Incorporate Smpmpmt draft

src/priv/preface.adoc

@@ -23,6 +23,7 @@ h|Extension h|Version
 |ext:smstateen[] |1.0
 |ext:smcsrind[] |1.0
 |ext:smepmp[] |1.0
+|ext:smpmpmt[] |1.0
 |ext:smcntrpmf[] |1.0
 |ext:smrnmi[] |1.0
 |ext:smcdeleg[] |1.0

src/priv/sm.adoc

@@ -7,6 +7,7 @@ Its contents are normative, but the presentation might appear disjoint.
 include::smstateen.adoc[]
 include::smcsrind.adoc[]
 include::smepmp.adoc[]
+include::smpmpmt.adoc[]
 include::smcntrpmf.adoc[]
 include::smrnmi.adoc[]
 include::smcdeleg.adoc[]

src/priv/smpmpmt.adoc

@@ -0,0 +1,58 @@
+[[ext:smpmpmt]]
+=== ext:smpmpmt[] PMP-based Memory Types Extension
+
+The extlink:svpbmt[] extension provides a mechanism to control memory
+attributes at the granularity of a virtual-memory page.
+Unfortunately, Svpbmt is only available for RV64, and only then when
+page-based virtual memory is in effect.
+
+The Smpmpmt extension provides a mechanism analogous to Svpbmt but associated
+with the PMP registers rather than page-table entries.
+A new WARL field, MT (Memory Type), is added in the unallocated bits 6--5 in
+each PMP configuration register.
+The MT field of the lowest-numbered PMP register that an access matches,
+following the existing PMP address-matching rules, overrides the PMAs for
+that access.
+If an access matches no PMP registers, or if the MT field of the matching PMP
+register contains the value PMA, then the access proceeds as though the
+Smpmpmt extension were not implemented.
+
+NOTE: The permissions checks associated with a PMP register are unrelated to
+whether an access is considered to match a PMP register.
+For example, even though a PMP register with its L bit clear does not enforce
+its permissions checks on M-mode accesses, M-mode accesses within the
+address range of that PMP register are still considered to match.
+Hence, that PMP register's MT field takes effect for matching M-mode accesses.
+
+The MT field is encoded as shown in <<ext:svpbmt>>.
+The encoding is identical to that of the PBMT field, and the meaning of each
+type is as specified by the Svpbmt extension.
+Refer to that specification for the semantics of each type.
+
+[[pmpmt]]
+.Encodings for MT field
+[%autowidth,float="center",align="center",cols="^,^,<",options="header"]
+|===
+|Mode |Value |Requested Memory Attributes
+|PMA +
+NC +
+IO +
+-
+|0 +
+1 +
+2 +
+3
+|None +
+Non-cacheable, idempotent, weakly-ordered (RVWMO), main memory +
+Non-cacheable, non-idempotent, strongly-ordered (I/O ordering), I/O +
+_Reserved for future standard use_
+|===
+
+The PMP-based memory types compose with the page-based memory types as follows.
+First, the PMP-based memory types override the PMAs, producing an intermediate
+set of attributes.
+Then, supervisor-level memory attributes (e.g. the page-based attributes
+described in the Svpbmt specification) override the intermediate attributes to
+produce a final set of attributes.
+As with Svpbmt, it is implementation-defined whether PMP-based memory types
+override additional PMAs not explicitly listed in <<ext:svpbmt>>.