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[[ztso]]
== "Ztso" Extension for Total Store Ordering, Version 1.0
This chapter defines the "Ztso" extension for the RISC-V Total Store
Ordering (RVTSO) memory consistency model. RVTSO is defined as a delta
from RVWMO, which is defined in <<rvwmo>>.
[NOTE]
====
_The Ztso extension is meant to facilitate the porting of code originally
written for the x86 or SPARC architectures, both of which use TSO by
default. It also supports implementations which inherently provide RVTSO
behavior and want to expose that fact to software._
====
RVTSO makes the following adjustments to RVWMO:
* All load operations behave as if they have an acquire-RCpc annotation
* All store operations behave as if they have a release-RCpc annotation.
* All AMOs behave as if they have both acquire-RCsc and release-RCsc
annotations.
[NOTE]
====
_These rules render all PPO rules except
<<overlapping-ordering, 4-7>> redundant. They also make
redundant any non-I/O fences that do not have both PW and SR set.
Finally, they also imply that no memory operation will be reordered past
an AMO in either direction._
_In the context of RVTSO, as is the case for RVWMO, the storage ordering
annotations are concisely and completely defined by PPO rules
<<overlapping-ordering, 5-7>>. In both of these
memory models, it is the <<ax-load>> that allows a hart to forward a value from its
store buffer to a subsequent (in program order) load—that is to say that
stores can be forwarded locally before they are visible to other harts._
====
Additionally, if the Ztso extension is implemented, then vector memory
instructions in the V extension and Zve family of extensions follow RVTSO at
the instruction level.
The Ztso extension does not strengthen the ordering of intra-instruction
element accesses.
In spite of the fact that Ztso adds no new instructions to the ISA, code
written assuming RVTSO will not run correctly on implementations not
supporting Ztso. Binaries compiled to run only under Ztso should
indicate as such via a flag in the binary, so that platforms which do
not implement Ztso can simply refuse to run them.
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