Memory hotplug support for Xen balloon driver allows expanding memory
available for the system above limit declared at system startup.
It is very useful on critical systems which require long
run without rebooting.

It's also very useful for non PV domains to obtain unpopulated physical
memory ranges to use in order to map foreign memory or grants.

Memory could be hotplugged in following steps:

1) target domain: ensure that memory auto online policy is in
effect by checking /sys/devices/system/memory/auto_online_blocks
file (should be 'online').

2) control domain: xl mem-max <target-domain> <maxmem>
where <maxmem> is >= requested memory size,

3) control domain: xl mem-set <target-domain> <memory>
where <memory> is requested memory size; alternatively memory
could be added by writing proper value to
/sys/devices/system/xen_memory/xen_memory0/target or
/sys/devices/system/xen_memory/xen_memory0/target_kb on the
target domain.

Alternatively, if memory auto onlining was not requested at step 1
the newly added memory can be manually onlined in the target domain
by doing the following:

for i in /sys/devices/system/memory/memory*/state; do \
[ "`cat "$i"`" = offline ] && echo online > "$i"; done

or by adding the following line to udev rules:

SUBSYSTEM=="memory", ACTION=="add", RUN+="/bin/sh -c '[ -f /sys$devpath/state ] && echo online > /sys$devpath/state'"