RISC-V Compliance Tests Aim to Address RISC-V Fragmentation

In the x86 world, one operating system image can usually run on all hardware thanks to clearly defined instruction sets, hardware and software requirements. Arm provides most flexibility in terms of peripherals, while having a fixed set of intrusions for a given architecture (e.g. Armv8, Armv7…), and this lead to fragmentation, so that in the past you had to customize your software with board files and other tweaks, and provide one binary per board, leading to lots of fragmentation. With device trees, things improved a bit, but there are still few images that will run on multiple boards without modifications.

RISC-V provides even more flexibility that Arm since you can mess up with the instructions set with designers able to add or remove instructions as they see fit for their application. One can easily imagine how this can lead to a complete mess with binary code only running on a subset of RISC-V platforms, and lots of compiler options to build the code for a particular RISC-V SoC.

RISC-V Fragmentation

Brian Bailey, Technology Editor/EDA for Semiconductor Engineering, goes into details about the challenges of RISC-V compliance in details, and I’ll try to summarize the key points in this post.

Allen Baum, system architect at Esperanto Technologies and chair of the RISC-V Compliance Task Group explains:

RISC-V is an open-source standard ISA with exceptional modularity and extensibility. Anyone can build an implementation and there are no license fees, except for commercial use of the trademarks.

Implementers are free to add custom extensions to boost capabilities and performance, while at the same time don’t have to include features that aren’t needed

Unconstrained flexibility, however, can lead to incompatibilities that could fragment the customer base and eliminate the incentives for cooperative ecosystem development.

That defines the issue, especially RISC-V is not controlled by one or two companies in like Arm and x86 worlds. Multiple companies will be designing their own RISC-V products, so they’ll each have to make sure their implementation complies with the specification. That’s why the RISC-V Compliance Task Group is working on developing RISC-V compliance tests in order to define rules to ensure software compatibility.

RISC-V Compliance Framework
RISC-V Consortium Compliance Framework. Source: Esperanto

The RISC-V compliance framework is said to have three major elements

  1. A modular set of test suites that exercise all aspects of the ISA
  2. Golden reference signatures that define correct execution results
  3. Frameworks that select and configure appropriate test suites based on both platform requirements and claimed device capabilities

Running the tests will report fail/pass results for each test. The framework is only part of the verification process, as explained in the documentation:

The goal of compliance tests is to check whether the processor under development meets the open RISC-V standards or not. It is considered as non-functional testing meaning that it doesn’t substitute for design verification. This can be interpreted as testing to check all important aspects of the specification but without focusing on details, for example, on all possible values of instruction operands or all combinations of possible registers.

The result that compliance tests provide to the user is an assurance that the specification has been interpreted correctly and the design under test (DUT) can be declared as RISC-V compliant.

The tools are still work-in-progress, tests to be performed still need to be define, and will go beyond just ISA compliance, for example Tilelink cache-coherent interconnect fabric may have to be added to the compliance tests. Those tools will help making sure the processors compliant with basic RISC-V specifications, but it’s unclear how all RISC-V extensions that may be vendors specific will be handled, and some may even end up into the standard RISC-V compliance tests.

Thanks to Blu for the tip.

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32 Comments
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blu
blu
2 years ago

I’d go further and say that the one-image-runs-everywhere standardization in the x86 world was partially a result of the strictly-controlled ISA, but much more so of the fact the IBM AT prevailed as the dominant platform and was copied by everybody, providing a homogeneous basic device space for a kernel to work with. That was in the beginning, before things like ACPI matured. Also, keep in mind a kernel never goes full swing with an ISA — on the contrary, it tries to restrict itself to the most essential ISA subset that could get the job done. Most other ISAs,… Read more »

blu
blu
2 years ago

Usually the only automatic thing about SIMD versions are basic CRT routines like memcpy — everything else has to be deliberately accounted for in the code, either by manually providing versioned routines for runtime dispatch, or by shipping different-targets builds of the app, either way leading to an explosion of support options, if the developer is trying to be truly exhaustive. Which leads to most devs not bothering.

dgp
dgp
2 years ago

>either by manually providing versioned routines for runtime dispatch,

Or you do like Android did and provide a build system that can build multiple versions of the performance critical code into shared libraries and have the OS load the best version of the code at runtime.

willy
willy
2 years ago

In addition, most of the time when you care about a few cycles justifying to use SIMD instructions, you lose all your gains in the test and conditional jump required to check their presence!

dgp
dgp
2 years ago

>you lose all your gains in the test and conditional jump required to check their presence!

Why? You check once at startup and load your shared library/update your function pointer/patch your code once.

blu
blu
2 years ago

That still requires N x different versions of the shared library and N x the maintenance time.

willy
willy
2 years ago

Very often you only need one or two instructions in the middle of a loop which itself is in the middle of a more complex code, and having to have two version simply requires copy-paste of the function to exist using the two flavors. If you use high-level functions only (e.g. image compression) then I totally agree with you since in this case you share almost nothing and the various implementations are quite different. But that’s not always the case.

blu
blu
2 years ago

For our product we usually end up with different target builds for the several SIMD levels we support. We are trying to be maximally inclusive and it’s still a PITA.

dgp
dgp
2 years ago

>Also, keep in mind a kernel never goes full swing with an ISA — on the contrary,
>it tries to restrict itself to the most essential ISA subset that could get the job done.

Linux has SSE etc optimised code in many areas. Check your dmesg output.. even on ARM systems the kernel usually patches itself on boot to take advantage of instructions that not all ARM cores have.

blu
blu
2 years ago

Which are those many areas? memcpy, crypto/raid and?

dgp
dgp
2 years ago

>memcpy

Referenced in over 6000 files. Optimised versions of memset etc which have similar amounts of usage across the whole code base are in arch/x86/lib.

>crypto

Which is at least 20 different optimized cipher implementations for x86 alone.

> and?

perf is “isa aware”. There are optimised decompressors. Interrupt flow has some optimisation for different processor generations. The RNG stuff has support for processor specific RNG instructions AFAIK.

Due to how the asm includes work ISA specific code will be used all over the place.

blu
blu
2 years ago

In other words memcpy and crypto. Thanks.

dgp
dgp
2 years ago

and timers and other stuff you’ll ignore.

blu
blu
2 years ago

Ah yes, the ‘and stuff’ argument. As usual, you’re a source of wisdom.

dgp
dgp
2 years ago

oooo next you’ll be calling me kid again. Sorry I wondered on to your infinite wisdom lawn old man.

FYI just the perf subsystem in Linux totally invalidates what you wrote either way but if it makes you feel better being right you can have gold star from me. 😉

blu
blu
2 years ago

FYI, you switched your own argument from ‘SSE etc is used in many other areas of the kernel’ (hint: SSE etc SIMD isn’t used ‘in many other areas’, as the SIMD RF is subject to context-size kernel restrictions) to ‘bbbbut timers and perf are ISA aware!’ Sorry, kido, you don’t get any stars from me.

dgp
dgp
2 years ago

You: >Also, keep in mind a kernel never goes full swing with an ISA — on the contrary Me: >SSE etc On the SSE part: One example of the kernel going “full swing”. One example means it’s not never. On the contrary there are multiple implementations of multiple ciphers for different SIMD extensions. Then there are multiple implementations of XOR for different SIMD extensions. You might not consider them to be important or significant but they are there. Optimisations of things like memcpy, io access macros etc are important even if they are small because almost all of the rest… Read more »

blu
blu
2 years ago

> One example of the kernel going “full swing”. One example means it’s not never. Erm, a full-swing ISA would be every single extension of the ISA circa the day of application. So unless you can demonstrate how the kernel as of today uses every single amd64 extension (that extends way beyond AXV2 and AES-NI, and includes things like AVX512) — yes, strictly speaking a kernel _never_ goes full swing. For a bunch of reasons, some of which we’ll discuss below. But perhaps you could’ve asked for a clarification of ‘never goes full swing’ first? In which case I’d have… Read more »

dgp
dgp
2 years ago

“these goal posts were made for moving and that’s just what they’ll do”

theguyuk
theguyuk
2 years ago

I remember the IBM clone days and WinChip, Cyrix, Amd, intel etc days. Where software compatibility mattered.

The best way to stop fragmentation is simply to put a very high licence fee for non standard meeting designs, with no licence fee for those designs that meet the standard. Also only allow design that meet the standard the right to call their design a RISC-V chip.

blu
blu
2 years ago

You cannot be an all-inclusive free ISA *and* put a high license fee for custom designs — the two philosophies are mutually-exclusive. RISC-V have already chosen their path. Now they have to provide the ‘details’ and make sure the chosen path is viable for commercial hw and sw vendors alike. In this respect RV32G and RV64G are considered as good sets to standardize upon, but I think RV64GV would be even better..

jqpabc123
jqpabc123
2 years ago

You cannot be an ISA without some significant method of standardization.

Otherwise, you’re just providing a quicker path to proprietary, non-ISA designs.

willy
willy
2 years ago

No, it’s software availability which makes the ISA : when everything is available for a given ISA, if some extensions are not used, nobody will implement them because they become useless. And there is a high incentive for adopting what works out of the box. The BIOS services are what made PCs succeed. Here if a rich boot loader was provided by default for a platform, allowing easy boot from various devices and switching between images + easy image updates from the BL itself, it would certainly be widely adopted. If that BL required, say, crypto extensions to validate an… Read more »

jqpabc123
jqpabc123
2 years ago

I think your last sentence is the best approach.

If it’s possible to make changes, someone can and will do it — either for fun or for profit. Lots of people don’t/won’t care about compatibility.

The trademark is the only available method to offer compatibility to those who care about it.

blu
blu
2 years ago

AFAIK the RV Foundation already does, or plans to do the right-to-brand compliance testing. So it’s not about the lack of methods for standardization — yes, those normally take comprehensive compliance suits and golden standards. It’s more about *what* should be standardized. As of now RISC-V is a large combination of extensions, and certifying against some of those for the right to bear the RISC-V logo means little. Is an RV64I-plus-a-metric-ton-of-proprietary-extensions still an RV64I? What if some of those extensions are non-orthogonal to the base set (example: x87 vs scalar SSE)? How strongly is the G set intended as an… Read more »

ade
ade
2 years ago

I wish they could also standardize what needs to be done in term of platform definition to have “one kernel to rule them all” like in x86 (and unlike arm), e.g. guarantee that any compliant device will have onboard permanent >256kB storage to store the bootloader+DT at a defined address…

blu
blu
2 years ago

Indeed, DTs need to be there from the start, no need to repeat the mistakes of other ISA vendors.

Diego
Diego
2 years ago

As I had to learn in the ARM world DTBs are non standard. Every new Kernel, even every patch can require a new or at least adjusted DTB…
Then Linux chases the DTS through a whole plethora of preprocessors, you can’t just use DTC on those without the full fledged Linux build system.

dgp
dgp
2 years ago

>I wish they could also standardize what needs to be done in term of platform definition to have >“one kernel to rule them all” like in x86 (and unlike arm), For Cortex A and better you should be able to boot a single kernel on any machine. Getting Linux running to the point it’ll run busybox on a totally new Cortex A machine is just a case of a few lines in the kernel source to register a new machine type and a device tree that contains at least the uart, arch timer and gic. (Usually easier said than done..)… Read more »

jqpabc123
jqpabc123
2 years ago

In other words, open source hardware works the same as open source software.

How easy is it to build a significant app that will run on any version of Linux without modification?

The good thing about open source is that anyone can modify it. The bad thing about open source is that lots of people modify it. It turns out that the biggest selling point is also the biggest drawback.

dgp
dgp
2 years ago

You do like Windows apps do and ship everything with the application. Be that static linking, shipping all the shared libraries, providing a docker image etc. How you do that without violating all the different licenses is another question though.

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