considers Custom Extensions to be "out of scope"; that "it's not their
problem, therefore there isn't a problem".
-The logical errors in this argument were quickly enumerated: namely
-that the RISC-V Foundation is not in control of the use-cases, such
-that binary-encoding is a hundred percent guaranteed to occur, and
-a hundred percent guaranteed to occur in *commodity* hardware where
-Debian, Fedora, SUSE and other distros will be hardest hit by the
-resultant chaos, and that will just be the more "visible" aspect of
-the underlying problem.
+The logical errors in this argument were quickly enumerated: namely that
+the RISC-V Foundation is not in control of the uses to which RISC-V is
+put, such that public global conflicts in binary-encoding are a hundred
+percent guaranteed to occur, and a hundred percent guaranteed to occur in
+*commodity* hardware where Debian, Fedora, SUSE and other distros will
+be hardest hit by the resultant chaos, and that will just be the more
+"visible" aspect of the underlying problem.
# Do nothing (Compliance too complex, therefore out of scope)
TBD (basically, may not be RV Foundation's "scope", still results in
problem, so not an option)
+The summary here was that Compliance testing of Custom Extensions is
+not just out-of-scope, but even if it was taken into account that
+binary-encoding meanings could change, it would still be out-of-scope.
+
+However at the time that this argument was made, it had not yet been
+appreciated fully the impact that revisions to the Standard would have,
+when billions of dollars worth of (older, legacy) RISC-V hardware had
+already been deployed.
+
Two interestingly diametrically-opposed equally valid arguments exist here:
* Whilst Compliance testing of Custom Extensions is definitely legitimately
TBD, basically same as mvend/march WARL except needs an extra CSR where
mv/ma doesn't.
+Out of the MISA discussion came a "MISA-like" proposal, which would
+take into account the flaws pointed out by trying to use "MISA":
+
+* The MISA-like CSR's meaning would be identified by compilers using the
+ mvendor-id/march-id tuple as a compiler target
+* Each custom-defined bit of the MISA-like CSR would (mutually-exclusively)
+ redirect binary encoding(s) to specific encodings
+* No Extension would *actually* be disabled: its internal state would
+ be left on (permanently) so that switching could be done inside
+ inner loops.
+
+Whilst it was the first "workable" solution it was also noted that the
+scheme is quite invasive: it requires an entirely new CSR to be added
+to the privileged spec. This does not completely fulfil the "minimum
+impact" requirement.
+
+Also interesting around the same time an additional discussion was
+raised that covered the *compiler* side of the same equation. This
+revolved around using mvendorid-marchid tuples at the compiler level,
+to be put into assembly output (by gcc), preserving the required
+*globally* unique identifying information for binutils to successfully
+turn the custom instruction into an actual binary-encoding (plus
+binary-encoding of the context-switching information). (**TBD, Jacob,
+separate page? review this para?**)
+
# mvendorid/marchid WARL
TBD paraphrase and clarify
-> In an earlier part of the thread someone kindly pointed out that MISA
-> already switches out entire sets of instructions [which interacts at the
-> "decode" phase]. However it was noted after a few days of investigating
+Coming out of the software-related proposal by Jacob, which hinged on
+the idea of a global gcc / binutils database that kept and coordinated
+architectural encodings, was to quite simply make the mvendorid and
+marchid CSRs have WARL (writeable) characteristics. For instances
+where mvendorid and marchid are readable, that would be taken to be
+a Standards-mandatory "declaration" that the architecture has *no*
+Custom Extensions.
+
+This incredibly simple non-invasive idea has some unique and distinct
+advantages over other proposals:
+
+* Existing designs - even though the specification is not finalised
+ (but has "frozen" aspects) - would be completely unaffected: the
+ change is to the "wording" of the specification to "retrospectively"
+ fit reality.
+* Unlike with the MISA idea this is *purely* at the "decode" phase:
+ no internal Extension state information is permitted to be disabled,
+ altered or destroyed as a direct result of writing to the
+ mvendor/march-id CSRs.
+* Compliance Testing may be carried out with a different vendorid/marchid
+ tuple set prior to a test, allowing a vendor to claim *Certified*
+ compatibility with *both* one (or more) legacy variants of the RISC-V
+ Specification *and* with a present one.
+* With sufficient care taken in the implementation an implementor
+ may have multiple interpretations of the same binary encoding within
+ an inner loop, with a single instruction (to the WARL register)
+ changing the meaning.
+
+A couple of points were made:
+
+* Compliance Testing may **fail** any system that has mvendorid/marchid
+ as WARL. This however is a clear case of "Compliance Tail Wagging Standard
+ Dog".
+* The redirection of meaning of certain binary encodings to multiple
+ engines was considered extreme, eyebrow-raising, and also (importantly)
+ potentially expensive, introducing significant latency at the decode
+ phase.
+
+On this latter point, it was observed that MISA already switches out entire
+sets of instructions (interacts at the "decode" phase). The difference
+between what MISA does and the mvendor/march-id WARL idea is that whilst
+MISA only switches instruction decoding on (or off), the WARL idea
+*redirects* encoding, to *different* engines, fortunately in a deliberately
+mutually-exclusive fashion.
+
> that particular lead that:
>
> * MISA Extension disabling is permitted (optionally) to DESTROY the state
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