"Hijacking" a PCU.

[Matt Thomas <matt%3am-software.com@localhost>]

One of the features added to NetBSD 6 was a machine-independent framework for 
dealing with "Per-CPU Units" (PCU).  These are things such as FPUs or SIMDs.  
By handing all the complex stuff (lazy switching), a lot of machine-dependent 
code goes away.

However, there is one disadvantage to using PCU.  If you want to use a PCU in 
the kernel (for faster copies for instance), PCU isn't very accommodating.

In the patch at http://www.netbsd.org/~matt/pcu-hijack.txt I've implemented the 
concept of hijacking a PCU for a kernel LWP to use.  If you assume that most 
LWPs won't be using the PCU, then using the PCU for the kernel should have 
little overhead since it shouldn't need to save/restore PCU state since there 
is no one else using it.  But one thing you don't want is lazy switching since 
the LWP that the hijacked PCU will be using it until it surrenders it.

To accomodate this, whenever a LWP that has hijacked a PCU is resumed and it no 
longer owns the PCU, it saves the current PCU state and immediately re-loads 
its own hijacked state.  When the PCU is surrendered, the PCU state is simply 
discarded since when the PCU was initially hijacked, any state that needed to 
saved was saved.

Using this feature, one could easily implement AltiVec or NEON based 
copyin/copyout/etc.