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SubjectRe: [PATCH] KVM: MMU: update comment on the number of page role combinations
On Tue, Nov 16, 2021, Paolo Bonzini wrote:
> 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

No, there are 18 from a gfn_track perspective. "smm" isn't counted because it's
in a separate memslot address space. The "mask below" is definitely vague on that
point though.

> 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.

Which is "in the neighborhood of 2^13" :-)

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