Questions on Nested Virtualization

Temporary forum to discuss issues and problems during a VirtualBox Beta or Release Candidate phase.

Re: Questions on Nested Virtualizaion

Postby ManWithNoName » 19. Apr 2020, 12:07

fth0 wrote:
klaus wrote:a key feature which makes the nested virtualization performance significantly better

If you're talking about VMCS Shadowing, I believe that's restricted to Intel CPUs with Intel vPro. The latter is the keyword to look for inside the Intel CPU specifications on https://ark.intel.com. For example, the (quite old) desktop CPU Intel i5-4690 has this feature, while the (much newer) mobile CPU Intel i5-8265U doesn't have it.


I don't find a link between the vpro features and the VMCS shadowing feature on the net.
I have found an INTEL doc which mentions that VMCS shadowing feature is included in some 7th core gen series. https://www.intel.com/content/dam/www/public/us/en/documents/product-briefs/7th-gen-core-family-mobile-brief.pdf (at the end of page 8 )
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Re: Questions on Nested Virtualizaion

Postby fth0 » 19. Apr 2020, 19:32

ManWithNoName wrote:I don't find a link between the vpro features and the VMCS shadowing feature on the net.

I'll admit that the only link I've found myself is a bit weak: Intels White Paper 4th Generation Intel® CoreTM vProTM Processors with Intel® VMCS Shadowing indicates that VMCS Shadowing was initially released with 4th Generation Intel Core vPro processors. That doesn't tell what happened in the later generations. The document which you've found, unfortunately doesn't show the VMCS Shadowing feature in the detailed tables near the end. In consequence, the real deal is only the CPU itself:

VirtualBox analyses the CPUID and several MSRs and writes it findings into the VBox.log file. Locate the following lines which are not consecutive, but nearby each other:
VBox.log file wrote:
00:00:01.668973 HM: MSR_IA32_VMX_PROCBASED_CTLS2      = 0x7cff00000000
00:00:01.668974 HM: EPT
00:00:01.668975 HM: UNRESTRICTED_GUEST
00:00:01.668977 HM: VMCS_SHADOWING (must be cleared)

Any line containing "(must be cleared)" tells you that the corresponding feature is not available, and you'll need all three of them for nested virtualization (at least in VirtualBox 6.1.4 and 6.1.6).
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