[Qemu-devel] [PATCH v3 for-2.3 2/2] pc: fix default VCPU to NUMA node mapping

[Igor Mammedov <imammedo@redhat.com>]

Since commit
   dd0247e0 pc: acpi: mark all possible CPUs as enabled in SRAT
Linux kernel actually tries to use CPU to Node mapping from
QEMU provided SRAT table instead of discarding it, and that
in some cases breaks build_sched_domains() which expects
sane mapping where cores/threads belonging to the same socket
are on the same NUMA node.

With current default round-robin mapping of VCPUs to nodes
guest ends-up with cores/threads belonging to the same socket
being on different NUMA nodes.

For example with following CLI:

   qemu-system-x86_64 -m 4G \
         -cpu Opteron_G3,vendor=AuthenticAMD \
         -smp 5,sockets=1,cores=4,threads=1,maxcpus=8 \
         -numa node,nodeid=0 -numa node,nodeid=1

2.6.32 based kernels will hang on boot due to incorrectly built
sched_group-s list in update_sd_lb_stats()

Replacing default mapping with a manual, where VCPUs belonging to
the same socket are on the same NUMA node, fixes the issue for
guests which can't handle nonsense topology i.e. changing CLI to:
  -numa node,nodeid=0,cpus=0-3 -numa node,nodeid=1,cpus=4-7

So instead of simply scattering VCPUs around nodes, provide
callback to map the same socket VCPUs to the same NUMA node,
which is what guests would expect from a sane hardware/BIOS.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
---
v3:
  - split out pc change into a separate patch
  - fix spelling mistakes
v2:
  - add machine callback cpu_index_to_socket_id() and use it
    instead of stub approach

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
---
 hw/i386/pc.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index 4b46c29..a52d2af 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -1851,6 +1851,14 @@ static void pc_machine_initfn(Object *obj)
                              NULL, NULL);
 }
 
+static unsigned pc_cpu_index_to_socket_id(unsigned cpu_index)
+{
+    unsigned pkg_id, core_id, smt_id;
+    x86_topo_ids_from_idx(smp_cores, smp_threads, cpu_index,
+                          &pkg_id, &core_id, &smt_id);
+    return pkg_id;
+}
+
 static void pc_machine_class_init(ObjectClass *oc, void *data)
 {
     MachineClass *mc = MACHINE_CLASS(oc);
@@ -1859,6 +1867,7 @@ static void pc_machine_class_init(ObjectClass *oc, void *data)
 
     pcmc->get_hotplug_handler = mc->get_hotplug_handler;
     mc->get_hotplug_handler = pc_get_hotpug_handler;
+    mc->cpu_index_to_socket_id = pc_cpu_index_to_socket_id;
     hc->plug = pc_machine_device_plug_cb;
     hc->unplug_request = pc_machine_device_unplug_request_cb;
     hc->unplug = pc_machine_device_unplug_cb;
-- 
1.8.3.1