now, I know a pcie 3.0 card works with pcie 2.0 but I don't know if it will knock back the performance or if it will even work. Thanks!
Update: Both of the pcie slots are x16
2 answers 2