How can I revert all open files in perforce?

I bet you just need to provide a filespec, try p4 revert -c <changelist> //...

For me the following worked as well

p4 revert //... : Revert every file you have open, in every one of your pending changelists, to its pre-opened state.

or

p4 revert -c default //... : Revert every file open in the default changelist to its pre-opened state

Taken from : p4 revert documentation