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qemu (1:4.2-1ubuntu1) focal; urgency=medium

In recent years the kernel and qemu support for nested virtualization got
much better. Therefore with QEMU 4.2 / Kernel 5.4 in Ubuntu 20.04 onwards
the former restriction that excluded nesting from being a supported
feature is lifted.

With kernel 4.20 nested virtualization became default enabled in the
kvm-intel.ko kernel module. It was enabled all the time for kvm-amd.ko.
And these days all common front end tools to KVM will pick default
cpu types capable of nesting. Therefore former tweaks to auto-enable nesting
for extra convenience could be dropped in qemu 1:4.2-1ubuntu1.

While no more needed going forward, for backward compatibility the default
guest CPU types (if you specify nothing, which isn't recomended in general)
will continue to add VMX/SVM capabilities.

Finally it might be worth to mention the constraint that live migration of a
level 1 guest that has nested level 2 guests running isn't ready yet. But
upstream development to stabilize that is ongoing already.

 -- Christian Ehrhardt <christian.ehrhardt@canonical.com>  Wed, 08 Jan 2020 16:18:01 +0100

qemu (1:2.10+dfsg-0ubuntu5) xenial; urgency=medium

Nested virtualization is a useful technology - it usually just works and
greatly facilitates many developer workflows. The phrase "it works until it
doesn't" is a correct description of its state.
But when it doesn’t work, it is very difficult to debug due to complex
interactions between virtualization layers.

Therefore it is important to realize that while Ubuntu adds code to make the
consumption of nested virtualization easier it should not be relied upon,
especially for production workloads.
Many projects - especially CI infrastructures use it quite heavily and are fine.
But one has to design the usage in a way to tolerate potential issues as
it is not a fully supported feature - neither by upstream nor by Ubuntu.

Note: Due to the default x86 virtual CPU types on Ubuntu having vmx (Intel)
and svm (AMD) enabled by default to make nested virt work out of the box
without any tweaking KVM reports on guest start a missing
feature of the "other" architecture respectively.
This non critical Message looks like (similarly for AMD with vmx):

  host doesn't support requested feature: CPUID.80000001H:ECX.svm [bit 2]

-- Christian Ehrhardt <christian.ehrhardt@canonical.com>  Thu, 11 Jan 2018 17:06:02 +0100