What to use under floor tile on 2nd floor

[Wipedout]

So I'm tiling the floor in the upstairs bathroom

What was under the current flooring is just the 2x6s that they covered the whole second floor with. 

It's relatively flat but far from perfect

So backer board I assume?  Bob Villa site says to cover the 2x6s with a thin layer of thin set then backer board then thin set and tile

Can I use Hardie backer? Or just concrete backer?

Any other suggestions/options.?

I'm replacing cabinets too. Should I do the whole floor in backer then install cabinets and then tile to the cabinets?

Thanks

[cams2705]

My home was built three years ago.  The builder used 3/4" tongue and groove OSB subfloor as a base and then concrete backer board over top of that.  They mud and taped all the joints and screw holes before laying the tile.

[MT Woodworker]

Plus 1 on the OSB then the concrete backer board
Make it water tight