Intel IA-32 - Manual (Page 298)

7-30 Vol. 3A
MULTIPLE-PROCESSOR MANAGEMENT
The performance counter interrupts, events, and precise event monitoring support can be set up
and allocated on a per thread (per logical processor) basis.
See Section 18.14, “Performance Monitoring and Hyper-Threading Technology,” for a discus-
sion of performance monitoring in the Intel Xeon processor MP.
7.8.8 IA32_MISC_ENABLE MSR
The IA32_MISC_ENABLE MSR (MSR address 1A0H) is shared between the logical proces-
sors in an IA-32 processor supporting Hyper-Threading Technology. Thus the architectural
features that this register controls are set the same for all the logical processors in the same phys-
ical package.
7.8.9 Memory Ordering
The logical processors in an IA-32 processor supporting Hyper-Threading Technology obey the
same rules for memory ordering as IA-32 processors without HT Technology (see Section 7.2,
“Memory Ordering”). Each logical processor uses a processor-ordered memory model that can
be further defined as “write-ordered with store buffer forwarding.” All mechanisms for strength-
ening or weakening the memory ordering model to handle special programming situations apply
to each logical processor.
7.8.10 Serializing Instructions
As a general rule, when a logical processor in an IA-32 processor supporting Hyper-Threading
Technology executes a serializing instruction, only that logical processor is affected by the oper-
ation. An exception to this rule is the execution of the WBINVD, INVD, and WRMSR instruc-
tions; and the MOV CR instruction when the state of the CD flag in control register CR0 is
modified. Here, both logical processors are serialized.
7.8.11 MICROCODE UPDATE Resources
In an IA-32 processor supporting Hyper-Threading Technology, the microcode update facilities
are shared between the logical processors; either logical processor can initiate an update. Each
logical processor has its own BIOS signature MSR (IA32_BIOS_SIGN_ID at MSR address
8BH). When a logical processor performs an update for the physical processor, the
IA32_BIOS_SIGN_ID MSRs for resident logical processors are updated with identical infor-
mation. If logical processors initiate an update simultaneously, the processor core provides the
necessary synchronization needed to insure that only one update is performed at a time.
Operating system microcode update drivers that adhere to Intel’s guidelines do not need to be
modified to run on an IA-32 processor supporting Hyper-Threading Technology.