add executive summary

[libreriscv.git] / isa_conflict_resolution.mdwn

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

**Executive Summary:** A non-invasive backwards-compatible change to make
mvendorid and marchid being read-only to be a formal declaration of an
architecture having no Custom Extensions, and being permitted to be
WARL in order to support multiple simultaneous architectures on the
same processor (or hart) permits not only backwards and forwards
compatibility with existing implementations of the RISC-V Standard,
not only permits seamless transitions to future versions of the
RISC-V Standard (something that is not possible at the moment), but
permanently fixes the problem of clashes in Custom Extension opcodes
on a global basis.

--------

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)

(Summary: 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)

(Summary: 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 (*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.

# Do nothing (Compliance too complex, therefore out of scope)

(Summary: 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

(Summary: MISA not suitable, leads to better idea)

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.
* 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.

# MISA-like

(Summary: basically same as mvend/march WARL except needs an extra CSR where
mv/ma doesn't. Along right lines, doesn't meet full requirements)

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 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 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
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

(Summary: the only idea that meets the full requirements.  Needs
 toolchain backup, but only when the first chip is released)

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 (and that
it conforms precisely to one and only one specific variant of the
RISC-V Specification).

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.

Implementations would therefore, in each Extension (assuming one separate
"decode" engine per Extension), simply have an extra (mutually-exclusively
enabled) wire in the AND gate for any given binary encoding, and in this
way there would actually be very little impact on the latency.  The assumption
here is that there are not dozens of Extensions vying for the same binary
encoding (at which point the Fabless Semi Company has other much more
pressing issues to deal with that make resolving encoding conflicts trivial
by comparison).

Also pointed out was that in certain cases pipeline stalls could be introduced
during the switching phase, if needed, just as they may be needed for
correct implementation of (mandatory) support for MISA.

**This is the only one of the proposals that meet the full requirements**

# ioctl-like

(Summary: good solid orthogonal idea.  See [[ioctl]] for full details)

==RB===

This proposal adds a standardised extension interface to the RV instruction set by introducing a fixed small number (e.g. 8) of "overloadable" R-type opcodes ext_ctl0, .. ext_ctl7. Each takes a process local interface cookie in rs1. Based on the cookie, the CPU routes the "overloaded" instructions to a "device" on or off the CPU that implements the actual semantics. 

The cookie is "opened" with an additional r-type instruction ext_open that takes a 20 bit identifier and "closed" with an  ext_close instruction. The implementing hardware device can use the cookie to reference internal state. Thus, interfaces may be statefull.

CPU's and devices may implement several interfaces, indeed, are expected to. E.g. a single hardware device might expose a functional interface with 6 overloaded instructions, expose configuration with two highly device specific management interfaces with 8 resp. 4 overloaded instructions, and respond to a standardised save state interface with 4 overloaded instructions.

Having a standardised overloadable interface simply avoids much of the need for isa extensions for hardware with non standard interfaces and semantics. This is analogous to the way that the standardised overloadable ioctl interface of the kernel almost completely avoids the need for extending the kernel with syscalls for the myriad of hardware devices with their specific interfaces and semantics.  

Since the rs1 input of the overloaded  ext_ctl instruction's are taken by the interface cookie, they are restricted in use compared to a normal R-type instruction (it is possible to pass 12 bits of additional info by or ing it with the cookie). Delegation is also expected to come at a small additional performance price compared to a "native" instruction. This should be an acceptable tradeoff in most cases. 

The expanded flexibility comes at the cost: the standard can specify the semantics of the delegation mechanism and the interfacing with the rest of the cpu, but the actual semantics of the overloaded instructions can only be defined by the designer of the interface. Likewise, a device can be conforming as far as delegation and interaction with the CPU is concerned, but whether the hardware is conforming to the semantics of the interface is outside the scope of spec. Being able to specify that semantics using the methods used for RV itself is clearly very valuable. One impetus for doing that is using it for purposes of its own, effectively freeing opcode space for other purposes. Also, some interfaces may become de facto or de jure standards themselves, necessitating hardware to implement competing interfaces. I.e., facilitating a free for all, may lead to standards proliferation. C'est la vie.  

The only "ISA-collisions" that can still occur are in the 20 bit (~10^6) interface identifier space, with 12 more bits to identify a device on a hart that implements the interface. One suggestion is setting aside 2^19 id's that are handed out for a small fee by a central (automated) registration (making sure the space is not just claimed), while the remaining 2^19 are used as a good hash on a long, plausibly globally unique human readable interface name. This gives implementors the choice between a guaranteed private identifier paying a fee, or relying on low probabilities. The interface identifier could also easily be extended to 42 bits on RV64. 


====End RB==

This proposal basically mirrors the concept of POSIX ioctls, providing
(arbitrarily) 8 functions (opcodes) whose meaning may be over-ridden
in an object-orientated fashion by calling an "open handle" (and close)
function (instruction) that switches (redirects) the 8 functions over to
different opcodes.


The "open handle" opcode takes a GUID (globally-unique identifier)
and an ioctl number, and stores the UUID in a table indexed by the
ioctl number:

    handle_global_state[8] # stores UUID or index of same 

    def open_handle(uuid, ioctl_num): 
          handle_global_state[ioctl_num] = uuid 

    def close_handle(ioctl_num): 
          handle_global_state[ioctl_num] = -1 # clear table entry

         
"Ioctls" (arbitrarily 8 separate R-type opcodes) then perform a redirect
based on what the global state for that numbered "ioctl" has been set to:

    def ioctl_fn0(*rargs): # star means "take all arguments as a tuple"
        if handle_global_state[0] == CUSTOMEXT1UUID: 
           CUSTOMEXT1_FN0(*rargs) # apply all arguments to function 
        elif handle_global_state[0] == CUSTOMEXT2UUID: 
           CUSTOMEXT2_FN0(*rargs) # apply all arguments to function 
        else:
            raise Exception("undefined opcode")

=== RB ==

not quite I think. It is more like

// Hardware, implementing interface with UUID 0xABCD

    def A_shutdown(cookie, data):
       ...

    def A_init(data)

    def A_do_stuff(cookie, data):
       ...

    def A_do_more_stuff(cookie, data):
       ...

    interfaceA = {
                  "shutdown": A_shutdown,
                  "init":     A_init,
                  "ctl0":     A_do_stuff, 
                  "ctl1":     A_do_more_stuff
                 }

// hardware implementing interface with UUID = 0x1234

    def B_do_things(cookie, data):
       ...
    def B_shutdown(cookie, data)
       ...

    interfaceB = {
                  "shutdown": B_shutdown,
                  "ctl0":     B_do_things
                 }


// The CPU being wired to the devices

    cpu_interfaces = {
                  0xABCD: interfaceA,
                  0x1234: interfaceB
                 }

// The functionality that the CPU must implement to use the extension interface

    cpu_open_handles = {}
  
    __handleId = 0
    def new_unused_handle_id()
        __handleId = __handleId + 1
        return __handleId
         
    def ext_open(uuid, data):
        interface = cpu_interface[uuid]
        if interface == NIL:
            raise Exception("No such interface")
        
        handleId = new_unused_handle_id()
        cpu_open_handles[handleId] = (interface, CurrentVirtualMemoryAddressSpace)

        cookie = A_init(data)                      # Here device takes over

        return (handle_id, cookie)

    def ext_close(handle, data):
        (handleId, cookie) = handle
        intf_VMA = cpu_open_handles[handleId]
        if intf_VMA == NIL:
             return -1

        (interface, VMA) = intf_VMA
        if VMA != CurrentVirtualMemoryAddressSpace:
             return -1
        assert(interface != NIL)
        shutdown = interface["shutdown"]
        if shutdown != NIL:

             err = interface.shutdown(cookie, data)  # Here device takes over

             if err != 0:
                 return err
        cpu_open_handles[handleId] = NIL
        return 0

    def ext_ctl0(handle, data):
        (handleId, cookie) = handle
        intf_VMA = cpu_open_handles[handleId]
        if intf_VMA == NIL:
             raise Exception("No such interface")   

        (interface, VMA) = intf_VMA
        if VMA != CurrentVirtualMemoryAddressSpace: 
             raise Exception("No such interface")  #Disclosing that the interface exists in different address is security hole 

        assert(interface != NIL)
        ctl0 = interface["ctl0"]
        if ctl0 == NIL:
            raise Exception("No such Instruction")

        return ctl0(cookie, data)                  # Here device takes over
       
        
The other ext_ctl's are similar. 
        
==End RB==



      
The proposal is functionally near-identical to that of the mvendor/march-id
except extended down to individual opcodes.  As such it could hypothetically
be proposed as an independent Standard Extension in its own right that extends
the Custom Opcode space *or* fits into the brownfield spaces within the
existing ISA opcode space *or* is used as the basis of an independent
Custom Extension in its own right.

==RB==
I really think it should be in browncode
==RB==

One of the reasons for seeking an extension of the Custom opcode space is
that the Custom opcode space is severely limited: only 2 opcodes are free
within the 32-bit space, and only four total remain in the 48 and 64-bit
space.

Despite the proposal (which is still undergoing clarification)
being worthwhile in its own right, and standing on its own merits and
thus definitely worthwhile pursuing, it is non-trivial and much more
invasive than the mvendor/march-id WARL concept.



# Comments, Discussion and analysis

TBD: placeholder as of 26apr2018

# Summary and Conclusion

In the early sections (those in the category "no action") it was established
in each case that the problem is not solved.  Avoidance of responsibility,
or conflation of "not our problem" with "no problem" does not make "problem"
go away.  Even "making it the Fabless Semiconductor's design problem" resulted
in a chip being *more costly to engineer as hardware **and** more costly
from a software-support perspective to maintain*... without actually
fixing the problem.

The first idea considered which could fix the problem was to just use
the pre-existing MISA CSR, however this was determined not to have
the right coverage (Standard Extensions only), and also crucially it
destroyed state.  Whilst unworkable it did lead to the first "workable"
solution, "MISA-like".

The "MISA-like" proposal, whilst meeting most of the requirements, led to
a better idea: "mvendor/march-id WARL", which, in combination with an offshoot
idea related to gcc and binutils, is the only proposal that fully meets the
requirements.

The "ioctl-like" idea *also* solves the problem, but, unlike the WARL idea
does not meet the full requirements to be "non-invasive" and "backwards
compatible" with pre-existing (pre-Standards-finalised) implementations.
It does however stand on its own merit as a way to extend the extremely
small Custom Extension opcode space, even if it itself implemented *as*
a Custom Extension into which *other* Custom Extensions are subsequently
shoe-horned.  This approach has the advantage that it requires no "approval"
from the RISC-V Foundation... but without the RISC-V Standard "approval"
guaranteeing no binary-encoding conflicts, still does not actually solve the
problem (if deployed as a Custom Extension for extending Custom Extensions).

Overall the mvendor/march-id WARL idea meets the three requirements,
and is the only idea that meets the three requirements:

* **Any proposal must be a minimal change with minimal (or zero) impact**
  (met through being purely a single backwards-compatible change to the
  wording of the specification: mvendor/march-id changes from read-only
  to WARL)
* **Any proposal should place no restriction on existing or future
  ISA encoding space**
  (met because it is just a change to one pre-existing CSR, as opposed
  to requiring additional CSRs or requiring extra opcodes or changes
  to existing opcodes)
* **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.**
  (met because existing implementations, with the exception of those
  that have Custom Extensions, come under the "vendor/arch-id read only
  is a formal declaration of an implementation 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).
* Making the mvendorid and marchid CSRs WARL solves the problem in a
  minimal to zero-disruptive backwards-compatible fashion that provides
  indefinite transparent *forwards*-compatibility.
* The retro-fitting cost onto existing implementations (even though the
  specification has not been finalised) is zero to negligeable
  (only changes to words in the specification required at this time:
  no vendor need discard existing designs, either being designed,
  taped out, or actually in production).
* 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,
  simultaneously), in order to support end-customer legacy scenarios and
  provide the same with a way to avoid "impossible-to-make" decisions that
  throw out ultra-costly multi-decade-investment in proprietary legacy
  software at the same as the (legacy) hardware.

-------

# 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.

# References

* <https://groups.google.com/a/groups.riscv.org/forum/#!topic/isa-dev/7bbwSIW5aqM>
* <https://groups.google.com/a/groups.riscv.org/forum/#!topic/isa-dev/InzQ1wr_3Ak%5B1-25%5D>