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

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Date:   Tue, 16 Nov 2021 13:28:17 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     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 11/16/21 12:07, Maxim Levitsky wrote:
>> - * But, even though there are 18 bits in the mask below, not all
>> combinations
>> + * But, even though there are 20 bits in the mask
>> below, not all combinations
> I to be honest counted 19 bits there (which includes the 'smm' bit),
> but I might have made a mistake. I do wonder maybe it is better to
> just remove that comment with explicit number?

Yes, they are 19.  But the explicit number is there to guide in
understanding how 19 goes down to 14 combinations.

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.

Paolo