docs/virt/kvm: Document configuring and running nested guests

This is a rewrite of this[1] Wiki page with further enhancements.  The
doc also includes a section on debugging problems in nested
environments, among other improvements.

[1] https://www.linux-kvm.org/page/Nested_GuestsSigned-off-by: Kashyap Chamarthy <kchamart@redhat.com>
Message-Id: <20200505112839.30534-1-kchamart@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virt/kvm/index.rst

@@ -28,3 +28,5 @@ KVM
    arm/index
 
    devices/index
+
+   running-nested-guests

Documentation/virt/kvm/running-nested-guests.rst

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