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docs: add info about factors affecting CPU compatibility

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While libvirt solves most of the problem of ensuring compatibility, when
there is incompatibility it can be hard for users to track down the
cause. Everything knows to check the physical CPU model, but there are a
surprisingly large number of other factors influencing compatibility.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
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Daniel P. Berrangé 2022-07-12 16:30:17 +01:00
parent 3ff558c1c1
commit 497d8555c4
1 changed files with 83 additions and 0 deletions
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docs/drvqemu.rst
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@ -416,6 +416,89 @@ following command should be run as root, prior to starting libvirtd
libvirt will then place each virtual machine in a cgroup at
``/dev/cgroup/libvirt/qemu/$VMNAME/``
Live migration compatibility
----------------------------
Many factors can affect the ability to live migrate a guest between a pair
of hosts. It is critical that when QEMU is started on the destination host,
the exposed guest machine ABI matches what was exposed by the existing QEMU
process on the source host. To facilitate this, when libvirt receives a
guest configuration document, it will attempt to expand any features that
were not specified, to ensure a stable guest machine ABI. Mostly this involves
adding address information to all devices, and adding controllers to attach
the devices to.
Certain features that affect the guest ABI, however, may only be known at the
time the guest is started and can be influenced by features of the host OS
and its hardware. This means that even if the guest XML configuration is the
same, it may still be impossible to migrate the guest between two hosts.
Migration CPU model compatibility
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The most common problems with migration compatibility surround the use of
the guest CPU ``host-model`` or ``host-passthrough`` modes. Both of these
modes attempt to expose the full host CPU featureset to the guest. The
``host-model`` mode attempts to expose as many features as possible while
retaining the ability to accurately check compatibility between hosts prior
to migration running. The ``host-passthrough`` mode attempts to expose the
host CPU as precisely as possible, but with the cost that it is not possible
for libvirt to check compatibility prior to migration.
If using ``host-model`` the target host hardware and software deployment
must expose a superset of the features of the source host CPU. If using
``host-passthrough`` the target host CPU and software deployment must
always expose a superset of the fetures, however, it is further strongly
recommended that the source and destination hosts be identical in every
way.
In both cases, there are a number of factors that will influence the CPU
features available to the guest
* **Physical CPU model** - the core constraint on what features are available.
Check ``/proc/cpuinfo`` for CPU model name.
* **Firmware revision (BIOS/UEFI/etc)** - firmware updates may bundle microcode
updates which arbitrarily add or remove CPU features, typically in response
to new hardware vulnerabilities. Check ``dmidecode`` for details on ``x86``
and ``aarch64`` platforms for firmware version, and ``/proc/cpuinfo`` for
associated microcode version (if not updated by the OS).
* **Firmware settings** - certain firmware settings can affect accessibility of
features. For example, turning on/off SMT/HT not only affects the number
of logical CPUs available to the OS, but can indirectly influence other
factors such as the number of performance counters available for use. Check
the firmware specific configuration interface.
* **Host kernel version** - the host kernel software version may have a
need to block certain physical CPU features from use in the guest. It can
also emulate certain features that may not exist in the silicon, for example,
x2apic. Check ``uname -r`` output for kernel version.
* **Host kernel settings** - the kernel command line options can be used to
block certain physical CPU features from use in the guest, for example,
``tsx=off``, ``l1tf=...`` or ``nosmt``. Check ``/proc/cmdline`` and
``/etc/modprobe.d/*.conf``.
* **microcode update version** - while the firmware will load the initial
microcode in to the CPU, the OS may ship packages providing newer microcode
updates since these can be deployed on a more timely manner than firmware
updates. These updates can arbitrarily load add or remove CPU features.
Check ``/proc/cpuinfo`` for microcode version.
* **QEMU version** - even when the kernel supports exposing a CPU feature to
the guest, an update in the QEMU emulator version will be required to unlock
its usage with a guest, except with ``host-passthrough``. Check the output
of ``$QEMU -version``.
* **libvirt version** - even when the kernel and QEMU support exposing a CPU
feature to the guest, an update in the libvirt version will be required to
unlock its usage with a guest, except with ``host-passthrough``. Check
``virsh version``.
* **Nested virtualization** - due to the limitations of nested virtualization,
a L1 nested host may not be able to expose the same featureset as a bare
metal host, even if everything else is the same.
The ``virsh capabilities`` output will provide information on the high level
CPU model, its features, microcode version. Most of the time this will provide
enough information to know whether the CPUs of two hosts will be compatible.
If there are unexpected differences though, checking the above list of
influencing factors can reveal where the difference arises from.
Import and export of libvirt domain XML configs
-----------------------------------------------