Introduce Virtio-loopback epsilon release:

Epsilon release introduces a new compatibility layer which make virtio-loopback
design to work with QEMU and rust-vmm vhost-user backend without require any
changes.

Signed-off-by: Timos Ampelikiotis <t.ampelikiotis@virtualopensystems.com>
Change-Id: I52e57563e08a7d0bdc002f8e928ee61ba0c53dd9

1 files changed, 410 insertions, 0 deletions

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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
+
+
+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 approximation 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.