Examining the Top Five Fallacies About RISC-V By David Patterson 12.13.2022

轩辕志瑜

原文：
In a little over a decade, RISC-V has arguably become at least the third most important instruction set architecture (ISA) for future applications of computing. In the next few years, it may become just as surprising to pick a proprietary ISA over the open RISC-V for a new project as it would be to pick a closed alternative to Ethernet or USB.

My colleagues at UC Berkeley and I predict that by the end of this decade, the dominant ISA for future product development will be the open RISC-V architecture. Companies around the world are already designing with RISC-V and the momentum is rapidly increasing, so this is a good time for the industry to take a closer look at RISC-V and examine some fallacies about it.

Fallacy No. 1: RISC-V is an open-source processor, like Linux is an open-source operating system.
Linux has a single-master open-source code base you can download, while RISC-V is an open specification of the hardware/software interface, for which there are many different implementations. A better analogy than Linux is Ethernet, as both Ethernet and RISC-V are free and open specifications.

Before the Ethernet standard, companies had their own proprietary local area networks. In 1980, Digital Equipment Corporation, Intel, and Xerox (DIX) joined forces to create a local network standard based on Ethernet. They also created an organization — IEEE 802.3 working group — that has advanced the Ethernet standard over the past four decades. Ethernet made rapid advances in cost and performance because many companies could build network products that ran the same software stack on top of the Ethernet standard.

The popular Universal Serial Bus (USB) also followed the Ethernet game plan by providing a free and open standard for peripheral interconnect that is embraced by many companies plus an organization to evolve it.

Like Ethernet and USB, RISC-V is an open standard (which is also run by a foundation) that lets many organizations design hardware, which fosters competition to improve its cost performance and develop a rich, shared software ecosystem that offers RISC-V products in many markets. Like Ethernet and USB, you can buy RISC-V hardware, build it yourself, license designs, or download open-source designs.

Fallacy No. 2: Picking an established, closed ISA is a safer business decision than picking the open RISC-V.
It’s easy to forget that a closed ISA is tied to the success of the company that owns it, and it can disappear if the company falters. For example, the once-popular DEC VAX, DEC Alpha, and Sun SPARC ISAs are extinct.

It’s also hard to remember that closed ISAs are intellectual property that can be sold to companies with different goals than its predecessors. For example, the MIPS ISA has had more than a half-dozen owners, and so far, the Arm ISA has had three: Acorn, ARM Holdings plc, and Softbank. By comparison, RISC-V is driven by the collective participation of hundreds of companies in a neutral open-standard organization, RISC-V International. Their collective interests determine the evolution of RISC-V through this nonprofit foundation.

Like Ethernet and USB, RISC-V is not tied to the fortunes of any one company, so it is a more prudent bet for a company’s software ecosystem development for the long haul.

Fallacy No. 3: Closed ISAs do not have fragmented software ecosystems.
Older closed ISAs have suffered from unforeseen incompatibilities over their long lifetimes. Examples include:

Despite trying to share the x86-64 ISA, AMD and Intel require different virtual machines.
Intel AVX-512 is significantly fragmented (e.g., the ML floating-point format BF16 comes and goes).
ARMv1 through ARMv7 use a 32-bit address space but are incompatible with ARMv8-A and successors, which offer both 32- and 64-bit address versions. ARMv8-M adds new features to the older 32-bit ISA but is incompatible with ARMv8-A.
No software environment is more fragmented than today’s system-on-chip (SoC) for edge devices. They include many incompatible ISAs and software stacks for the many types and brands of processors (application CPUs, embedded CPUs, DSPs, ML accelerators, and ISPs). One reason is because these processors use closed ISAs that cannot be used for third-party IP, so each processor block has its own ISA.

Fallacy No. 4: RISC-V’s modularity leads to a more fragmented software ecosystem than those of closed ISAs.
This fallacy has been raised since my colleagues and I began advocating for RISC-V, so it’s not been neglected. Some market segments require a stable ISA and even binary compatibility, which RISC-V addresses with profiles. They specify a set of ISA choices from the standard extensions that capture the most value for most users in a market, enabling the software community to focus resources on building an appropriate software ecosystem. Similarly, hardware vendors structure their offerings around standard profiles to ensure their designs will have mainstream software support. For example, RISC-V offers them for 64-bit address UNIX systems. Profiles are the foundation upon which portable apps and OSes can be built.

Beyond profiles, the RISC-V ISA offers the exciting possibility of a common base ISA with custom enhancements and a shared software stack across the many processors of an SoC. RISC-V potentially could dramatically reduce the fragmentation of today’s SoC software ecosystems.

Fallacy No. 5: Given the points above, RISC-V cannot become the dominant ISA.
As long as there are both 32-bit and 64-bit address versions, there is no technical disagreement that a single base ISA could be used everywhere from embedded systems to supercomputers; the main argument is a business one, of whether it should be a closed ISA or an open ISA. If we do achieve a lingua franca for computing, it seems self-evident that it would be too dangerous for the fate of the entire information technology industry to be tied to the fortunes of a single company. It would be much safer if we could instead depend upon a free and open standard, just as we did for networking and peripheral interconnect.

译文：（译文来自IC大家谈公众号， 估计用的是机器翻译。 已校正一些翻译）

在十多年的时间里，RISC-V 可以说至少已成为未来计算应用程序中第三重要的指令集架构 (ISA)。在接下来的几年里，为新项目选择专有 ISA 而不是开放式 RISC-V 可能会像选择以太网或 USB 的封闭替代方案一样令人惊讶。

我和我在加州大学伯克利分校的同事预测，到本世纪末，未来产品开发的主导 ISA 将是开放的 RISC-V 架构。世界各地的公司已经在使用 RISC-V 进行设计，而且势头正在迅速增强，因此现在是业界仔细研究 RISC-V 并检查一些关于它的谬误的好时机。

谬误一：RISC-V 是开源处理器，就像 Linux 是开源操作系统一样。

Linux是有单一主分支的开源代码库，谁都可以下载。而 RISC-V 是一个开放的硬件/软件接口规范，有许多不同的实现。比 Linux 更好的类比是以太网，因为以太网和 RISC-V 都是免费和开放的规范。

在以太网标准出现之前，每个公司都拥有自己专有的局域网。1980 年，Digital Equipment Corporation、Intel 和 Xerox (DIX) 联手创建了基于以太网的本地网络标准。他们还创建了一个组织——IEEE 802.3 工作组——该工作组在过去四十年中推动了以太网标准的发展。以太网在成本和性能方面取得了快速进步，因为许多公司可以构建在以太网标准之上运行相同软件堆栈的网络产品。

流行的通用串行总线 (USB) 也遵循以太网游戏计划，为外围设备互连提供免费和开放的标准，许多公司和组织都接受该标准以对其进行改进。

与以太网和 USB 一样，RISC-V 是一种开放标准（也由基金会管理），允许许多组织设计硬件，从而促进竞争以提高其性价比并开发提供 RISC-V 产品的丰富的共享软件生态系统在许多市场。与以太网和 USB 一样，您可以购买 RISC-V 硬件、自行构建、许可设计或下载开源设计。

谬误二：选择成熟的封闭式 ISA 比选择开放式 RISC-V 更安全。

有一个事我们很容易忘记，那就是封闭的 ISA 与拥有它的公司的成功息息相关，如果公司步履蹒跚，它就会消失。例如，曾经流行的 DEC VAX、DEC Alpha 和 Sun SPARC ISA 已经绝迹。

另一个很难让人记住的事情是——封闭式 ISA 是一种知识产权，可以出售给目标与其前身不同的公司。例如，MIPS ISA 拥有超过六家所有者，到目前为止，Arm ISA 拥有三个东家：Acorn、ARM Holdings plc 和 Softbank。相比之下，RISC-V 是由数百家公司共同参与的中立开放标准组织 RISC-V International 推动的。他们的集体利益通过这个非营利基金会决定了 RISC-V 的发展。

与以太网和 USB 一样，RISC-V 不与任何一家公司的命运挂钩，因此对于公司软件生态系统的长期发展来说，这是一种更为谨慎的选择。

谬误三：封闭的 ISA 没有分散的软件生态系统。

较旧的封闭式 ISA 在其较长的生命周期中遭受了无法预料的不兼容性。例子包括：

• 尽管试图共享 x86-64 ISA，但 AMD 和 Intel 需要不同的虚拟机。
• 英特尔 AVX-512 非常分散（例如，ML 浮点格式 BF16 来来去去）。
• ARMv1 到 ARMv7 使用 32 位地址空间，但与提供 32 位和 64 位地址版本的 ARMv8-A 和后续版本不兼容。ARMv8-M 为旧的 32 位 ISA 添加了新功能，但与 ARMv8-A 不兼容。
  
  

没有比当今边缘设备的片上系统 (SoC) 更分散的软件环境了。它们包括许多不兼容的 ISA 和软件堆栈，适用于多种类型和品牌的处理器（应用 CPU、嵌入式 CPU、DSP、ML 加速器和 ISP）。一个原因是因为这些处理器使用不能用于第三方 IP 的封闭式 ISA，因此每个处理器块都有自己的 ISA。

谬误四：相比封闭式 ISA， RISC-V 的模块化导致的软件生态系统更加分散。

自从我和我的同事开始提倡 RISC-V以来，这个谬论就一直存在，所以它并没有被忽视。一些细分市场需要稳定的 ISA 甚至二进制兼容性，RISC-V 通过配置文件解决了这些问题。他们从标准扩展中指定了一组 ISA 选择，为市场上的大多数用户获取最大价值，使软件社区能够将资源集中在构建合适的软件生态系统上。同样，硬件供应商围绕标准配置文件构建他们的产品，以确保他们的设计将获得主流软件支持。例如，RISC-V 为 64 位地址的 UNIX 系统提供它们。配置文件是构建便携式应用程序和操作系统的基础。

除了配置文件之外，RISC-V ISA 还提供了令人兴奋的可能性，即具有自定义增强功能的通用基础 ISA 和跨 SoC 的许多处理器的共享软件堆栈。RISC-V 可能会显著减少当今 SoC 软件生态系统的碎片化。

谬误五：鉴于以上几点，RISC-V 不可能成为主导 ISA。

只要同时存在 32 位和 64 位地址版本，就可以在从嵌入式系统到超级计算机的任何地方使用单一基础 ISA，这在技术上没有分歧；主要争论是商业争论，即它应该是封闭式 ISA 还是开放式 ISA。如果我们真的实现了计算的通用语言，那么不言而喻的是，将整个信息技术行业的命运与一家公司的命运联系在一起就太危险了。如果我们可以依赖一个免费和开放的标准，就像我们对网络和外围互连所做的那样，那将会更加安全。