Hotplugging cpu, memory and host devices to a running virtual machine with QMP

gistfile1.txt

Hotplugging cpu:

Start QEMU with QMP socket available and with startup amount of CPUs less than maxcpus:
./qemu-system-x86_64 -qmp unix:/tmp/qmp-sock,server,nowait -smp 2,maxcpus=8

Connect to qmp socket and add cpu:
socat /tmp/qmp-sock -
{ "execute": "cpu-add", "arguments": { "id": 2 } }

Hotpluggable Cpu's can be queried as:

{ "execute": "query-hotpluggable-cpus" }

Online newly added CPU as :
echo 1 > /sys/devices/system/cpu/cpu1/online

Hotplugging memory:
Existing memory can be queried with :
{ "execute": "query-memory-devices" }
{"return": [{"type": "dimm", "data": {"memdev": "/objects/mem0", "hotplugged": false, "addr": 4294967296, "hotpluggable": true, "size": 23592
9600, "slot": 0, "node": 0, "id": "nv0"}}]}

To add memory, start qemu with startup memory less than max:
./qemu-system-x86_64 -qmp unix:/tmp/qmp-sock,server,nowait -m 2G,slots=2,maxmem=3G

Add memory with following qmp command:
{ "execute": "object-add", "arguments": { "qom-type": "memory-backend-ram",  "id": "mem3", "props": { "size": 1048576 } } }

The above command creates a memory backend device.To create a front-end pc-dimm device and insert it into the first empty slot,
use command:
{ "execute": "device_add", "arguments": { "memdev" : "mem3", "id" :"dimm3", "driver": "pc-dimm"}}    
{"return": {}}

Hotplugging virtio disk:
The following add a backend disk:
{"execute":"blockdev-add","arguments":{"driver":"raw","node-name":"disk1","file":{"driver":"file","filename":"/dev/mapper/docker-8:2-3016900"}}}
{"return": {}}

To add guest visible frontend virtio-block device:
{ "execute": "device_add", "arguments": { "driver": "virtio-blk", "drive": "disk1", "id": "virtio0" } }

Block devices can be queried as:
{"execute" :"query-block"}

Note: For hot-plugging disks, kernel needs to be compiled with CONFIG_HOTPLUG_PCI=y,CONFIG_HOTPLUG_PCI_ACPI=y