[Qemu-devel] [PATCH for 3.1 v3] spapr: Fix ibm, max-associativity-domains property number of nodes

[Serhii Popovych <spopovyc@redhat.com>]

Laurent Vivier reported off by one with maximum number of NUMA nodes
provided by qemu-kvm being less by one than required according to
description of "ibm,max-associativity-domains" property in LoPAPR.

It appears that I incorrectly treated LoPAPR description of this
property assuming it provides last valid domain (NUMA node here)
instead of maximum number of domains.

  ### Before hot-add

  (qemu) info numa
  3 nodes
  node 0 cpus: 0
  node 0 size: 0 MB
  node 0 plugged: 0 MB
  node 1 cpus:
  node 1 size: 1024 MB
  node 1 plugged: 0 MB
  node 2 cpus:
  node 2 size: 0 MB
  node 2 plugged: 0 MB

  $ numactl -H
  available: 2 nodes (0-1)
  node 0 cpus: 0
  node 0 size: 0 MB
  node 0 free: 0 MB
  node 1 cpus:
  node 1 size: 999 MB
  node 1 free: 658 MB
  node distances:
  node   0   1
    0:  10  40
    1:  40  10

  ### Hot-add

  (qemu) object_add memory-backend-ram,id=mem0,size=1G
  (qemu) device_add pc-dimm,id=dimm1,memdev=mem0,node=2
  (qemu) [   87.704898] pseries-hotplug-mem: Attempting to hot-add 4 ...
  <there is no "Initmem setup node 2 [mem 0xHEX-0xHEX]">
  [   87.705128] lpar: Attempting to resize HPT to shift 21
  ... <HPT resize messages>

  ### After hot-add

  (qemu) info numa
  3 nodes
  node 0 cpus: 0
  node 0 size: 0 MB
  node 0 plugged: 0 MB
  node 1 cpus:
  node 1 size: 1024 MB
  node 1 plugged: 0 MB
  node 2 cpus:
  node 2 size: 1024 MB
  node 2 plugged: 1024 MB

  $ numactl -H
  available: 2 nodes (0-1)
  ^^^^^^^^^^^^^^^^^^^^^^^^
             Still only two nodes (and memory hot-added to node 0 below)
  node 0 cpus: 0
  node 0 size: 1024 MB
  node 0 free: 1021 MB
  node 1 cpus:
  node 1 size: 999 MB
  node 1 free: 658 MB
  node distances:
  node   0   1
    0:  10  40
    1:  40  10

After fix applied numactl(8) reports 3 nodes available and memory
plugged into node 2 as expected.

>From David Gibson:
------------------
  Qemu makes a distinction between "non NUMA" (nb_numa_nodes == 0) and
  "NUMA with one node" (nb_numa_nodes == 1).  But from a PAPR guests's
  point of view these are equivalent.  I don't want to present two
  different cases to the guest when we don't need to, so even though the
  guest can handle it, I'd prefer we put a '1' here for both the
  nb_numa_nodes == 0 and nb_numa_nodes == 1 case.

This consolidates everything discussed previously on mailing list.

Fixes: da9f80fbad21 ("spapr: Add ibm,max-associativity-domains property")
Reported-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Serhii Popovych <spopovyc@redhat.com>
---
 hw/ppc/spapr.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index 7afd1a1..2ee7201 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -1033,7 +1033,7 @@ static void spapr_dt_rtas(sPAPRMachineState *spapr, void *fdt)
         cpu_to_be32(0),
         cpu_to_be32(0),
         cpu_to_be32(0),
-        cpu_to_be32(nb_numa_nodes ? nb_numa_nodes - 1 : 0),
+        cpu_to_be32(nb_numa_nodes ? nb_numa_nodes : 1),
     };
 
     _FDT(rtas = fdt_add_subnode(fdt, 0, "rtas"));
-- 
1.8.3.1