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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
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