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+
+NUMA mechanics for sPAPR (pseries machines)
+============================================
+
+NUMA in sPAPR works different than the System Locality Distance
+Information Table (SLIT) in ACPI. The logic is explained in the LOPAPR
+1.1 chapter 15, "Non Uniform Memory Access (NUMA) Option". This
+document aims to complement this specification, providing details
+of the elements that impacts how QEMU views NUMA in pseries.
+
+Associativity and ibm,associativity property
+--------------------------------------------
+
+Associativity is defined as a group of platform resources that has
+similar mean performance (or in our context here, distance) relative to
+everyone else outside of the group.
+
+The format of the ibm,associativity property varies with the value of
+bit 0 of byte 5 of the ibm,architecture-vec-5 property. The format with
+bit 0 equal to zero is deprecated. The current format, with the bit 0
+with the value of one, makes ibm,associativity property represent the
+physical hierarchy of the platform, as one or more lists that starts
+with the highest level grouping up to the smallest. Considering the
+following topology:
+
+::
+
+    Mem M1 ---- Proc P1    |
+    -----------------      | Socket S1  ---|
+          chip C1          |               |
+                                           | HW module 1 (MOD1)
+    Mem M2 ---- Proc P2    |               |
+    -----------------      | Socket S2  ---|
+          chip C2          |
+
+The ibm,associativity property for the processors would be:
+
+* P1: {MOD1, S1, C1, P1}
+* P2: {MOD1, S2, C2, P2}
+
+Each allocable resource has an ibm,associativity property. The LOPAPR
+specification allows multiple lists to be present in this property,
+considering that the same resource can have multiple connections to the
+platform.
+
+Relative Performance Distance and ibm,associativity-reference-points
+--------------------------------------------------------------------
+
+The ibm,associativity-reference-points property is an array that is used
+to define the relevant performance/distance  related boundaries, defining
+the NUMA levels for the platform.
+
+The definition of its elements also varies with the value of bit 0 of byte 5
+of the ibm,architecture-vec-5 property. The format with bit 0 equal to zero
+is also deprecated. With the current format, each integer of the
+ibm,associativity-reference-points represents an 1 based ordinal index (i.e.
+the first element is 1) of the ibm,associativity array. The first
+boundary is the most significant to application performance, followed by
+less significant boundaries. Allocated resources that belongs to the
+same performance boundaries are expected to have relative NUMA distance
+that matches the relevancy of the boundary itself. Resources that belongs
+to the same first boundary will have the shortest distance from each
+other. Subsequent boundaries represents greater distances and degraded
+performance.
+
+Using the previous example, the following setting reference points defines
+three NUMA levels:
+
+* ibm,associativity-reference-points = {0x3, 0x2, 0x1}
+
+The first NUMA level (0x3) is interpreted as the third element of each
+ibm,associativity array, the second level is the second element and
+the third level is the first element. Let's also consider that elements
+belonging to the first NUMA level have distance equal to 10 from each
+other, and each NUMA level doubles the distance from the previous. This
+means that the second would be 20 and the third level 40. For the P1 and
+P2 processors, we would have the following NUMA levels:
+
+::
+
+  * ibm,associativity-reference-points = {0x3, 0x2, 0x1}
+
+  * P1: associativity{MOD1, S1, C1, P1}
+
+  First NUMA level (0x3) => associativity[2] = C1
+  Second NUMA level (0x2) => associativity[1] = S1
+  Third NUMA level (0x1) => associativity[0] = MOD1
+
+  * P2: associativity{MOD1, S2, C2, P2}
+
+  First NUMA level (0x3) => associativity[2] = C2
+  Second NUMA level (0x2) => associativity[1] = S2
+  Third NUMA level (0x1) => associativity[0] = MOD1
+
+  P1 and P2 have the same third NUMA level, MOD1: Distance between them = 40
+
+Changing the ibm,associativity-reference-points array changes the performance
+distance attributes for the same associativity arrays, as the following
+example illustrates:
+
+::
+
+  * ibm,associativity-reference-points = {0x2}
+
+  * P1: associativity{MOD1, S1, C1, P1}
+
+  First NUMA level (0x2) => associativity[1] = S1
+
+  * P2: associativity{MOD1, S2, C2, P2}
+
+  First NUMA level (0x2) => associativity[1] = S2
+
+  P1 and P2 does not have a common performance boundary. Since this is a one level
+  NUMA configuration, distance between them is one boundary above the first
+  level, 20.
+
+
+In a hypothetical platform where all resources inside the same hardware module
+is considered to be on the same performance boundary:
+
+::
+
+  * ibm,associativity-reference-points = {0x1}
+
+  * P1: associativity{MOD1, S1, C1, P1}
+
+  First NUMA level (0x1) => associativity[0] = MOD0
+
+  * P2: associativity{MOD1, S2, C2, P2}
+
+  First NUMA level (0x1) => associativity[0] = MOD0
+
+  P1 and P2 belongs to the same first order boundary. The distance between then
+  is 10.
+
+
+How the pseries Linux guest calculates NUMA distances
+=====================================================
+
+Another key difference between ACPI SLIT and the LOPAPR regarding NUMA is
+how the distances are expressed. The SLIT table provides the NUMA distance
+value between the relevant resources. LOPAPR does not provide a standard
+way to calculate it. We have the ibm,associativity for each resource, which
+provides a common-performance hierarchy,  and the ibm,associativity-reference-points
+array that tells which level of associativity is considered to be relevant
+or not.
+
+The result is that each OS is free to implement and to interpret the distance
+as it sees fit. For the pseries Linux guest, each level of NUMA duplicates
+the distance of the previous level, and the maximum amount of levels is
+limited to MAX_DISTANCE_REF_POINTS = 4 (from arch/powerpc/mm/numa.c in the
+kernel tree). This results in the following distances:
+
+* both resources in the first NUMA level: 10
+* resources one NUMA level apart: 20
+* resources two NUMA levels apart: 40
+* resources three NUMA levels apart: 80
+* resources four NUMA levels apart: 160
+
+
+Consequences for QEMU NUMA tuning
+---------------------------------
+
+The way the pseries Linux guest calculates NUMA distances has a direct effect
+on what QEMU users can expect when doing NUMA tuning. As of QEMU 5.1, this is
+the default ibm,associativity-reference-points being used in the pseries
+machine:
+
+ibm,associativity-reference-points = {0x4, 0x4, 0x2}
+
+The first and second level are equal, 0x4, and a third one was added in
+commit a6030d7e0b35 exclusively for NVLink GPUs support. This means that
+regardless of how the ibm,associativity properties are being created in
+the device tree, the pseries Linux guest will only recognize three scenarios
+as far as NUMA distance goes:
+
+* if the resources belongs to the same first NUMA level = 10
+* second level is skipped since it's equal to the first
+* all resources that aren't a NVLink GPU, it is guaranteed that they will belong
+  to the same third NUMA level, having distance = 40
+* for NVLink GPUs, distance = 80 from everything else
+
+In short, we can summarize the NUMA distances seem in pseries Linux guests, using
+QEMU up to 5.1, as follows:
+
+* local distance, i.e. the distance of the resource to its own NUMA node: 10
+* if it's a NVLink GPU device, distance: 80
+* every other resource, distance: 40
+
+This also means that user input in QEMU command line does not change the
+NUMA distancing inside the guest for the pseries machine.
