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
+percent guaranteed to occur (*outside* of the control and remit of the
+RISC-V Foundation), 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.
* MISA was only designed to cover Standard Extensions.
* There is nothing to prevent multiple Extensions being enabled
that wish to simultaneously interpret the same binary encoding.
+* There is nothing in the MISA specification which permits
+ *future* versions (bug-fixes) of the RISC-V ISA to be "switched in".
Overall, whilst the MISA concept is a step in the right direction it's
a hundred percent unsuitable for solving the problem.
* 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.
+ be left on (permanently) so that switching of ISA decoding
+ could be done inside inner loops without adverse impact on
+ performance.
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.
+scheme is invasive: it requires an entirely new CSR to be added
+to the privileged spec (thus making existing implementations redundant).
+This does not 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
TBD paraphrase and clarify
-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.
+Coming out of the software-related proposal by Jacob Bachmeyer, which
+hinged on the idea of a globally-maintained gcc / binutils database
+that kept and coordinated architectural encodings (curated by the Free
+Software Foundation), 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:
that have Custom Extensions, come under the "vendor/arch-id read only
is a declaration of having no Custom Extensions" fall-back category)
+So to summarise:
+
+* The consequences of not tackling this are severe: the RISC-V Foundation
+ cannot take a back seat. If it does, clear historical precedent shows
+ 100% what the outcome will be (1).
+* The retro-fitting cost onto existing implementations (even though the
+ specification has not been finalised) is negligeable
+ (changes to words in the specification)
+* The benefits are clear (pain-free transition path for vendors to safely
+ upgrade over time; no fights over Custom opcode space; no hassle for
+ software toolchain; no hassle for GNU/Linux Distros)
+* The implementation details are clear (and problem-free except for
+ vendors who insist on deploying dozens of conflicting Custom Extensions:
+ an extreme unlikely outlier).
+* Compliance Testing is straightforward and allows vendors to seek and
+ obtain *multiple* Compliance Certificates with past, present and future
+ variants of the RISC-V Standard (in the exact same processor), in order
+ support legacy customers and provide same customers with a way to avoid
+ "impossible-to-make" decisions that throw out ultra-expensive multi-decade
+ proprietary legacy software at the same as the hardware.
+
# Conversation Exerpts
The following conversation exerpts are taken from the ISA-dev discussion
projects / libreriscv.git / blobdiff