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

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Date:   Fri, 11 Jun 2021 18:49:25 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.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 11/06/21 18:45, Sean Christopherson wrote:
>> 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.
> 

Yes the problem is the

         if (new_role.as_u64 == g_context->mmu_role.as_u64)
                 return;

and the patch lets you preserve the optimization instead of dropping it. 
  I was wondering if I was missing something else because of the "when 
walking the guest page tables" remark.

Paolo