[PATCH v2 13/13] kbase: Document virtio-mem

This commit adds new memorydevices.rst page which should serve all models of memory devices. Yet, I'm documenting virtio-mem quirks only. Signed-off-by: Michal Privoznik <mprivozn(a)redhat.com&gt; --- docs/kbase/index.rst | 4 + docs/kbase/memorydevices.rst | 160 +++++++++++++++++++++++++++++++++++ docs/kbase/meson.build | 1 + 3 files changed, 165 insertions(+) create mode 100644 docs/kbase/memorydevices.rst diff --git a/docs/kbase/index.rst b/docs/kbase/index.rst index 532804fe05..45450bf33b 100644 --- a/docs/kbase/index.rst +++ b/docs/kbase/index.rst @@ -46,6 +46,10 @@ Usage `PCI topology <../pci-addresses.html>`__ Addressing schemes for PCI devices +`Memory devices <memorydevices.html>`__ + Memory devices and their use + + Internals / Debugging --------------------- diff --git a/docs/kbase/memorydevices.rst b/docs/kbase/memorydevices.rst new file mode 100644 index 0000000000..23adf54e16 --- /dev/null +++ b/docs/kbase/memorydevices.rst @@ -0,0 +1,160 @@ +============== +Memory devices +============== + +.. contents:: + +Basics +====== + +Memory devices can be divided into two families: DIMMs and NVDIMMs. The former +is typical RAM memory: it's volatile and thus its contents doesn't survive +reboots nor guest shut downs and power ons. The latter retains its contents +across reboots or power outages. + +In Libvirt, there are two models for DIMMs: + +* ``dimm`` model: + + :: + + <memory model='dimm'> + <target> + <size unit='KiB'>523264</size> + <node>0</node> + </target> + <address type='dimm' slot='0'/> + </memory> + +* ``virtio-mem`` model: + + :: + + <memory model='virtio-mem'> + <target> + <size unit='KiB'>1048576</size> + <node>0</node> + <block unit='KiB'>2048</block> + <requested unit='KiB'>524288</requested> + </target> + <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/> + </memory> + +Then there are two models for NVDIMMs: + +* ``nvidmm`` model: + + :: + + <memory model='nvdimm'> + <source> + <path>/tmp/nvdimm</path> + </source> + <target> + <size unit='KiB'>523264</size> + <node>0</node> + </target> + <address type='dimm' slot='0'/> + </memory> + +* ``virtio-pmem`` model: + + :: + + <memory model='virtio-pmem' access='shared'> + <source> + <path>/tmp/virtio_pmem</path> + </source> + <target> + <size unit='KiB'>524288</size> + </target> + <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/> + </memory> + + +Please not that (maybe somewhat surprisingly) virtio models go onto PCI bus +instead of DIMM slots. + +Furthermore, DIMMs can have ``<source/>`` element which configures backend for +devices. For NVDIMMs the element is mandatory and reflects where the contents +is saved. + +See https://libvirt.org/formatdomain.html#elementsMemory + +``virtio-mem`` model +==================== + +The ``virtio-mem`` model can be viewed as revised memory balloon. It offers +memory hotplug and hotunplug solution (without the actual hotplug of the +device). It solves problems that memory balloon can't solve on its own and thus +is more flexible than DIMM + balloon solution. ``virtio-mem`` is NUMA aware, +and thus memory can be inflated/deflated only for a subset of guest NUMA nodes. +Also, it works with chunks that are either exposed to guest or taken back from +it. + +See https://virtio-mem.gitlab.io/ + +Under the hood, ``virtio-mem`` device is split into chunks of equal size which +are then exposed to the guest. Either all of them or only a portion depending +on user's request. Therefore there are three important sizes for +``virtio-mem``. All are to be found under ``<target/>`` element: + +#. The maximum size the device can ever offer, exposed under ``<size/>`` +#. The size a single block, exposed under ``<block/>`` +#. The current size exposed to the guest, exposed under ``<requested/>`` + +For instance, the following example the maximum size is 4GiB, the block size is +2MiB and only 1GiB should be exposed to the guest: + + :: + + <memory model='virtio-mem'> + <target> + <size unit='KiB'>4194304</size> + <block unit='KiB'>2048</block> + <requested unit='KiB'>1048576</requested> + </target> + </memory> + +Please note that ``<requested/>`` must be an integer multiple of ``<block/>`` +size or zero (memory completely deflated) and has to be less or equal to +``<size/>`` (memory completely inflated). Furthermore, QEMU recommends the +``<block/>`` size to be as big as a Transparent Huge Page (usually 2MiB). + +To change the size exposed to the guest, users should pass memory device XML +with nothing but ``<requested/>`` changed into the +``virDomainUpdateDeviceFlags()`` API. For user's convenience this can be done +via virsh too: + + :: + + # virsh update-memory-device $dom --requested size 2GiB + +If there are two or more ``<memory/>`` devices then ``--alias`` shall be used +to tell virsh which memory device should be updated. + +For running guests there is fourth size that can be found under ``<target/>``: + + :: + + <actual unit='KiB'>2097152</actual> + +The ``<actual/>`` reflects the actual size consumed by the guest. In general it +can differ from ``<requested/>``. Reasons include guest kernel missing +``virtio-mem`` module and thus being unable to take offered memory, or guest +kernel being unable to free memory and allow deflation. Since ``<actual/>`` +only reports size to users, the element is never parsed. It is formatted only +into live XML. + +Since changing actual allocation requires cooperation with guest kernel, +requests for change are not instant. Therefore, libvirt emits +``VIR_DOMAIN_EVENT_ID_MEMORY_DEVICE_SIZE_CHANGE`` event whenever actual +allocation changed. + +Please not that using ``virtio-mem`` with memory balloon is not possible, +currently. The real reason is that libvirt's memory accounting isn't ready and +mixing these two would be confusing to users. Libvirt exposes current value of +memory balloon under ``<currentMemory/>`` but if it were to account for +``<actual/>`` too then it would be impossible to learn true size of the +balloon. Also it might result in mistakenly trying to deflate ``virtio-mem`` +via ``setmem`` command. diff --git a/docs/kbase/meson.build b/docs/kbase/meson.build index 7b4e7abbd3..7f6adba915 100644 --- a/docs/kbase/meson.build +++ b/docs/kbase/meson.build @@ -9,6 +9,7 @@ docs_kbase_files = [ 'locking-lockd', 'locking', 'locking-sanlock', + 'memorydevices', 'migrationinternals', 'qemu-passthrough-security', 'rpm-deployment', -- 2.26.2