From the spec
Any SBI function, requiring a hart mask, must take the following two arguments:
unsigned long hart_mask is a scalar bit-vector containing hartids unsigned long hart_mask_base is the starting hartid from which the bit-vector must be computed.
hart_mask_base can be set to -1 to indicate that hart_mask shall be ignored and all available harts must be considered.
PS: I'll fix the TODO below, I just don't remember why did I put it there. Probably it is not needed anymore