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Power Management
4.2.3
Requesting Low-Power Idle States
The primary software interfaces for requesting low-power idle states are through the
MWAIT instruction with sub-state hints and the HLT instruction (for C1 and C1E).
However, software may make C-state requests using the legacy method of I/O reads
from the ACPI-defined processor clock control registers, referred to as P_LVLx. This
method of requesting C-states provides legacy support for operating systems that
initiate C-state transitions using I/O reads.
For legacy operating systems, P_LVLx I/O reads are converted within the processor to
the equivalent MWAIT C-state request. Therefore, P_LVLx reads do not directly result in
I/O reads to the system. The feature, known as I/O MWAIT redirection, must be
enabled in the BIOS.
Note:
The P_LVLx I/O Monitor address needs to be set up before using the P_LVLx I/O read
interface. Each P-LVLx is mapped to the supported MWAIT(Cx) instruction as follows:
Table 4-4.
P_LVLx to MWAIT Conversion
P_LVLx
P_LVL2
P_LVL3
The BIOS can write to the C-state range field of the PMG_IO_CAPTURE MSR to restrict
the range of I/O addresses that are trapped and emulate MWAIT like functionality. Any
P_LVLx reads outside of this range does not cause an I/O redirection to MWAIT(Cx) like
request. They fall through like a normal I/O instruction.
Note:
When P_LVLx I/O instructions are used, MWAIT substates cannot be defined. The
MWAIT substate is always zero if I/O MWAIT redirection is used. By default, P_LVLx I/O
redirections enable the MWAIT 'break on EFLAGS.IF' feature that triggers a wakeup on
an interrupt, even if interrupts are masked by EFLAGS.IF.
4.2.4
Core C-states
The following are general rules for all core C-states, unless specified otherwise:
• A core C-State is determined by the lowest numerical thread state (such that,
Thread 0 requests C1E while thread1 requests C3, resulting in a core C1E state).
See
Table
• A core transitions to C0 state when:
— an interrupt occurs.
— there is an access to the monitored address if the state was entered using an
MWAIT instruction.
• For core C1/C1E, and core C3, an interrupt directed toward a single thread wakes
only that thread. However, since both threads are no longer at the same core C-
state, the core resolves to C0.
• For core C6, an interrupt coming into either thread wakes both threads into C0
state.
• Any interrupt coming into the processor package may wake any core.
Datasheet, Volume 1
MWAIT(Cx)
MWAIT(C3)
MWAIT(C6)
C6. No sub-states allowed
4-3.
Notes
39
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