[PATCH] hw/arm/virt: Expose empty NUMA nodes through ACPI

[Gavin Shan]

The empty NUMA nodes, where no memory resides, aren't exposed
through ACPI SRAT table. It's not user preferred behaviour because
the corresponding memory node devices are missed from the guest
kernel as the following example shows, and memory can't be hot
added to these empty NUMA nodes at later point.

  /home/gavin/sandbox/qemu.main/build/qemu-system-aarch64 \
  -accel kvm -machine virt,gic-version=host               \
  -cpu host -smp 4,sockets=2,cores=2,threads=1            \
  -m 1024M,slots=16,maxmem=64G                            \
  -object memory-backend-ram,id=mem0,size=512M            \
  -object memory-backend-ram,id=mem1,size=512M            \
  -numa node,nodeid=0,cpus=0-1,memdev=mem0                \
  -numa node,nodeid=1,cpus=2-3,memdev=mem1                \
  -numa node,nodeid=2                                     \
  -numa node,nodeid=3                                     \
     :
  guest# ls /sys/devices/system/node | grep node
  node0
  node1
  node2

This exposes these empty NUMA nodes through ACPI SRAT table. With
this applied, the corresponding memory node devices can be found
from the guest. Note that the hotpluggable capability is explicitly
given to these empty NUMA nodes for sake of completeness.

  guest# ls /sys/devices/system/node | grep node
  node0
  node1
  node2
  node3

Signed-off-by: Gavin Shan <gshan@redhat.com>
---
 hw/arm/virt-acpi-build.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/hw/arm/virt-acpi-build.c b/hw/arm/virt-acpi-build.c
index 674f902652..a4c95b2f64 100644
--- a/hw/arm/virt-acpi-build.c
+++ b/hw/arm/virt-acpi-build.c
@@ -526,6 +526,7 @@ build_srat(GArray *table_data, BIOSLinker *linker, 
VirtMachineState *vms)
     const CPUArchIdList *cpu_list = mc->possible_cpu_arch_ids(ms);
     AcpiTable table = { .sig = "SRAT", .rev = 3, .oem_id = vms->oem_id,
                         .oem_table_id = vms->oem_table_id };
+    MemoryAffinityFlags flags;
 
     acpi_table_begin(&table, table_data);
     build_append_int_noprefix(table_data, 1, 4); /* Reserved */
@@ -547,12 +548,15 @@ build_srat(GArray *table_data, BIOSLinker *linker, 
VirtMachineState *vms)
 
     mem_base = vms->memmap[VIRT_MEM].base;
     for (i = 0; i < ms->numa_state->num_nodes; ++i) {
-        if (ms->numa_state->nodes[i].node_mem > 0) {
-            build_srat_memory(table_data, mem_base,
-                              ms->numa_state->nodes[i].node_mem, i,
-                              MEM_AFFINITY_ENABLED);
-            mem_base += ms->numa_state->nodes[i].node_mem;
+        if (ms->numa_state->nodes[i].node_mem) {
+            flags = MEM_AFFINITY_ENABLED;
+        } else {
+            flags = MEM_AFFINITY_ENABLED | MEM_AFFINITY_HOTPLUGGABLE;
         }
+
+        build_srat_memory(table_data, mem_base,
+                          ms->numa_state->nodes[i].node_mem, i, flags);
+        mem_base += ms->numa_state->nodes[i].node_mem;
     }
 
     if (ms->nvdimms_state->is_enabled) {
-- 
2.23.0