Re: tracking P->V for unmanaged device pages

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

   Date: Thu, 26 Mar 2015 15:30:01 +0100
   From: "J. Hannken-Illjes" <hannken%eis.cs.tu-bs.de@localhost>

   On 26 Mar 2015, at 13:13, Taylor R Campbell <campbell+netbsd-tech-kern%mumble.net@localhost> wrote:

   > The attached patch implements the following extension to pmap(9) on
   > x86 and uses it in DRM[*].  The implementation uses a linear list of
   > pv-tracked ranges, since it is expected to be short (one to three
   > elements).  The list is managed with pserialize(9) so it adds no
   > locking overhead to existing pmap operations that need to look up
   > entries in it.

   If you plan to use pserialize here it is time to change
   pserialize_perform() to not kpause() every xc_broadcast.

   Currently pserialize_perform() takes at least two ticks to complete.

No objection to making pserialize(9) faster, but for this P->V
tracking code, pserialize_perform happens only when you are, e.g.,
detaching a device -- which is about the least performance-critical
path I can imagine.