Re: Introducing non-RAM managed page ("device page")

[Eduardo Horvath <eeh%NetBSD.org@localhost>]

On Tue, 15 Sep 2009, der Mouse wrote:

> > I'm curious why you want to get the VM subsystem involved in this
> > process.  The VM subsystem manages the virtualization of main memory,
> > hence the name.  [...]
> 
> > We've handled mmap()ing device memory forever without getting VM
> > involved.
> 
> How have you managed that?  On most architectures - all I'm familiar
> with, certainly - all memory accesses go through the VM hardware (well,
> all do if any do), so if you want to mmap a device you have to
> _somehow_ poke the VM hardware to set up the mapping.

The VM subsystem does not fiddle with the MMU.  pmap does.  The VM 
subsystem just keeps track of how pages are used.  It's been this way 
since at least BSD 4.4.

Eduardo