Nested virtualization (not) enabled

Interesting thing I learned a few days ago:

kvm: Nested Virtualization enabled

does not always mean that nested virtualization is being used.

If you use qemu’s software emulation (more often known as TCG) then it emulates a generic-looking AMD CPU with SVM (AMD’s virtualization feature).

AMD virtualization easily supports nesting (unlike Intel’s VT which is a massive PITA to nest), and when the KVM module is loaded, it notices the “AMD” host CPU with SVM and willingly enables nested virt. There’s actually a little bit of benefit to this because it avoids a second layer of TCG being needed if you did run a L2 guest in there (although it’s still going to be slow).

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