Specification Update

Errata
28 Specification Update
AZ22. Performance Monitoring Events for Retired Instructions (C0H) May Not
Be Accurate
Problem: The INST_RETIRED performance monitor may miscount retired instructions as follows:
• Repeat string and repeat I/O operations are not counted when a hardware interrupt
is received during or after the last iteration of the repeat flow.
• VMLAUNCH and VMRESUME instructions are not counted.
• HLT and MWAIT instructions are not counted. The following instructions, if executed
during HLT or MWAIT events, are also not counted:
a) RSM from a C-state SMI during an MWAIT instruction.
b) RSM from an SMI during a HLT instruction.
Implication: There may be a smaller than expected value in the INST_RETIRED performance
monitoring counter. The extent to which this value is smaller than expected is
determined by the frequency of the above cases.
Workaround: None identified.
Status: For the steppings affected, see the Summary Tables of Changes.
AZ23. Returning to Real Mode from SMM with EFLAGS.VM Set May Result in
Unpredictable System Behavior
Problem: Returning back from SMM mode into real mode while EFLAGS.VM is set in SMRAM may
result in unpredictable system behavior.
Implication: If SMM software changes the values of the EFLAGS.VM in SMRAM, it may result in
unpredictable system behavior. Intel has not observed this behavior in commercially
available software.
Workaround: SMM software should not change the value of EFLAGS.VM in SMRAM.
Status: For the steppings affected, see the Summary Tables of Changes.