So, a regular volume pedal is just like the volume knob on your bass--the only difference is you "turn" it by rocking your foot back and forth. For slow passages this works great. For faster passages, you'll break your foot trying.
Another option is an effect that automatically swells the beginning of each note. The old Boss "Slow Gear" is the best known one, but there are new ones by VFE, Pigtronix, and others. Maybe somebody else can chime in with more specific names. Unfotunately those automatic ones also don't work so great on fast passages, although it depends on your settings, because the pedal is "listening" to whether you're playing a note or not, and during fast playing it doesn't "hear" any rests.