docs: adding NUMA documentation for pseries

This patch adds a new documentation file, ppc-spapr-numa.rst,
informing what developers and user can expect of the NUMA distance
support for the pseries machine, up to QEMU 5.1.

In the (hopefully soon) future, when we rework the NUMA mechanics
of the pseries machine to at least attempt to contemplate user
choice, this doc will be extended to inform about the new
support.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20200803133440.825276-1-danielhb413@gmail.com>
Tested-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

2 files changed, 192 insertions, 0 deletions

diff --git a/docs/specs/index.rst b/docs/specs/index.rst
index 426632a475..1b0eb979d5 100644
--- a/docs/specs/index.rst
+++ b/docs/specs/index.rst
@@ -12,6 +12,7 @@ Contents:
 
    ppc-xive
    ppc-spapr-xive
+   ppc-spapr-numa
    acpi_hw_reduced_hotplug
    tpm
    acpi_hest_ghes
diff --git a/docs/specs/ppc-spapr-numa.rst b/docs/specs/ppc-spapr-numa.rst
new file mode 100644
index 0000000000..e762038022
--- /dev/null
+++ b/docs/specs/ppc-spapr-numa.rst
@@ -0,0 +1,191 @@
+
+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.