[PATCH v3 4/5] spapr_numa: consider user input when defining associativity

[Daniel Henrique Barboza]

This patch puts all the pieces together to finally allow user
input when defining the NUMA topology of the spapr guest.

For each NUMA node A, starting at node id 0, the new
spapr_numa_define_associativity_domains() will:

- get the distance between node A and B = A + 1
- get the correspondent NUMA level for this distance
- assign the associativity domain for A and B for the given
NUMA level, using the lowest associativity domain value between
them
- if there is more NUMA nodes, increment B and repeat

Since we always start at the first node (id = 0) and go in
ascending order, we are prioritizing any previous associativity
already calculated. This is necessary because neither QEMU, nor
the pSeries kernel, supports multiple associativity domains for
each resource, meaning that we have to decide which associativity
relation is relevant. Another side effect is that the first
NUMA node, node 0, will always have an associativity array
full of zeroes. This is intended - in fact, the Linux kernel
expects it (see [1] for more info).

Ultimately, all of this results in a best effort approximation for
the actual NUMA distances the user input in the command line. Given
the nature of how PAPR itself interprets NUMA distances versus the
expectations risen by how ACPI SLIT works, there might be better
algorithms but, in the end, it'll also result in another way to
approximate what the user really wanted.

To keep this commit message no longer than it already is, the next
patch will update the existing documentation in ppc-spapr-numa.rst
with more in depth details and design considerations/drawbacks.

[1] 
https://lore.kernel.org/linuxppc-dev/5e8fbea3-8faf-0951-172a-b41a2138fbcf@gmail.com/

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
---
 hw/ppc/spapr_numa.c | 120 +++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 119 insertions(+), 1 deletion(-)

diff --git a/hw/ppc/spapr_numa.c b/hw/ppc/spapr_numa.c
index 16badb1f4b..f3d43ceb1e 100644
--- a/hw/ppc/spapr_numa.c
+++ b/hw/ppc/spapr_numa.c
@@ -37,12 +37,118 @@ static bool spapr_numa_is_symmetrical(MachineState *ms)
     return true;
 }
 
+/*
+ * This function will translate the user distances into
+ * what the kernel understand as possible values: 10
+ * (local distance), 20, 40, 80 and 160, and return the equivalent
+ * NUMA level for each. Current heuristic is:
+ *  - local distance (10) returns numa_level = 0x4
+ *  - distances between 11 and 30 inclusive -> rounded to 20,
+ *    numa_level = 0x3
+ *  - distances between 31 and 60 inclusive -> rounded to 40,
+ *    numa_level = 0x2
+ *  - distances between 61 and 120 inclusive -> rounded to 80,
+ *    numa_level = 0x1
+ *  - everything above 120 returns numa_level = 0 to indicate that
+ *    there is no match. This will be calculated as disntace = 160
+ *    by the kernel (as of v5.9)
+ */
+static uint8_t spapr_numa_get_numa_level(uint8_t distance)
+{
+    uint8_t rounded_distance = 160;
+    uint8_t numa_level;
+
+    if (distance > 11 && distance <= 30) {
+        rounded_distance = 20;
+    } else if (distance > 31 && distance <= 60) {
+        rounded_distance = 40;
+    } else if (distance > 61 && distance <= 120) {
+        rounded_distance = 80;
+    }
+
+    switch (rounded_distance) {
+    case 10:
+        numa_level = 0x4;
+        break;
+    case 20:
+        numa_level = 0x3;
+        break;
+    case 40:
+        numa_level = 0x2;
+        break;
+    case 80:
+        numa_level = 0x1;
+        break;
+    default:
+        numa_level = 0;
+    }
+
+    return numa_level;
+}
+
+static void spapr_numa_define_associativity_domains(SpaprMachineState *spapr)
+{
+    MachineState *ms = MACHINE(spapr);
+    NodeInfo *numa_info = ms->numa_state->nodes;
+    int nb_numa_nodes = ms->numa_state->num_nodes;
+    int src, dst;
+
+    for (src = 0; src < nb_numa_nodes; src++) {
+        for (dst = src; dst < nb_numa_nodes; dst++) {
+            /*
+             * This is how the associativity domain between A and B
+             * is calculated:
+             *
+             * - get the distance between them
+             * - get the correspondent NUMA level for this distance
+             * - the arrays were initialized with their own numa_ids,
+             * and we're calculating the distance in node_id ascending order,
+             * starting from node 0. This will have a cascade effect in the
+             * algorithm because the associativity domains that node 0 defines
+             * will be carried over to the other nodes, and node 1
+             * associativities will be carried over unless there's already a
+             * node 0 associativity assigned, and so on. This happens because
+             * we'll assign assoc_src as the associativity domain of dst
+             * as well, for the given NUMA level.
+             *
+             * The PPC kernel expects the associativity domains of node 0 to
+             * be always 0, and this algorithm will grant that by default.
+             */
+            uint8_t distance = numa_info[src].distance[dst];
+            uint8_t n_level = spapr_numa_get_numa_level(distance);
+            uint32_t assoc_src;
+
+            /*
+             * n_level = 0 means that the distance is greater than our last
+             * rounded value (120). In this case there is no NUMA level match
+             * between src and dst and we can skip the remaining of the loop.
+             *
+             * The Linux kernel will assume that the distance between src and
+             * dst, in this case of no match, is 10 (local distance) doubled
+             * for each NUMA it didn't match. We have MAX_DISTANCE_REF_POINTS
+             * levels (4), so this gives us 10*2*2*2*2 = 160.
+             *
+             * This logic can be seen in the Linux kernel source code, as of
+             * v5.9, in arch/powerpc/mm/numa.c, function __node_distance().
+             */
+            if (n_level == 0) {
+                continue;
+            }
+
+            assoc_src = spapr->numa_assoc_array[src][n_level];
+            spapr->numa_assoc_array[dst][n_level] = assoc_src;
+        }
+    }
+
+}
+
 void spapr_numa_associativity_init(SpaprMachineState *spapr,
                                    MachineState *machine)
 {
     SpaprMachineClass *smc = SPAPR_MACHINE_GET_CLASS(spapr);
     int nb_numa_nodes = machine->numa_state->num_nodes;
     int i, j, max_nodes_with_gpus;
+    bool using_legacy_numa = spapr_machine_using_legacy_numa(spapr);
 
     /*
      * For all associativity arrays: first position is the size,
@@ -56,6 +162,17 @@ void spapr_numa_associativity_init(SpaprMachineState *spapr,
     for (i = 0; i < nb_numa_nodes; i++) {
         spapr->numa_assoc_array[i][0] = cpu_to_be32(MAX_DISTANCE_REF_POINTS);
         spapr->numa_assoc_array[i][MAX_DISTANCE_REF_POINTS] = cpu_to_be32(i);
+
+        /*
+         * Fill all associativity domains of non-zero NUMA nodes with
+         * node_id. This is required because the default value (0) is
+         * considered a match with associativity domains of node 0.
+         */
+        if (!using_legacy_numa && i != 0) {
+            for (j = 1; j < MAX_DISTANCE_REF_POINTS; j++) {
+                spapr->numa_assoc_array[i][j] = cpu_to_be32(i);
+            }
+        }
     }
 
     /*
@@ -85,7 +202,7 @@ void spapr_numa_associativity_init(SpaprMachineState *spapr,
      * 1 NUMA node) will not benefit from anything we're going to do
      * after this point.
      */
-    if (spapr_machine_using_legacy_numa(spapr)) {
+    if (using_legacy_numa) {
         return;
     }
 
@@ -95,6 +212,7 @@ void spapr_numa_associativity_init(SpaprMachineState *spapr,
         exit(EXIT_FAILURE);
     }
 
+    spapr_numa_define_associativity_domains(spapr);
 }
 
 void spapr_numa_write_associativity_dt(SpaprMachineState *spapr, void *fdt,
-- 
2.26.2