linux-kernel - Re: [PATCH] KVM: MMU: update comment on the number of page role combinations

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Message-ID: <c6a42ab2-34be-b75e-e474-4ca8a80cea48@redhat.com>
Date:   Thu, 18 Nov 2021 08:08:54 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: MMU: update comment on the number of page role
 combinations

On 11/17/21 23:25, Sean Christopherson wrote:
>>
>> Here is a better writeup:
>>
>>   *   - invalid shadow pages are not accounted, so the bits are effectively 18
>>   *   - quadrant will only be used if gpte_is_8_bytes is zero (non-PAE paging);
>>   *     execonly and ad_disabled are only used for nested EPT which has
>>   *     gpte_is_8_bytes=1.  Therefore, 2 bits are always unused.
>>   *   - the 4 bits of level are effectively limited to the values 2/3/4/5,
>>   *     as 4k SPs are not tracked (allowed to go unsync).  In addition non-PAE
>>   *     paging has exactly one upper level, making level effectively redundant
>>   *     when gpte_is_8_bytes=0.
>>   *   - on top of this, smep_andnot_wp and smap_andnot_wp are only set if cr0_wp=0,
>>   *     therefore these three bits only give rise to 5 possibilities.
>>
>> FWIW, the full count becomes 6400 unless I screwed up the math.
> Which is "in the neighborhood of 2^13":-)
> 

2^12 if SMM is not counted.

Paolo