The alternatives are the L0 emulates a hypervisor-privileged mode to the L1 which would be a performance cost (and does not really avoid the problem since the L0 would have to emulate vmenter/vmrun and therefore be aware of the L2 anyway, or the hardware would have to provide some nested virtualization facility which seems like it would be complicated and expensive for little benefit, though I could be mistaken.

Does anything do a "real" nested virtualization in hardware? s390 might but I know ~nothing about it and it probably does not expose its bare metal hardware layers to Linux/KVM anyway.

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nijave 16 hours ago | parent | next [-]

IBM mainframes might but I have a very shallow understanding.

Found this https://sixe.eu/news/kvm-nested-vms-ibm-power-linux-lpar

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bonzini 16 hours ago | parent [-]

That's exactly the same as x86. Nested virtualization support is almost entirely in the hypervisor.

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nijave 12 hours ago | parent [-]

x86 doesn't have a firmware hypervisor that allows splitting into LPARs.

That's showing 3 layers of nesting: LPAR, hypervisor, nested hypervisor

	▲	bonzini 6 hours ago \| parent [-]
		On POWER, LPARs and the first level of hypervisor do not use any LPAR-specific nesting support in the processor. It's all handled by the firmware, not the hardware.

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bonzini 16 hours ago | parent | prev [-]

No, even s390 needs shadow paging.

Intel and AMD both have some small amount of acceleration of nested virtualization, respectively with shadow VMCS and virtualized VMLOAD/VMSAVE.