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From:   Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH v2 1/3] KVM: x86: Allow CPU to force vendor-specific TDP
 level
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Date:   Tue, 10 Aug 2021 11:23:48 +0200
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On 09/08/21 23:49, Jim Mattson wrote:
> Doesn't this break legacy type 2 hypervisors that don't know anything
> about 5-level NPT and don't have any control over whether or not the
> host uses 5-level paging?

Yes, where "legacy" probably means "all released versions of all of 
them", including KVM.  Host support for LA57 was merged in 4.13, while 
KVM started supporting 5-level page tables in EPT in 4.14 and even then 
just returned PT64_ROOT_LEVEL (i.e. 4) for the maximum NPT level.

So all Linux versions up to 5.13, which has "KVM: x86: Prevent KVM SVM 
from loading on kernels with 5-level paging", will break horribly. 
Better backport that patch to stable...

Paolo