In implementations of ARM64 architecture, at most there could be
a CPU topology hierarchy like "sockets/dies/clusters/cores/threads"
defined. For example, some ARM64 server chip Kunpeng 920 totally
has 2 sockets, 2 NUMA nodes (also represent CPU dies range) in each
socket, 6 clusters in each NUMA node, 4 CPU cores in each cluster.

Clusters within the same NUMA share the L3 cache data and cores
within the same cluster share a L2 cache and a L3 cache tag.
Given that designing a vCPU topology with cluster level for the
guest can gain scheduling performance improvement, let's support
this new parameter on ARM virt machines.

After this, we can define a 4-level CPU topology hierarchy like:
cpus=*,maxcpus=*,sockets=*,clusters=*,cores=*,threads=*.

Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
---
 hw/arm/virt.c   |  1 +
 qemu-options.hx | 10 ++++++++++
 2 files changed, 11 insertions(+)

diff --git a/hw/arm/virt.c b/hw/arm/virt.c
index 369552ad45..b2129f7ccd 100644
--- a/hw/arm/virt.c
+++ b/hw/arm/virt.c
@@ -2698,6 +2698,7 @@ static void virt_machine_class_init(ObjectClass *oc, void *data)
     hc->unplug_request = virt_machine_device_unplug_request_cb;
     hc->unplug = virt_machine_device_unplug_cb;
     mc->nvdimm_supported = true;
+    mc->smp_props.clusters_supported = true;
     mc->auto_enable_numa_with_memhp = true;
     mc->auto_enable_numa_with_memdev = true;
     mc->default_ram_id = "mach-virt.ram";
diff --git a/qemu-options.hx b/qemu-options.hx
index 0f26f7dad7..74d335e4c3 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -277,6 +277,16 @@ SRST
 
         -smp 16,sockets=2,dies=2,cores=2,threads=2,maxcpus=16
 
+    The following sub-option defines a CPU topology hierarchy (2 sockets
+    totally on the machine, 2 clusters per socket, 2 cores per cluster,
+    2 threads per core) for ARM virt machines which support sockets/clusters
+    /cores/threads. Some members of the option can be omitted but their values
+    will be automatically computed:
+
+    ::
+
+        -smp 16,sockets=2,clusters=2,cores=2,threads=2,maxcpus=16
+
     Historically preference was given to the coarsest topology parameters
     when computing missing values (ie sockets preferred over cores, which
     were preferred over threads), however, this behaviour is considered
-- 
2.19.1