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

From: Paolo Bonzini
Date: Thu Nov 18 2021 - 02:12:22 EST

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.