Some MSRs are shared, some aren't. One must always check this in
the IA32 Volume 3 manual. The three SYSENTER MSRs are not shared.
However, no-one has yet proven that writing to these in the context
switch path has acceptable performance -- remember, there is _no_
a priori reason to assume _anything_ about performance on P4s,
you really do need to measure things before taking design decisions.
Manfred had a version with fixed MSR values and the varying data
in memory. Maybe that's actually faster.
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