Clarify Svpbmt non-leaf PTEs and other wording

Thanks to @scottj97 for pointing out the unclear wording.

1 files changed, 13 insertions, 7 deletions

diff --git a/src/supervisor.tex b/src/supervisor.tex
index 18732fb..60d6122 100644
--- a/src/supervisor.tex
+++ b/src/supervisor.tex
@@ -2280,7 +2280,7 @@ algorithm in Section~\ref{sv32algorithm}, except that:
 {\bf Warning! This draft specification is likely to change before being
 accepted as standard by the RISC-V Foundation.}
 
-In Sv39, Sv48, and Sv57, bits 62--61 of the page table entry indicate the use
+In Sv39, Sv48, and Sv57, bits 62--61 of a leaf page table entry indicate the use
 of page-based memory types that override the PMA(s) for the associated memory
 pages.  The encoding for the PBMT bits is captured in Table~\ref{pbmt}.
 
@@ -2307,6 +2307,9 @@ Future extensions may provide more and/or finer-grained control over which PMAs
 can be overridden.
 \end{commentary}
 
+For non-leaf PTEs, bits 62--61 are reserved for future standard use and must be
+cleared by software for forward compatibility.
+
 If the underlying physical memory attribute for a page is I/O, then accesses to
 that page with PBMT=1 will obey RVWMO or RVTSO rather than I/O strong ordering
 rules, and accesses to such pages are considered main memory rather than I/O
@@ -2361,9 +2364,12 @@ performed.  Such optimizations will likely improve performance when applied
 with adequate care.
 \end{commentary}
 
-When two-level paging is enabled within the H extension, the page-based memory
-types are applied in two stages.  First, the G-stage PTE PBMT bits (if enabled)
-are applied to the attributes in the PMA to produce an intermediate set of
-attributes.  Second, the VS-stage PTE PBMT bits (if enabled) are applied to
-these intermediate attributes to produce the final set of attributes used by
-accesses to the page in question.
+When two-stage address translation is enabled within the H extension, the
+page-based memory types are also applied in two stages.  First, if
+{\tt hgatp}.MODE is not equal to zero, the G-stage PTE PBMT bits are applied to
+the attributes in the PMA to produce an intermediate set of attributes.
+Otherwise, the PMAs serve as the intermediate attributes.  Second, if
+{\tt vsatp}.MODE is not equal to zero, the VS-stage PTE PBMT bits are applied
+to the intermediate attributes to produce the final set of attributes used by
+accesses to the page in question.  Otherwise, the intermediate attributes are
+used as the final set of attributes.