How do you hotplug-enable new CPU and RAM in a virtual machine?

I am running Ubuntu in a virtual machine and I'd like to add CPU and RAM without rebooting the device. How can I hotplug this new virtual hardware?

These can be enabled through the use of the /sys filesystem using root credentials.

For the CPU, you change the 0 to a 1 in to the appropriate file of the pattern: /sys/devices/system/cpu/cpu*/online.

For the RAM, you find the state in the files /sys/devices/system/memory/memory*/state and change the contents from offline to online.

The script below will turn all CPU and RAM online for you.

#!/bin/bash
# Based on script by William Lam - http://engineering.ucsb.edu/~duonglt/vmware/

# Bring CPUs online
for CPU_DIR in /sys/devices/system/cpu/cpu[0-9]*
do
    CPU=${CPU_DIR##*/}
    echo "Found cpu: '${CPU_DIR}' ..."
    CPU_STATE_FILE="${CPU_DIR}/online"
    if [ -f "${CPU_STATE_FILE}" ]; then
        if grep -qx 1 "${CPU_STATE_FILE}"; then
            echo -e "\t${CPU} already online"
        else
            echo -e "\t${CPU} is new cpu, onlining cpu ..."
            echo 1 > "${CPU_STATE_FILE}"
        fi
    else 
        echo -e "\t${CPU} already configured prior to hot-add"
    fi
done

# Bring all new Memory online
for RAM in $(grep line /sys/devices/system/memory/*/state)
do
    echo "Found ram: ${RAM} ..."
    if [[ "${RAM}" == *":offline" ]]; then
        echo "Bringing online"
        echo $RAM | sed "s/:offline$//"|sed "s/^/echo online > /"|source /dev/stdin
    else
        echo "Already online"
    fi
done

Instead of operating the kernel parameters, you can automatically enable hotplugged CPU or memory by using udev rules:

/etc/udev/rules.d/94-hotplug-cpu-mem.rules:

ACTION=="add", SUBSYSTEM=="cpu", ATTR{online}=="0", ATTR{online}="1"

ACTION=="add", SUBSYSTEM=="memory", ATTR{state}=="offline", ATTR{state}="online"

Tested on CentOS 6/7, Ubuntu 14.

Debian 7 crashed for unknown reason. Further testing would be required.