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Subject: Re: [PATCH 10/54] KVM: x86/mmu: Replace EPT shadow page shenanigans
 with simpler check
To:     Sean Christopherson <seanjc@google.com>
Cc:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Yu Zhang <yu.c.zhang@linux.intel.com>,
        Maxim Levitsky <mlevitsk@redhat.com>
References: <20210622175739.3610207-1-seanjc@google.com>
 <20210622175739.3610207-11-seanjc@google.com>
 <8ce36922-dba0-9b53-6f74-82f3f68b443c@redhat.com>
 <YNNegF8RcF3vX2Sh@google.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <df77b8e9-b2bb-b085-0789-909a8b9d44c3@redhat.com>
Date:   Wed, 23 Jun 2021 18:41:17 +0200
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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