279 lines
9.5 KiB
ReStructuredText
279 lines
9.5 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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==============================
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Running nested guests with KVM
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==============================
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A nested guest is the ability to run a guest inside another guest (it
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can be KVM-based or a different hypervisor). The straightforward
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example is a KVM guest that in turn runs on a KVM guest (the rest of
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this document is built on this example)::
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.----------------. .----------------.
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| L2 | | L2 |
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| (Nested Guest) | | (Nested Guest) |
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|----------------'--'----------------|
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| L1 (Guest Hypervisor) |
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| KVM (/dev/kvm) |
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.------------------------------------------------------.
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| L0 (Host Hypervisor) |
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| KVM (/dev/kvm) |
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|------------------------------------------------------|
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| Hardware (with virtualization extensions) |
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'------------------------------------------------------'
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Terminology:
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- L0 – level-0; the bare metal host, running KVM
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- L1 – level-1 guest; a VM running on L0; also called the "guest
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hypervisor", as it itself is capable of running KVM.
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- L2 – level-2 guest; a VM running on L1, this is the "nested guest"
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.. note:: The above diagram is modelled after the x86 architecture;
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s390x, ppc64 and other architectures are likely to have
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a different design for nesting.
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For example, s390x always has an LPAR (LogicalPARtition)
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hypervisor running on bare metal, adding another layer and
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resulting in at least four levels in a nested setup — L0 (bare
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metal, running the LPAR hypervisor), L1 (host hypervisor), L2
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(guest hypervisor), L3 (nested guest).
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This document will stick with the three-level terminology (L0,
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L1, and L2) for all architectures; and will largely focus on
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x86.
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Use Cases
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---------
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There are several scenarios where nested KVM can be useful, to name a
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few:
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- As a developer, you want to test your software on different operating
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systems (OSes). Instead of renting multiple VMs from a Cloud
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Provider, using nested KVM lets you rent a large enough "guest
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hypervisor" (level-1 guest). This in turn allows you to create
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multiple nested guests (level-2 guests), running different OSes, on
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which you can develop and test your software.
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- Live migration of "guest hypervisors" and their nested guests, for
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load balancing, disaster recovery, etc.
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- VM image creation tools (e.g. ``virt-install``, etc) often run
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their own VM, and users expect these to work inside a VM.
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- Some OSes use virtualization internally for security (e.g. to let
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applications run safely in isolation).
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Enabling "nested" (x86)
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-----------------------
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From Linux kernel v4.20 onwards, the ``nested`` KVM parameter is enabled
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by default for Intel and AMD. (Though your Linux distribution might
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override this default.)
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In case you are running a Linux kernel older than v4.19, to enable
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nesting, set the ``nested`` KVM module parameter to ``Y`` or ``1``. To
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persist this setting across reboots, you can add it in a config file, as
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shown below:
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1. On the bare metal host (L0), list the kernel modules and ensure that
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the KVM modules::
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$ lsmod | grep -i kvm
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kvm_intel 133627 0
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kvm 435079 1 kvm_intel
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2. Show information for ``kvm_intel`` module::
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$ modinfo kvm_intel | grep -i nested
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parm: nested:bool
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3. For the nested KVM configuration to persist across reboots, place the
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below in ``/etc/modprobed/kvm_intel.conf`` (create the file if it
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doesn't exist)::
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$ cat /etc/modprobe.d/kvm_intel.conf
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options kvm-intel nested=y
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4. Unload and re-load the KVM Intel module::
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$ sudo rmmod kvm-intel
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$ sudo modprobe kvm-intel
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5. Verify if the ``nested`` parameter for KVM is enabled::
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$ cat /sys/module/kvm_intel/parameters/nested
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Y
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For AMD hosts, the process is the same as above, except that the module
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name is ``kvm-amd``.
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Additional nested-related kernel parameters (x86)
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-------------------------------------------------
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If your hardware is sufficiently advanced (Intel Haswell processor or
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higher, which has newer hardware virt extensions), the following
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additional features will also be enabled by default: "Shadow VMCS
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(Virtual Machine Control Structure)", APIC Virtualization on your bare
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metal host (L0). Parameters for Intel hosts::
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$ cat /sys/module/kvm_intel/parameters/enable_shadow_vmcs
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Y
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$ cat /sys/module/kvm_intel/parameters/enable_apicv
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Y
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$ cat /sys/module/kvm_intel/parameters/ept
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Y
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.. note:: If you suspect your L2 (i.e. nested guest) is running slower,
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ensure the above are enabled (particularly
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``enable_shadow_vmcs`` and ``ept``).
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Starting a nested guest (x86)
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-----------------------------
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Once your bare metal host (L0) is configured for nesting, you should be
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able to start an L1 guest with::
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$ qemu-kvm -cpu host [...]
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The above will pass through the host CPU's capabilities as-is to the
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gues); or for better live migration compatibility, use a named CPU
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model supported by QEMU. e.g.::
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$ qemu-kvm -cpu Haswell-noTSX-IBRS,vmx=on
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then the guest hypervisor will subsequently be capable of running a
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nested guest with accelerated KVM.
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Enabling "nested" (s390x)
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-------------------------
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1. On the host hypervisor (L0), enable the ``nested`` parameter on
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s390x::
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$ rmmod kvm
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$ modprobe kvm nested=1
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.. note:: On s390x, the kernel parameter ``hpage`` is mutually exclusive
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with the ``nested`` paramter — i.e. to be able to enable
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``nested``, the ``hpage`` parameter *must* be disabled.
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2. The guest hypervisor (L1) must be provided with the ``sie`` CPU
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feature — with QEMU, this can be done by using "host passthrough"
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(via the command-line ``-cpu host``).
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3. Now the KVM module can be loaded in the L1 (guest hypervisor)::
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$ modprobe kvm
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Live migration with nested KVM
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------------------------------
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Migrating an L1 guest, with a *live* nested guest in it, to another
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bare metal host, works as of Linux kernel 5.3 and QEMU 4.2.0 for
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Intel x86 systems, and even on older versions for s390x.
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On AMD systems, once an L1 guest has started an L2 guest, the L1 guest
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should no longer be migrated or saved (refer to QEMU documentation on
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"savevm"/"loadvm") until the L2 guest shuts down. Attempting to migrate
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or save-and-load an L1 guest while an L2 guest is running will result in
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undefined behavior. You might see a ``kernel BUG!`` entry in ``dmesg``, a
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kernel 'oops', or an outright kernel panic. Such a migrated or loaded L1
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guest can no longer be considered stable or secure, and must be restarted.
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Migrating an L1 guest merely configured to support nesting, while not
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actually running L2 guests, is expected to function normally even on AMD
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systems but may fail once guests are started.
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Migrating an L2 guest is always expected to succeed, so all the following
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scenarios should work even on AMD systems:
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- Migrating a nested guest (L2) to another L1 guest on the *same* bare
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metal host.
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- Migrating a nested guest (L2) to another L1 guest on a *different*
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bare metal host.
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- Migrating a nested guest (L2) to a bare metal host.
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Reporting bugs from nested setups
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-----------------------------------
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Debugging "nested" problems can involve sifting through log files across
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L0, L1 and L2; this can result in tedious back-n-forth between the bug
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reporter and the bug fixer.
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- Mention that you are in a "nested" setup. If you are running any kind
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of "nesting" at all, say so. Unfortunately, this needs to be called
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out because when reporting bugs, people tend to forget to even
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*mention* that they're using nested virtualization.
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- Ensure you are actually running KVM on KVM. Sometimes people do not
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have KVM enabled for their guest hypervisor (L1), which results in
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them running with pure emulation or what QEMU calls it as "TCG", but
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they think they're running nested KVM. Thus confusing "nested Virt"
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(which could also mean, QEMU on KVM) with "nested KVM" (KVM on KVM).
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Information to collect (generic)
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The following is not an exhaustive list, but a very good starting point:
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- Kernel, libvirt, and QEMU version from L0
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- Kernel, libvirt and QEMU version from L1
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- QEMU command-line of L1 -- when using libvirt, you'll find it here:
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``/var/log/libvirt/qemu/instance.log``
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- QEMU command-line of L2 -- as above, when using libvirt, get the
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complete libvirt-generated QEMU command-line
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- ``cat /sys/cpuinfo`` from L0
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- ``cat /sys/cpuinfo`` from L1
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- ``lscpu`` from L0
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- ``lscpu`` from L1
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- Full ``dmesg`` output from L0
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- Full ``dmesg`` output from L1
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x86-specific info to collect
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Both the below commands, ``x86info`` and ``dmidecode``, should be
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available on most Linux distributions with the same name:
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- Output of: ``x86info -a`` from L0
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- Output of: ``x86info -a`` from L1
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- Output of: ``dmidecode`` from L0
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- Output of: ``dmidecode`` from L1
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s390x-specific info to collect
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Along with the earlier mentioned generic details, the below is
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also recommended:
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- ``/proc/sysinfo`` from L1; this will also include the info from L0
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