[Qemu-devel] [PULL 32/36] spapr: Fix default NUMA node allocation for threads

[David Gibson <david@gibson.dropbear.id.au>]

At present, if guest numa nodes are requested, but the cpus in each node
are not specified, spapr just uses the default behaviour or assigning each
vcpu round-robin to nodes.

If smp_threads != 1, that will assign adjacent threads in a core to
different NUMA nodes.  As well as being just weird, that's a configuration
that can't be represented in the device tree we give to the guest, which
means the guest and qemu end up with different ideas of the NUMA topology.

This patch implements mc->cpu_index_to_socket_id in the spapr code to
make sure vcpus get assigned to nodes only at the socket granularity.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
---
 hw/ppc/spapr.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index 533c7ad..59fc814 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -2162,6 +2162,13 @@ static HotplugHandler *spapr_get_hotpug_handler(MachineState *machine,
     return NULL;
 }
 
+static unsigned spapr_cpu_index_to_socket_id(unsigned cpu_index)
+{
+    /* Allocate to NUMA nodes on a "socket" basis (not that concept of
+     * socket means much for the paravirtualized PAPR platform) */
+    return cpu_index / smp_threads / smp_cores;
+}
+
 static void spapr_machine_class_init(ObjectClass *oc, void *data)
 {
     MachineClass *mc = MACHINE_CLASS(oc);
@@ -2183,6 +2190,7 @@ static void spapr_machine_class_init(ObjectClass *oc, void *data)
     mc->get_hotplug_handler = spapr_get_hotpug_handler;
     hc->plug = spapr_machine_device_plug;
     hc->unplug = spapr_machine_device_unplug;
+    mc->cpu_index_to_socket_id = spapr_cpu_index_to_socket_id;
 
     smc->dr_lmb_enabled = false;
     fwc->get_dev_path = spapr_get_fw_dev_path;
-- 
2.4.3