[Qemu-devel] [RFC PATCH v5 6/6] spapr: Don't allow memory hotplug to memory less nodes

[Bharata B Rao <bharata@linux.vnet.ibm.com>]

Currently PowerPC kernel doesn't allow hot-adding memory to memory-less
node, but instead will silently add the memory to the first node that has
some memory. This causes two unexpected behaviours for the user.

Memory gets hotplugged to a different node than what the user specified.
Since pc-dimm subsystem in QEMU still thinks that memory belongs to
memory-less node, a reboot will set things accordingly and the previously
hotplugged memory now ends in the right node. This appears as if some
memory moved from one node to another.

So until kernel starts supporting memory hotplug to memory-less
nodes, just prevent such attempts upfront in QEMU.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
---
 hw/ppc/spapr.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index f4792b8..003ef14 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -2139,6 +2139,28 @@ static void spapr_machine_device_plug(HotplugHandler *hotplug_dev,
             return;
         }
 
+        /*
+         * Currently PowerPC kernel doesn't allow hot-adding memory to
+         * memory-less node, but instead will silently add the memory
+         * to the first node that has some memory. This causes two
+         * unexpected behaviours for the user.
+         *
+         * - Memory gets hotplugged to a different node than what the user
+         *   specified.
+         * - Since pc-dimm subsystem in QEMU still thinks that memory belongs
+         *   to memory-less node, a reboot will set things accordingly
+         *   and the previously hotplugged memory now ends in the right node.
+         *   This appears as if some memory moved from one node to another.
+         *
+         * So until kernel starts supporting memory hotplug to memory-less
+         * nodes, just prevent such attempts upfront in QEMU.
+         */
+        if (nb_numa_nodes && !numa_info[node].node_mem) {
+            error_setg(errp, "Can't hotplug memory to memory-less node %d",
+                       node);
+            return;
+        }
+
         spapr_memory_plug(hotplug_dev, dev, node, errp);
     }
 }
-- 
2.1.0