allow coexistence of Ztso and Ssdtso

changes.adoc

@@ -1,5 +1,9 @@
 == Revision history
 
+Changes from v1.0-draft2 to v1.0-draft3:
+
+* Allow co-existence of Ssdtso and Ztso
+
 Changes from v1.0-draft1 to v1.0-draft2:
 
 * Changed controlling CSR to `menvcfg`, `henvcfg`, and `senvcfg`.

header.adoc

@@ -3,7 +3,7 @@ Authors: Dr. Philipp Tomsich (VRULL), Dr. Zhao Siqi (T-Head)
 :docgroup: RISC-V Fast Track
 :description: RISC-V Dynamic Switching of TSO-mode (Ssdtso) extension
 :company: RISC-V.org
-:revdate: 10/2023
+:revdate: 12/2023
 :revnumber: 1.0
 :revremark: This document is under development. Expect potential changes. Visit http://riscv.org/spec-state for further details.
 :revinfo:

ssdtso.adoc

@@ -20,13 +20,11 @@ The intended use-cases for Ssdtso are
 
 === Dependencies and Incompatibilities
 
-The Ssdto extension conflicts with Ztso.
+For its intended use case (i.e., to provide an provide an on-demand RVTSO execution capability for processors otherwise operating in RVWMO), Ssdtso will be used on cores that do not implement Ztso.
 
-[NOTE]
-====
-Ztso provides RVTSO semantics across all privilege levels and harts of an implementation.
-In contrast to this, Ssdtso assumes that RVWMO is the default (at boot) mode of operation, that individual harts can be switched to RVTSO, and that the M-mode execution environment can not switched to RVTSO.
-====
+However, the Ssdto extension does not conflict with Ztso (i.e., a core that operates in RVTSO mode at all times) and can be implemented on such a core for compatibility:
+ * the RVTSO mode is always-on on Ztso implementations, and
+ * the DTSO bit in `menvcfg`, `senvcfg` and `henvcfg` will accurately reflect whether the core currently operates in RVTSO mode.
 
 [NOTE]
 ====
@@ -37,8 +35,10 @@ To allow the free composition of extensions for unforeseen use-cases, no formal
 
 === State controlling whether dynamic TSO is enabled
 
-The dynamic-RVTSO behaviour is controlled by bit 8 (`DTSO`) of `menvcfg, senvcfg, henvcfg` for all lower privilege levels.
-Implementations providing Ssdtso are required to default to RVWMO semantics: the bits controlling dynamic-RVTSO behaviour are initialized to 0 on reset.
+The dynamic-RVTSO behaviour is controlled by bit 8 (`DTSO`) of `menvcfg`, `senvcfg`, and `henvcfg` for all lower privilege levels.
+Implementations providing Ssdtso initialize the `DTSO`-bit according to their default memory mode:
+ * implementations that implement only Ssdtso, and not Ztso, default to use RVWMO semantics natively: the bits controlling dynamic-RVTSO behaviour are initialized to 0 on reset
+ * implementations that implement both Ssdtso and Ztso, default to use RVTSO semantics natively: the bits controlling dynamic-RVTSO behaviour are initialized to 1 on reset
 
 If the DTSO control bit in the respective envcfg is set, all lower-privilege modes will operate in dynamic-RVTSO mode:
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