CONFIG_XEN_BALLOON_MEMORY_HOTPLUG: Memory hotplug support for Xen balloon driver

General informations

The Linux kernel configuration item CONFIG_XEN_BALLOON_MEMORY_HOTPLUG:

• prompt: Memory hotplug support for Xen balloon driver
• type: bool
• depends on: CONFIG_XEN_BALLOON && CONFIG_MEMORY_HOTPLUG
• defined in drivers/xen/Kconfig
• found in Linux kernels: 3.1–3.19, 4.0–4.20, 5.0–5.19, 6.0–6.12, 6.13-rc+HEAD

Help text

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'"

Hardware

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The data is retrived from:

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