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"Hijacking" a PCU.
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
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