The reason I'm asking is that there are several errata  for the
Pentium Pro processor which affect control register CR4 and SMM.
In particular, errata 26 implies that CR4.PCE must not be set to
allow the RDPMC instruction in user-mode, and errata 56 implies
that all CR4 settings may be lost after transiting to and from SMM.
 Pentium Pro Processor Specification Update, January 1999.
Intel document number 242689-035.
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