docs/specs/ppc-spapr-numa.rst - qemu - Git at Google


 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


 pseries NUMA mechanics
 ======================

 Starting in QEMU 5.2, the pseries machine considers user input when setting NUMA
 topology of the guest. The overall design is:

 * ibm,associativity-reference-points is set to {0x4, 0x3, 0x2, 0x1}, allowing
   for 4 distinct NUMA distance values based on the NUMA levels

 * ibm,max-associativity-domains supports multiple associativity domains in all
   NUMA levels, granting user flexibility

 * ibm,associativity for all resources varies with user input

 These changes are only effective for pseries-5.2 and newer machines that are
 created with more than one NUMA node (disconsidering NUMA nodes created by
 the machine itself, e.g. NVLink 2 GPUs). The now legacy support has been
 around for such a long time, with users seeing NUMA distances 10 and 40
 (and 80 if using NVLink2 GPUs), and there is no need to disrupt the
 existing experience of those guests.

 To bring the user experience x86 users have when tuning up NUMA, we had
 to operate under the current pseries Linux kernel logic described in
 `How the pseries Linux guest calculates NUMA distances`_. The result
 is that we needed to translate NUMA distance user input to pseries
 Linux kernel input.

 Translating user distance to kernel distance
 --------------------------------------------

 User input for NUMA distance can vary from 10 to 254. We need to translate
 that to the values that the Linux kernel operates on (10, 20, 40, 80, 160).
 This is how it is being done:

 * user distance 11 to 30 will be interpreted as 20
 * user distance 31 to 60 will be interpreted as 40
 * user distance 61 to 120 will be interpreted as 80
 * user distance 121 and beyond will be interpreted as 160
 * user distance 10 stays 10

 The reasoning behind this aproximation is to avoid any round up to the local
 distance (10), keeping it exclusive to the 4th NUMA level (which is still
 exclusive to the node_id). All other ranges were chosen under the developer
 discretion of what would be (somewhat) sensible considering the user input.
 Any other strategy can be used here, but in the end the reality is that we'll
 have to accept that a large array of values will be translated to the same
 NUMA topology in the guest, e.g. this user input:

 ::

       0   1   2
   0  10  31 120
   1  31  10  30
   2 120  30  10

 And this other user input:

 ::

       0   1   2
   0  10  60  61
   1  60  10  11
   2  61  11  10

 Will both be translated to the same values internally:

 ::

       0   1   2
   0  10  40  80
   1  40  10  20
   2  80  20  10

 Users are encouraged to use only the kernel values in the NUMA definition to
 avoid being taken by surprise with that the guest is actually seeing in the
 topology. There are enough potential surprises that are inherent to the
 associativity domain assignment process, discussed below.


 How associativity domains are assigned
 --------------------------------------

 LOPAPR allows more than one associativity array (or 'string') per allocated
 resource. This would be used to represent that the resource has multiple
 connections with the board, and then the operational system, when deciding
 NUMA distancing, should consider the associativity information that provides
 the shortest distance.

 The spapr implementation does not support multiple associativity arrays per
 resource, neither does the pseries Linux kernel. We'll have to represent the
 NUMA topology using one associativity per resource, which means that choices
 and compromises are going to be made.

 Consider the following NUMA topology entered by user input:

 ::

       0   1   2   3
   0  10  40  20  40
   1  40  10  80  40
   2  20  80  10  20
   3  40  40  20  10

 All the associativity arrays are initialized with NUMA id in all associativity
 domains:

 * node 0: 0 0 0 0
 * node 1: 1 1 1 1
 * node 2: 2 2 2 2
 * node 3: 3 3 3 3


 Honoring just the relative distances of node 0 to every other node, we find the
 NUMA level matches (considering the reference points {0x4, 0x3, 0x2, 0x1}) for
 each distance:

 * distance from 0 to 1 is 40 (no match at 0x4 and 0x3, will match
   at 0x2)
 * distance from 0 to 2 is 20 (no match at 0x4, will match at 0x3)
 * distance from 0 to 3 is 40 (no match at 0x4 and 0x3, will match
   at 0x2)

 We'll copy the associativity domains of node 0 to all other nodes, based on
 the NUMA level matches. Between 0 and 1, a match in 0x2, we'll also copy
 the domains 0x2 and 0x1 from 0 to 1 as well. This will give us:

 * node 0: 0 0 0 0
 * node 1: 0 0 1 1

 Doing the same to node 2 and node 3, these are the associativity arrays
 after considering all matches with node 0:

 * node 0: 0 0 0 0
 * node 1: 0 0 1 1
 * node 2: 0 0 0 2
 * node 3: 0 0 3 3

 The distances related to node 0 are accounted for. For node 1, and keeping
 in mind that we don't need to revisit node 0 again, the distance from
 node 1 to 2 is 80, matching at 0x1, and distance from 1 to 3 is 40,
 match in 0x2. Repeating the same logic of copying all domains up to
 the NUMA level match:

 * node 0: 0 0 0 0
 * node 1: 1 0 1 1
 * node 2: 1 0 0 2
 * node 3: 1 0 3 3

 In the last step we will analyze just nodes 2 and 3. The desired distance
 between 2 and 3 is 20, i.e. a match in 0x3:

 * node 0: 0 0 0 0
 * node 1: 1 0 1 1
 * node 2: 1 0 0 2
 * node 3: 1 0 0 3


 The kernel will read these arrays and will calculate the following NUMA topology for
 the guest:

 ::

       0   1   2   3
   0  10  40  20  20
   1  40  10  40  40
   2  20  40  10  20
   3  20  40  20  10

 Note that this is not what the user wanted - the desired distance between
 0 and 3 is 40, we calculated it as 20. This is what the current logic and
 implementation constraints of the kernel and QEMU will provide inside the
 LOPAPR specification.

 Users are welcome to use this knowledge and experiment with the input to get
 the NUMA topology they want, or as closer as they want. The important thing
 is to keep expectations up to par with what we are capable of provide at this
 moment: an approximation.

 Limitations of the implementation
 ---------------------------------

 As mentioned above, the pSeries NUMA distance logic is, in fact, a way to approximate
 user choice. The Linux kernel, and PAPR itself, does not provide QEMU with the ways
 to fully map user input to actual NUMA distance the guest will use. These limitations
 creates two notable limitations in our support:

 * Asymmetrical topologies aren't supported. We only support NUMA topologies where
   the distance from node A to B is always the same as B to A. We do not support
   any A-B pair where the distance back and forth is asymmetric. For example, the
   following topology isn't supported and the pSeries guest will not boot with this
   user input:

 ::

       0   1
   0  10  40
   1  20  10


 * 'non-transitive' topologies will be poorly translated to the guest. This is the
   kind of topology where the distance from a node A to B is X, B to C is X, but
   the distance A to C is not X. E.g.:

 ::

       0   1   2   3
   0  10  20  20  40
   1  20  10  80  40
   2  20  80  10  20
   3  40  40  20  10

   In the example above, distance 0 to 2 is 20, 2 to 3 is 20, but 0 to 3 is 40.
   The kernel will always match with the shortest associativity domain possible,
   and we're attempting to retain the previous established relations between the
   nodes. This means that a distance equal to 20 between nodes 0 and 2 and the
   same distance 20 between nodes 2 and 3 will cause the distance between 0 and 3
   to also be 20.


 Legacy (5.1 and older) pseries NUMA mechanics
 =============================================

 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

 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

 This also means that user input in QEMU command line does not change the
 NUMA distancing inside the guest for the pseries machine.

	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


	pseries NUMA mechanics
	======================

	Starting in QEMU 5.2, the pseries machine considers user input when setting NUMA
	topology of the guest. The overall design is:

	* ibm,associativity-reference-points is set to {0x4, 0x3, 0x2, 0x1}, allowing
	for 4 distinct NUMA distance values based on the NUMA levels

	* ibm,max-associativity-domains supports multiple associativity domains in all
	NUMA levels, granting user flexibility

	* ibm,associativity for all resources varies with user input

	These changes are only effective for pseries-5.2 and newer machines that are
	created with more than one NUMA node (disconsidering NUMA nodes created by
	the machine itself, e.g. NVLink 2 GPUs). The now legacy support has been
	around for such a long time, with users seeing NUMA distances 10 and 40
	(and 80 if using NVLink2 GPUs), and there is no need to disrupt the
	existing experience of those guests.

	To bring the user experience x86 users have when tuning up NUMA, we had
	to operate under the current pseries Linux kernel logic described in
	`How the pseries Linux guest calculates NUMA distances`_. The result
	is that we needed to translate NUMA distance user input to pseries
	Linux kernel input.

	Translating user distance to kernel distance
	--------------------------------------------

	User input for NUMA distance can vary from 10 to 254. We need to translate
	that to the values that the Linux kernel operates on (10, 20, 40, 80, 160).
	This is how it is being done:

	* user distance 11 to 30 will be interpreted as 20
	* user distance 31 to 60 will be interpreted as 40
	* user distance 61 to 120 will be interpreted as 80
	* user distance 121 and beyond will be interpreted as 160
	* user distance 10 stays 10

	The reasoning behind this aproximation is to avoid any round up to the local
	distance (10), keeping it exclusive to the 4th NUMA level (which is still
	exclusive to the node_id). All other ranges were chosen under the developer
	discretion of what would be (somewhat) sensible considering the user input.
	Any other strategy can be used here, but in the end the reality is that we'll
	have to accept that a large array of values will be translated to the same
	NUMA topology in the guest, e.g. this user input:

	::

	0 1 2
	0 10 31 120
	1 31 10 30
	2 120 30 10

	And this other user input:

	::

	0 1 2
	0 10 60 61
	1 60 10 11
	2 61 11 10

	Will both be translated to the same values internally:

	::

	0 1 2
	0 10 40 80
	1 40 10 20
	2 80 20 10

	Users are encouraged to use only the kernel values in the NUMA definition to
	avoid being taken by surprise with that the guest is actually seeing in the
	topology. There are enough potential surprises that are inherent to the
	associativity domain assignment process, discussed below.


	How associativity domains are assigned
	--------------------------------------

	LOPAPR allows more than one associativity array (or 'string') per allocated
	resource. This would be used to represent that the resource has multiple
	connections with the board, and then the operational system, when deciding
	NUMA distancing, should consider the associativity information that provides
	the shortest distance.

	The spapr implementation does not support multiple associativity arrays per
	resource, neither does the pseries Linux kernel. We'll have to represent the
	NUMA topology using one associativity per resource, which means that choices
	and compromises are going to be made.

	Consider the following NUMA topology entered by user input:

	::

	0 1 2 3
	0 10 40 20 40
	1 40 10 80 40
	2 20 80 10 20
	3 40 40 20 10

	All the associativity arrays are initialized with NUMA id in all associativity
	domains:

	* node 0: 0 0 0 0
	* node 1: 1 1 1 1
	* node 2: 2 2 2 2
	* node 3: 3 3 3 3


	Honoring just the relative distances of node 0 to every other node, we find the
	NUMA level matches (considering the reference points {0x4, 0x3, 0x2, 0x1}) for
	each distance:

	* distance from 0 to 1 is 40 (no match at 0x4 and 0x3, will match
	at 0x2)
	* distance from 0 to 2 is 20 (no match at 0x4, will match at 0x3)
	* distance from 0 to 3 is 40 (no match at 0x4 and 0x3, will match
	at 0x2)

	We'll copy the associativity domains of node 0 to all other nodes, based on
	the NUMA level matches. Between 0 and 1, a match in 0x2, we'll also copy
	the domains 0x2 and 0x1 from 0 to 1 as well. This will give us:

	* node 0: 0 0 0 0
	* node 1: 0 0 1 1

	Doing the same to node 2 and node 3, these are the associativity arrays
	after considering all matches with node 0:

	* node 0: 0 0 0 0
	* node 1: 0 0 1 1
	* node 2: 0 0 0 2
	* node 3: 0 0 3 3

	The distances related to node 0 are accounted for. For node 1, and keeping
	in mind that we don't need to revisit node 0 again, the distance from
	node 1 to 2 is 80, matching at 0x1, and distance from 1 to 3 is 40,
	match in 0x2. Repeating the same logic of copying all domains up to
	the NUMA level match:

	* node 0: 0 0 0 0
	* node 1: 1 0 1 1
	* node 2: 1 0 0 2
	* node 3: 1 0 3 3

	In the last step we will analyze just nodes 2 and 3. The desired distance
	between 2 and 3 is 20, i.e. a match in 0x3:

	* node 0: 0 0 0 0
	* node 1: 1 0 1 1
	* node 2: 1 0 0 2
	* node 3: 1 0 0 3


	The kernel will read these arrays and will calculate the following NUMA topology for
	the guest:

	::

	0 1 2 3
	0 10 40 20 20
	1 40 10 40 40
	2 20 40 10 20
	3 20 40 20 10

	Note that this is not what the user wanted - the desired distance between
	0 and 3 is 40, we calculated it as 20. This is what the current logic and
	implementation constraints of the kernel and QEMU will provide inside the
	LOPAPR specification.

	Users are welcome to use this knowledge and experiment with the input to get
	the NUMA topology they want, or as closer as they want. The important thing
	is to keep expectations up to par with what we are capable of provide at this
	moment: an approximation.

	Limitations of the implementation
	---------------------------------

	As mentioned above, the pSeries NUMA distance logic is, in fact, a way to approximate
	user choice. The Linux kernel, and PAPR itself, does not provide QEMU with the ways
	to fully map user input to actual NUMA distance the guest will use. These limitations
	creates two notable limitations in our support:

	* Asymmetrical topologies aren't supported. We only support NUMA topologies where
	the distance from node A to B is always the same as B to A. We do not support
	any A-B pair where the distance back and forth is asymmetric. For example, the
	following topology isn't supported and the pSeries guest will not boot with this
	user input:

	::

	0 1
	0 10 40
	1 20 10


	* 'non-transitive' topologies will be poorly translated to the guest. This is the
	kind of topology where the distance from a node A to B is X, B to C is X, but
	the distance A to C is not X. E.g.:

	::

	0 1 2 3
	0 10 20 20 40
	1 20 10 80 40
	2 20 80 10 20
	3 40 40 20 10

	In the example above, distance 0 to 2 is 20, 2 to 3 is 20, but 0 to 3 is 40.
	The kernel will always match with the shortest associativity domain possible,
	and we're attempting to retain the previous established relations between the
	nodes. This means that a distance equal to 20 between nodes 0 and 2 and the
	same distance 20 between nodes 2 and 3 will cause the distance between 0 and 3
	to also be 20.


	Legacy (5.1 and older) pseries NUMA mechanics
	=============================================

	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

	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

	This also means that user input in QEMU command line does not change the
	NUMA distancing inside the guest for the pseries machine.