From 8447ac2f7cec468e7f8710a35c1518c07e78905e Mon Sep 17 00:00:00 2001
From: Ravi Sahita <ravi@rivosinc.com>
Date: Mon, 14 Oct 2024 14:01:38 -0700
Subject: [PATCH 1/2] add valid bit to MTTL3 entry

Signed-off-by: Ravi Sahita <ravi@rivosinc.com>
---
 chapter4.adoc | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/chapter4.adoc b/chapter4.adoc
index a98a315..b5486d4 100644
--- a/chapter4.adoc
+++ b/chapter4.adoc
@@ -59,10 +59,17 @@ The following MTT L3 entry accomodates a 56 bit PAW:
 ....
 {reg: [
   {bits:  44, name: 'MTTL2 PPN'},
-  {bits:  20, name: 'ZERO'},
+  {bits:  19, name: 'ZERO'},
+  {bits:   1, name: 'VALID'},
 ], config:{lanes: 1, hspace:1024}}
 ....
 
+The `VALID` bit determines the validity of the `MTTL3` entry. If this bit is
+set, the `MTTL3` entry is valid and the `MTTL2 PPN` holds the next level of the
+MTT, and the `ZERO` field must be 0. If the `VALID` bit is clear, the entry is
+invalid, and all other bits in the entry are don't-cares and may be used freely
+by software.
+
 The following are the `MTTL2` entry by XLEN:
 
 [caption="Figure {counter:image}: ", reftext="Figure {image}"]
@@ -243,8 +250,8 @@ corresponding to the original access type.
 an access-fault exception corresponding to the original access type.
 
 4. If any bits or encodings that are reserved for future standard use are
-set within _mpte_, stop and raise an access-fault exception corresponding to
-the original access type.
+set within _mpte_, or _mpte_ is a not-valid `MTTL3` entry, stop and raise an
+access-fault exception corresponding to the original access type.
 
 5. Otherwise, the _mpte_ is valid. If (_i_=1) or (_i_=2 and _mpte.type_ is not
 `MTT_L1_DIR`), go to step 6. Otherwise, the _mpte_ is a pointer to the next

From a9bb6f1437524862368fbddf006ed30bd391ee93 Mon Sep 17 00:00:00 2001
From: Ravi Sahita <ravi@rivosinc.com>
Date: Mon, 14 Oct 2024 22:38:28 -0700
Subject: [PATCH 2/2] update bit position of valid bit in MTTL3

Signed-off-by: Ravi Sahita <ravi@rivosinc.com>
---
 chapter4.adoc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/chapter4.adoc b/chapter4.adoc
index b5486d4..8e87adb 100644
--- a/chapter4.adoc
+++ b/chapter4.adoc
@@ -59,8 +59,8 @@ The following MTT L3 entry accomodates a 56 bit PAW:
 ....
 {reg: [
   {bits:  44, name: 'MTTL2 PPN'},
-  {bits:  19, name: 'ZERO'},
   {bits:   1, name: 'VALID'},
+  {bits:  19, name: 'ZERO'},
 ], config:{lanes: 1, hspace:1024}}
 ....