[PATCH v3] KVM: MMU: update comment on the number of page role combinations

Paolo Bonzini
2021-11-18T11:42:00Z

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 | 32 ++++++++++++++++++++------------
1 file changed, 20 insertions(+), 12 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 6ac61f85e07b..55f280e96b59 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -291,19 +291,27 @@ 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.
+ * There are 19 bits in the mask below, and the page tracking code only uses
+ * 16 bits per gfn in kvm_arch_memory_slot to count whether a page is tracked.
+ * However, not all combinations of modes and flags are possible. First
+ * of all, invalid shadow pages pages are not accounted, and "smm" is constant
+ * in a given memslot (because memslots are per address space, and SMM uses
+ * a separate address space). Of the remaining 2^17 possibilities:
*
- * - 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.
+ * - 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
+ * given (as_id, gfn) pair is a bit less than 2^12.
*/
union kvm_mmu_page_role {
u32 word;
--
2.27.0