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x86_64 and CX8

is someone aware of a x86_64 CPU which does *not* support cmpxchg8b ?
The instruction seems mandatory since i686-class CPUs, and it seems that
since windows XP cmpxchg8b is not optional.

in amd64/amd64/spl.S we have 2 implementations of spllower(), which seems
overkill if all x86-64 CPUs supports cmpxchg8b ...

The reason I'm asking is I want to switch XENPV to cmpxchg8b versions
of spllower and mutex_exit, and we can't use x86_patch() here.

Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference

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