linux-kernel - Re: [PATCH v2 1/3] KVM: x86: Allow CPU to force vendor-specific TDP level

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Message-ID: <400f8ca7-8f82-308b-3427-b644144cfa5c@redhat.com>
Date:   Tue, 10 Aug 2021 11:23:48 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Yu Zhang <yu.c.zhang@...ux.intel.com>,
        Wei Huang <wei.huang2@....com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, vkuznets@...hat.com,
        wanpengli@...cent.com, joro@...tes.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, x86@...nel.org, hpa@...or.com
Subject: Re: [PATCH v2 1/3] KVM: x86: Allow CPU to force vendor-specific TDP
 level

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