Re: tracking P->V for unmanaged device pages

[Taylor R Campbell <campbell+netbsd-tech-kern%mumble.net@localhost>]

   Date: Wed, 8 Apr 2015 12:29:13 +0900
   From: Masao Uebayashi <uebayasi%gmail.com@localhost>

   Considering these, PV should be kept in struct vm_physseg, instead of
   struct vm_page, ideally.

The only difficulty with that is that you still need a fast way to go
from a struct vm_page to the PV tracking record.  Since vm_page has no
pointer to vm_physseg, you'd need to add one in struct vm_page_md.
But then you might as well just include the PV tracking record in the
struct vm_page_md anyway.

   PV should be also an MI struct, like struct vm_pv.

No objection here.  I think everyone would like to see MI code for PV
tracking instead of having copypasta in every pmap.  But someone has
to do the work.

   If you once have such a right design, you no longer need to have a
   separate API to track PV, because pmap_enter() is given sufficient
   information to decide whether PV is tracked or not; P/V addresses and
   flags telling how to map the new virtual address (cache enabled, PAT
   used, etc.).

In my pmap_pv API, there's no need to change pmap_enter.  All you need
to do is to call pmap_pv_track on driver attach to mark the pages as
device pages.  Then pmap can do the bookkeeping inside.

It sounds like what you're proposing is to replace pmap_pv_track by a
version of uvm_page_physload that will not allocate struct vm_page for
each page in the region, but only struct vm_pv.  I have no objection
to this either, but someone has to do the work.