[PATCH v2] KVM: MMU: update comment on the number of page role combinations

[Paolo Bonzini <pbonzini@redhat.com>]

Fix the number of bits in the role, and simplify the explanation of
why several bits or combinations of bits are redundant.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 arch/x86/include/asm/kvm_host.h | 30 ++++++++++++++++++------------
 1 file changed, 18 insertions(+), 12 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index e977634333d4..f69b100b23f4 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -291,19 +291,25 @@ struct kvm_kernel_irq_routing_entry;
  * the number of unique SPs that can theoretically be created is 2^n, where n
  * is the number of bits that are used to compute the role.
  *
- * But, even though there are 18 bits in the mask below, not all combinations
- * of modes and flags are possible.  The maximum number of possible upper-level
- * shadow pages for a single gfn is in the neighborhood of 2^13.
+ * But, even though there are 19 bits in the mask below, not all combinations
+ * of modes and flags are possible:
  *
- *   - invalid shadow pages are not accounted.
- *   - level is effectively limited to four combinations, not 16 as the number
- *     bits would imply, as 4k SPs are not tracked (allowed to go unsync).
- *   - level is effectively unused for non-PAE paging because there is exactly
- *     one upper level (see 4k SP exception above).
- *   - quadrant is used only for non-PAE paging and is exclusive with
- *     gpte_is_8_bytes.
- *   - execonly and ad_disabled are used only for nested EPT, which makes it
- *     exclusive with quadrant.
+ *   - invalid shadow pages are not accounted, so the bits are effectively 18
+ *
+ *   - quadrant will only be used if gpte_is_8_bytes=0 (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 completely 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.
+ *
+ * Therefore, the maximum number of possible upper-level shadow pages for a
+ * single gfn is a bit less than 2^13.
  */
 union kvm_mmu_page_role {
 	u32 word;
-- 
2.27.0