linux-kernel - Re: [PATCH] KVM: x86/mmu: Calculate and check "full" mmu

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Date:   Fri, 11 Jun 2021 16:45:31 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/mmu: Calculate and check "full" mmu_role for
 nested MMU

On Fri, Jun 11, 2021, Paolo Bonzini wrote:
> On 11/06/21 00:00, Sean Christopherson wrote:
> > things like the number of levels in the guest's page tables are
> > surprisingly important when walking the guest page tables
> 
> Along which path though?  I would have naively expected those to be driven
> only by the context->root_level.

The functional code is driven by context->root_level, but if KVM doesn't include
the level in the mmu_role then it will fail to update context->root_level when
L2 changes from 32-bit PAE to 64-bit.  If all the CR0/CR4/EFER bits remain the
same, only the level will differ.  Without this patch, role.level is always '0'
for the nested MMU.