Specification Update

Errata
84 Specification Update
W125. CPL-Qualified BTS May Report Incorrect Branch-From
Instruction Address
Problem: CPL (Current Privilege Level)-qualified BTS (Branch Trace Store) may
report incorrect branch-from instruction address under the following
conditions:
• Either BTS_OFF_OS [9] or BTS_OFF_USR [10] is selected in IA32_DEBUGCTLC
MSR (1D9H).
• Privilege-level transitions occur between CPL > 0 and CPL 0 or vice versa.
Implication: Due to this erratum, the from address reported by BTS may be
incorrect for the described conditions.
Workaround: None identified
Status: For the steppings affected, see the Summary Tables of Changes.
W126. PEBS Does Not Always Differentiate between CPL-Qualified
Events
Problem: Performance monitoring counter configured to sample PEBS (Precise Event
Based Sampling) events at a certain privilege level may count samples
at the wrong privilege level.
Implication: Performance monitoring counter may be higher than expected for CPL-
qualified events. Do not use performance monitoring counters for
precise event sampling when the precise event is dependent on the CPL
value.
Workaround: None identified.
Status: For the steppings affected, see the Summary Tables of Changes.
W127. PMI May Be Delayed to Next PEBS Event
Problem: After a PEBS (Precise Event-Based Sampling) event, the PEBS index is
compared with the PEBS threshold, and the index is incremented with
every event. If PEBS index is equal to the PEBS threshold, a PMI
(Performance Monitoring Interrupt) should be issued. Due to this
erratum, the PMI may be delayed by one PEBS event.
Implication: Debug Store Interrupt Service Routines may observe delay of PMI
occurrence by one PEBS event.
Workaround: None identified.
Status: For the steppings affected, see the Summary Tables of Changes.