Spell familiar

This came up in a recent game with Tom, and I think I might of got the rule wrong.

Rule says "A model with a spell familiar my re-roll failed Psychic tests."

So trying to cast a 2 warp charge skill with two dice and you only pass one roll and miss the other. Would you have to re roll both dice or would you keep the passed roll and just re roll the failed roll?

I've been doing it as you reroll all of the dice. Rolling X amount of dice to try pass is the psychic test, so to me that says you gotta reroll em all.

the kicker is that you can't reroll perils cuz they are generally successes...

Yup.. all the dice. 
It will say specifcally if you're allowed to roll one of the dice.. like if you have Fleet and you charge, you can choose which dice you may reroll. 

I think it might actually clarify it in the first few pages of the book.