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# Resolving ISA conflicts and providing a pain-free RISC-V Standards Upgrade Path

In a lengthy thread that ironically was full of conflict indicative
of the future direction in which RISC-V will go if left unresolved,
multiple Custom Extensions were noted to be permitted free rein to
introduce global binary-encoding conflict with no means of resolution
described or endorsed by the RISC-V Standard: a practice that has known
disastrous and irreversible consequences for any architecture that
permits such practices (1).

Much later on in the discussion it was realised that there is also no way
within the current RISC-V Specification to transition to improved versions
of the standard, regardless of whether the fixes are absolutely critical
show-stoppers or whether they are just keeping the standard up-to-date (2).

With no transition path there is guaranteed to be tension and conflict
within the RISC-V Community over whether revisions should be made:
should existing legacy designs be prioritised, mutually-exclusively over
future designs (and what happens during the transition period is absolute
chaos, with the compiler toolchain, software ecosystem and ultimately
the end-users bearing the full brunt of the impact).  If several
overlapping revisions are required that have not yet transitioned out
of use (which could take well over two decades to occur) the situation
becomes disastrous for the credibility of the entire RISC-V ecosystem.

It was also pointed out that Compliance is an extremely important factor
to take into consideration, and that Custom Extensions (as being optional)
effectively and quite reasonably fall entirely outside of the scope of
Compliance Testing.  At this point in the discussion however it was not
yet noted the stark problem that the *mandatory* RISC-V Specification
also faces, by virtue of there being no transitional way to bring in
show-stopping critical alterations.

To put this into perspective, just taking into account hardware costs
alone: with production mask charges for 28nm being around USD $1.5m,
engineering development costs and licensing of RTLs for peripherals
being of a similar magnitude, no manufacturer is going to back away
from selling a "flawed" or "legacy" product (whether it complies with
the RISC-V Specification or not) without a bitter fight.

It was also pointed out that there will be significant software tool
maintenance costs for manufacturers, meaning that the probability will
be extremely high that they will refuse to shoulder such costs, and
will publish and continue to publish (and use) hopelessly out-of-date
unpatched tools.  This practice is well-known to result in security
flaws going unpatched, with one of many immediate undesirable consequences
being that product in extremely large volume gets discarded into landfill.

**All and any of the issues that were discussed, and all of those that
were not, can be avoided by providing a hardware-level runtime-enabled
forwards and backwards compatible transition path between *all* parts
(mandatory or not) of current and future revisions of the RISC-V ISA
Standard.**

The rest of the discussion - indicative as it was of the stark mutually
exclusive gap being faced by the RISC-V ISA Standard given that it does
not cope with the problem - was an effort by two groups in two clear
camps: one that wanted things to remain as they are, and another that
made efforts to point out that the consequences of not taking action
are clearly extreme and irreversible (which, unfortunately, given the
severity, some of the first group were unable to believe, despite there
being clear historical precedent for the exact same mistake being made in
other architectures, and the consequences on the same being absolutely
clear).

However after a significant amount of time, certain clear requirements came
out of the discussion:

* Any proposal must be a minimal change with minimal (or zero) impact
* Any proposal should place no restriction on existing or future
  ISA encoding space
* Any proposal should take into account that there are existing implementors
  of the (yet to be finalised but still "partly frozen") Standard who may
  resist, for financial investment reasons, efforts to make any change
  (at all) that could cost them immediate short-term profits.

Several proposals were put forward (and some are still under discussion)

* "Do nothing": problem is not severe: no action needed.
* "Do nothing": problem is out-of-scope for RISC-V Foundation.
* "Do nothing": problem complicates Compliance Testing (and is out of scope)
* "MISA": the MISA CSR enables and disables extensions already: use that
* "MISA-like": a new CSR which switches in and out new encodings
  (without destroying state)
* "mvendorid/marchid WARL": switching the entire "identity" of a machine
* "ioctl-like": a OO proposal based around the linux kernel "ioctl" system.

Each of these will be discussed below in their own sections.

# Do nothing (no problem exists)

TBD (basically not an option).

There were several solutions offered that fell into this category.
A few of them are listed in the introduction; more are listed below,
and it was exhaustively (and exhaustingly) established that none of
them are workable.

Initially it was pointed out that Fabless Semiconductor companies could
simply license multiple Custom Extensions and a suitable RISC-V core, and
modify them accordingly.  The Fabless Semi Company would be responsible
for paying the NREs on re-developing the test vectors (as the extension
licensers would be extremely unlikely to do that without payment), and
given that said Companies have an "integration" job to do, it would
be reasonable to expect them to have such additional costs as well.

The costs of this approach were outlined and discussed as being
disproportionate and extreme compared to the actual likely cost of
licensing the Custom Extensions in the first place.  Additionally it
was pointed out that not only hardware NREs would be involved but
custom software tools (compilers and more) would also be required
(and maintained separately, on the basis that upstream would not
accept them except under extreme pressure, and then only with
prejudice).

All similar schemes involving customisation of the custom extensions
were likewise rejected, but not before the customisation process was
mistakenly conflated with tne *normal* integration process of developing
a custom processor (Bus Architectures, Cache layouts, peripheral layouts).

The most compelling hardware-related reason (excluding the severe impact on
the software ecosystem) for rejecting the customisation-of-customisation
approach was the case where Extensions were using an instruction encoding
space (48-bit, 64-bit) *greater* than that which the chosen core could
cope with (32-bit, 48-bit).

Overall, none of the options presented were feasible, and, in addition,
with no clear leadership from the RISC-V Foundation on how to avoid
global world-wide encoding conflict, even if they were followed through,
still would result in the failure of the RISC-V ecosystem due to
irreversible global conflicting ISA binary-encoding meanings (POWERPC's
Altivec / SPE nightmare).

This in addition to the case where the RISC-V Foundation wishes to
fix a critical show-stopping update to the Standard, post-release,
where billions of dollars have been spent on deploying RISC-V in the
field.

# Do nothing (out of scope)

TBD (basically, may not be RV Foundation's "scope", still results in
problem, so not an option)

This was one of the first arguments presented: The RISC-V Foundation
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 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
  out of scope, Compliance testing of simultaneous legacy (old revisions of
  ISA Standards) and current (new revisions of ISA Standard) definitely
  is not.  Efforts to reduce *Compliance Testing* complexity is therefore
  "Compliance Tail Wagging Standard Dog".
* Beyond a certain threshold, complexity of Compliance Testing is so
  burdensome that it risks outright rejection of the entire Standard.

Meeting these two diametrically-opposed perspectives requires that the
solution be very, very simple.

# MISA

TBD, basically MISA not suitable

MISA permits extensions to be disabled by masking out the relevant bit.
Hypothetically it could be used to disable one extension, then enable
another that happens to use the same binary encoding.

*However*:

* MISA Extension disabling is permitted (optionally) to **destroy**
  the state information.  Thus it is totally unsuitable for cases
  where instructions from different Custom extensions are needed in
  quick succession.
* 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.
 
Overall, whilst the MISA concept is a step in the right direction it's
a hundred percent unsuitable for solving the problem.

# MISA-like

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

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
> information (which means that it *has* to be re-initialised just to be
> safe... mistake in the standard, there), and * MISA was only designed
> to cover Standard Extensions.
> 
> So the practice of switching extensions in and out - and the resultant
> "disablement" and "enablement" at the *instruction decode phase* is
> *already* a hard requirement as part of conforming with the present
> RISC-V Specification.
> 
> Around the same MISA discussion, someone else also kindly pointed out
> that one solution to the heavyweight nature of the switching would
> be to deliberately introduce a pipeline stall whilst the switching is
> occurring: I can see the sense in that approach, even if I don't know the
> full details of what each implementor might choose to do.  They may even
> choose two, or three, or N pipeline stalls: it really doesn't matter,
> as it's an implementors' choice (and problem to solve).
> 
> So yes it's pretty heavy-duty... and also already required.
> 
> For the case where "legacy" variants of the RISC-V Standard are
> backwards-forwards-compatibly supported over a 10-20 year period
> in Industrial and Military/Goverment-procurement scenarios (so that
> the impossible-to-achieve pressure is off to get the spec ABSOLUTELY
> correct, RIGHT now), nobody would expect a seriously heavy-duty amount
> of instruction-by-instruction switching: it'd be used pretty much once
> and only once at boot-up (or once in a Hypervisor Virtual Machine client)
> and that's it.
> 
> I can however foresee instances where implementors would actually
> genuinely want a bank of operations to be carried out using one extension,
> followed immediately by another bank from a (conflicting binary-encoding)
> extension, in an inner loop: Software-defined MPEG / MP4 decode to call
> DCT block decode Custom Extension followed immediately by Custom Video
> Processing Extension followed immediately by Custom DSP Processing
> Extension to do YUV-to-RGB conversion for example is something that
> is clearly desirable.  Solving that one would be entiiirely their
> problem... and the RISC-V Specification really really should give them
> the space to do that in a clear-cut unambiguous way.

# ioctl-like

TBD - [[ioctl]] for full details, summary kept here

# Discussion and analysis

TBD

# Conclusion

TBD

# Conversation Exerpts

The following conversation exerpts are taken from the ISA-dev discussion

## (1) Albert Calahan on SPE / Altiven conflict in POWERPC

> Yes. Well, it should be blocked via legal means. Incompatibility is
> a disaster for an architecture.
> 
> The viability of PowerPC was badly damaged when SPE was
> introduced. This was a vector instruction set that was incompatible
> with the AltiVec instruction set. Software vendors had to choose,
> and typically the choice was "neither". Nobody wants to put in the
> effort when there is uncertainty and a market fragmented into
> small bits.
> Note how Intel did not screw up. When SSE was added, MMX remained.
> Software vendors could trust that instructions would be supported.
> Both MMX and SSE remain today, in all shipping processors. With very
> few exceptions, Intel does not ship chips with missing functionality.
> There is a unified software ecosystem.
> 
> This goes beyond the instruction set. MMU functionality also matters.
> You can add stuff, but then it must be implemented in every future CPU.
> You can not take stuff away without harming the architecture.

## (2) Luke Kenneth Casson Leighton on Standards backwards-compatibility

> For the case where "legacy" variants of the RISC-V Standard are
> backwards-forwards-compatibly supported over a 10-20 year period in
> Industrial and Military/Goverment-procurement scenarios (so that the
> impossible-to-achieve pressure is off to get the spec ABSOLUTELY
> correct, RIGHT now), nobody would expect a seriously heavy-duty amount
> of instruction-by-instruction switching: it'd be used pretty much once
> and only once at boot-up (or once in a Hypervisor Virtual Machine
> client) and that's it.

## (3) Allen Baum on Standards Compliance

> Putting my compliance chair hat on: One point that was made quite
> clear to me is that compliance will only test that an implementation
> correctly implements the portions of the spec that are mandatory, and
> the portions of the spec that are optional and the implementor claims
> it is implementing. It will test nothing in the custom extension space,
> and doesn't monitor or care what is in that space.