.. SPDX-License-Identifier: GPL-2.0

===================================== SRAT - Static Resource Affinity Table

The System/Static Resource Affinity Table describes resource (CPU, Memory) affinity to "Proximity Domains". This table is technically optional, but for performance information (see "HMAT") to be enumerated by linux it must be present.

There is a careful dance between the CEDT and SRAT tables and how NUMA nodes are created. If things don't look quite the way you expect - check the SRAT Memory Affinity entries and CEDT CFMWS to determine what your platform actually supports in terms of flexible topologies.

The SRAT may statically assign portions of a CFMWS SPA range to a specific proximity domains. See linux numa creation for more information about how this presents in the NUMA topology.

Proximity Domain

A proximity domain is ROUGHLY equivalent to "NUMA Node" - though a 1-to-1 mapping is not guaranteed. There are scenarios where "Proximity Domain 4" may map to "NUMA Node 3", for example. (See "NUMA Node Creation")

Memory Affinity

Generally speaking, if a host does any amount of CXL fabric (decoder) programming in BIOS - an SRAT entry for that memory needs to be present.

Example ::

     Subtable Type : 01 [Memory Affinity]
            Length : 28
  Proximity Domain : 00000001          <- NUMA Node 1
         Reserved1 : 0000
      Base Address : 000000C050000000  <- Physical Memory Region
    Address Length : 0000003CA0000000
         Reserved2 : 00000000

Flags (decoded below) : 0000000B Enabled : 1 Hot Pluggable : 1 Non-Volatile : 0

Generic Port Affinity

The Generic Port Affinity subtable provides an association between a proximity domain and a device handle representing a Generic Port such as a CXL host bridge. With the association, latency and bandwidth numbers can be retrieved from the SRAT for the path between CPU(s) (initiator) and the Generic Port. This is used to construct performance coordinates for hotplugged CXL DEVICES, which cannot be enumerated at boot by platform firmware.

Example ::

     Subtable Type : 06 [Generic Port Affinity]
            Length : 20               <- 32d, length of table
          Reserved : 00
Device Handle Type : 00               <- 0 - ACPI, 1 - PCI
  Proximity Domain : 00000001
     Device Handle : ACPI0016:01
             Flags : 00000001         <- Bit 0 (Enabled)
          Reserved : 00000000

The Proximity Domain is matched up to the :doc:HMAT <hmat> SSLBI Target Proximity Domain List for the related latency or bandwidth numbers. Those performance numbers are tied to a CXL host bridge via the Device Handle. The driver uses the association to retrieve the Generic Port performance numbers for the whole CXL path access coordinates calculation.