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

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Message-Id: <20211118114039.1733976-1-pbonzini@redhat.com>
Date:   Thu, 18 Nov 2021 06:40:39 -0500
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     seanjc@...gle.com, mlevitsk@...hat.com
Subject: [PATCH v3] KVM: MMU: update comment on the number of page role combinations

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@...hat.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