linux-kernel - Re: [PATCH 10/54] KVM: x86/mmu: Replace EPT shadow page shenanigans with simpler check

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Date:   Wed, 23 Jun 2021 18:41:17 +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,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Subject: Re: [PATCH 10/54] KVM: x86/mmu: Replace EPT shadow page shenanigans
 with simpler check

On 23/06/21 18:17, Sean Christopherson wrote:
>> What the commit message doesn't say is, did we miss this
>> opportunity all along, or has there been a change since commit
>> 47c42e6b4192 ("KVM: x86: fix handling of role.cr4_pae and rename it
>> to 'gpte_size'", 2019-03-28) that allows this?
>
> The code was wrong from the initial "unsync" commit.  The 4-byte vs.
> 8-byte check papered over the real bug, which was that the roles were
> not checked for compabitility.  I suspect that the bug only
> manisfested as an observable problem when the GPTE sizes mismatched,
> thus the PAE check was added.

I meant that we really never needed is_ept_sp, and you could have used 
the simpler check already at the time you introduced gpte_is_8_bytes. 
But anyway I think we're in agreement.

Paolo