Advanced Configuration and Power Interface Specification

Advanced Configuration and Power Interface Specification
Table 5-77 Flags – GICC Affinity Structure
Field
Byte
Length
Byte
Offset
Description
Enabled 1 0 If clear, the OSPM ignores the contents of the GICC Affinity
Structure. This allows system firmware to populate the SRAT with
a static number of structures but only enable them as necessary.
Reserved 31 1 Must be zero.
5.2.17 System Locality Distance Information Table (SLIT)
This optional table provides a matrix that describes the relative distance (memory latency) between
all System Localities, which are also referred to as Proximity Domains. Systems employing a Non
Uniform Memory Access (NUMA) architecture contain collections of hardware resources including
for example, processors, memory, and I/O buses, that comprise what is known as a “NUMA node”.
Processor accesses to memory or I/O resources within the local NUMA node is generally faster than
processor accesses to memory or I/O resources outside of the local NUMA node.
The value of each Entry[i,j] in the SLIT table, where i represents a row of a matrix and j represents a
column of a matrix, indicates the relative distances from System Locality / Proximity Domain i to
every other System Locality j in the system (including itself).
The i,j row and column values correlate to the value returned by the _PXM object in the ACPI
namespace. See Section 6.2.14, “_PXM (Proximity)” for more information.
The entry value is a one-byte unsigned integer. The relative distance from System Locality i to
System Locality j is the i*N + j entry in the matrix, where N is the number of System Localities.
Except for the relative distance from a System Locality to itself, each relative distance is stored
twice in the matrix. This provides the capability to describe the scenario where the relative distances
for the two directions between System Localities is different.
The diagonal elements of the matrix, the relative distances from a System Locality to itself are
normalized to a value of 10. The relative distances for the non-diagonal elements are scaled to be
relative to 10. For example, if the relative distance from System Locality i to System Locality j is
2.4, a value of 24 is stored in table entry i*N+ j and in j*N+ i, where N is the number of System
Localities.
If one locality is unreachable from another, a value of 255 (0xFF) is stored in that table entry.
Distance values of 0-9 are reserved and have no meaning.
Table 5-78 SLIT Format
Field
Byte
Length
Byte
Offset
Description
Header
Signature 4 0 ‘SLIT’. Signature for the System Locality Distance Information
Table.
Length 4 4 Length, in bytes, of the entire System Locality Distance
Information Table.
Revision 1 8 1
Checksum 1 9 Entire table must sum to zero.
162 April, 2015 Version 6.0