> Consider using pmap_extract(pm, ...), 

Ok, If kernel behaves so, then AVPT is mandatory because VPT
always belongs to current process.  (readers should note that
ASID was invented for nearly zero-cost context switch)

VPT is 4MB in size, 2MB + 1MB hole + 1MB.  The last segment is
for KSEG2 which is in charge to handle kernel memory.  It's shared
across all processes.  So, if AVPT case is limited for pmap_kernel(),
it's necessity is pretty limited since pmap_kernel() is always available.
I came to the thought 64bit pmap will differ radically so I have to revisit.

> So, what to do you propose to do if the kernel can't provide an optimal 
> VA base for a given mapping?

Go UNCACHED road to enjoy bumpy ride.  User wisely insists to take
incompatible VA.

Toru Nishimura/ALKYL Technology