Why do hosting companies typically disable nested KVM virtualization?

Question

I have the impression that most virtual servers provide nowadays use KVM, but that they disable nested KVM virtualization. Hence I, as a consumer, have to resort to tools like (the awesome) systemd-nspawn to subdivide my virtual server into further nested containers.

What are reasons for hosting companies to disabled nested guests? It it just because of fears that this flexibility might undermine their business model, or are there additional technical reasons?

Answer 1

Nested KVM is not a feature complete, well-tested, and stable feature, despite recent advancements.

• Intel x86 only enabled it by default circa Linux 4.20, relatively recently.
• Level 2 migration does not work on architectures other than IBM Z.
• Level 3 guests are expected to perform horribly, if they function at all.
• Red Hat will only support this on RHEL 8 on Intel at the moment, not AMD or POWER or IBM Z
• Migrating a level 1 guest is undefined, it might oops or panic.
• x86 guests need to be started with hardware virtualization features (vmx) which may limit migration options.

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Ask your hosting provider to enable nested. Use cases do exist, lab environments, custom hypervisors.

Keep in mind that the amount of engineering still required to get to bare metal stability is considerable. The answer from your hypervisor and OS support may be no.