Using %cr2 to reference "current"

["H. Peter Anvin" <hpa@zytor.com>]

2.4.13-ac8 uses %cr2 rather than (%esp & 0xfffe0000) to get "current".
I've been trying to figure out the point of this... writing a control
register is microcode on all the x86 implementations I know (and you
have to re-set it after every pagefault), and reading one probably is
one on most (not Transmeta, but...)

On the other hand, %esp is a GPR and available to the core directly,
and so are usually plain immediates.

Is using %cr2 really faster than the old implementation, or is there
another reason?  It seems that the alignment constraints on the stack
still remains, since the %esp solution still remains in places...

It might also be worth considering a segment-register based
implementation instead.  The reason we're not using %fs and %gs in the
kernel anymore is because of the setup slowness, but perhaps using
them (use %fs since it's much more likely to be NULL and thus faster
to restore) would be faster than using %cr2?

	-hpa
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