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[PATCH v5] hw/arm/virt: Don't create device-tree node for empty NUMA nod

From: Gavin Shan
Subject: [PATCH v5] hw/arm/virt: Don't create device-tree node for empty NUMA node
Date: Fri, 15 Oct 2021 20:42:46 +0800

The empty NUMA node, where no memory resides, are allowed. For
example, the following command line specifies two empty NUMA nodes.
With this, QEMU fails to boot because of the conflicting device-tree
node names, as the following error message indicates.

  /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
  qemu-system-aarch64: FDT: Failed to create subnode /memory@80000000: 

As specified by linux device-tree binding document, the device-tree
nodes for these empty NUMA nodes shouldn't be generated. However,
the corresponding NUMA node IDs should be included in the distance
map. The memory hotplug through device-tree on ARM64 isn't existing
so far and it's not necessary to require the user to provide a distance
map. Furthermore, the default distance map Linux generates may even be
sufficient. So this simply skips populating the device-tree nodes for
these empty NUMA nodes to avoid the error, so that QEMU can be started

Signed-off-by: Gavin Shan <gshan@redhat.com>
v5: Improved commit log and comments as Drew suggested.
 hw/arm/boot.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/hw/arm/boot.c b/hw/arm/boot.c
index 57efb61ee4..74ad397b1f 100644
--- a/hw/arm/boot.c
+++ b/hw/arm/boot.c
@@ -599,10 +599,23 @@ int arm_load_dtb(hwaddr addr, const struct arm_boot_info 
+    /*
+     * We drop all the memory nodes which correspond to empty NUMA nodes
+     * from the device tree, because the Linux NUMA binding document
+     * states they should not be generated. Linux will get the NUMA node
+     * IDs of the empty NUMA nodes from the distance map if they are needed.
+     * This means QEMU users may be obliged to provide command lines which
+     * configure distance maps when the empty NUMA node IDs are needed and
+     * Linux's default distance map isn't sufficient.
+     */
     if (ms->numa_state != NULL && ms->numa_state->num_nodes > 0) {
         mem_base = binfo->loader_start;
         for (i = 0; i < ms->numa_state->num_nodes; i++) {
             mem_len = ms->numa_state->nodes[i].node_mem;
+            if (!mem_len) {
+                continue;
+            }
             rc = fdt_add_memory_node(fdt, acells, mem_base,
                                      scells, mem_len, i);
             if (rc < 0) {

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