From: Alain D D Williams Date: Wed, 19 Aug 2020 17:23:06 +0000 (+0100) Subject: Bring into sync X-Git-Url: https://git.libre-soc.org/?p=libresoc-isa-manual.git;a=commitdiff_plain;h=af5427eae9dba2cdbf3609e19cd619178b4dd5bb Bring into sync --- diff --git a/powerpc-add/.gitignore b/powerpc-add/.gitignore new file mode 100644 index 0000000..ea1472e --- /dev/null +++ b/powerpc-add/.gitignore @@ -0,0 +1 @@ +output/ diff --git a/powerpc-add/src/intro.tex b/powerpc-add/src/intro.tex new file mode 100644 index 0000000..2f98bcf --- /dev/null +++ b/powerpc-add/src/intro.tex @@ -0,0 +1,217 @@ +% +\chapter{Introduction} + +\section{Why has Libre-SOC chosen PowerPC ?} + +For a hybrid CPU-VPU-GPU, intended for mass-volume adoption in tablets, +netbooks, chromebooks and industrial embedded (SBC) systems, our choice was +between Nyuzi, MIAOW, RISC-V, PowerPC, MIPS and OpenRISC. + +Of all the options, the PowerPC architecture is more complete and far more +mature. It also has a deeper adoption by Linux distributions. + +Following IBM's release of the Power Architecture instruction set to the Linux +Foundation in August 2019 the barrier to using it is no more than that of using +RISC-V. We are encouraged that the OpenPOWER Foundation is supportive of what +we are doing and helping, e.g by putting us in touch with people who can help +us. + +\subsection{Summary} + +\vspace{-0.1in} +\begin{itemize} +\parskip 0pt +\itemsep 1pt + +\item + We propose the standardisation of the way that the PowerPC Instruction Set + Architecture (PPC ISA) is extended, enabling many different flavours within a + well supported family to co-exist, long-term, without conflict, right across + the board. + +\item + + This is about more than just our project. Our proposals will facilitate the + use of PPC in novel or niche applications without breaking the PPC ISA into + incompatible islands. + +\item + + PPC will gain a competitive market advantage by removing the need for + separate VPU or GPU functions in RTL or ASICs thus enabling lower cost + systems. Libre-SOC's project is to extend the PPC to integrate the GPU and + VPU functionality directly as part of the PPC ISA (example: Broadcom + VideoCore IV being based around extensions to an ARC core). + +\item + + Libre-SOC's extensions will be easily adopted, as the standard GNU/Linux + distributions will very deliberately run unmodified on our ISA, including + full compatibility with illegal instruction trap requirements. + +\end{itemize} + + +\subsection{One CPU multiple ISAs} + +This is a quick overview of the way that we would like to add changes that we +are proposing to the PowerPC instruction set (ISA). It is based on a Open +Standardisation of the way that existing "mode switches", already found in the +POWER instruction set, are added: + +\begin{itemize} +\parskip 0pt +\itemsep 1pt + +\item + + FPSCR's "NI" bit, setting non-IEEE754 FP mode + +\item + + MSR's "LE" bit (and associated HILE bit), setting little-endian mode + +\item + + MSR's "SF" bit, setting either 32-bit or 64-bit mode + +\item + + PCR's "compatibility" bits 60-62, V2.05 V2.06 V2.07 mode + +\end{itemize} + +[It is well-noted that unless each "mode switch" bit is set, any alternative +(additional) instructions (and functionality) are completely inaccessible, and +will result in "illegal instruction" traps being thrown. This is recognised as +being critically important.] + +These bits effectively create multiple, incompatible run-time switchable ISAs +within one CPU. They are selectable for the needs of the individual program (or +OS) being run. + +All of these bits are set by an instruction, that, once set, radically changes +the entire behaviour and characteristics of subsequent instructions. + +With these (and other) long-established precedents already in POWER, there is +therefore essentially conceptually nothing new about what we propose: we simply +seek that the process by which such "switching" is added is formalised and +standardised, such that we (and others, including IBM itself) have a clear, +well-defined standards-non-disruptive, atomic and non-intrusive path to extend +the POWER ISA for use in markets that it presently cannot enter. + +We advocate that some of "mode-setting" (escape-sequencing) bits be binary +encoded, some unary encoded, and that some space marked for "offical" use, some +"experimental", some "custom" and some "reserved". The available space in a +suitably-chosen SPR to be formalised, and recommend the OpenPOWER Foundation be +given the IANA-like role in atomically allocating mode bits. + +The IANA-like atomic role ensures that new PCR mode bits are allocated +world-wide unique. In combination with a mandatory illegal instruction +exception to be thrown on any system not supporting any given mode, the +opportunity exists for all systems to trap and emulate all other systems and +thus retain some semblance of interoperability. (Contrast this with either +allocating the same mode bit(s) to two (or more) designers, or not making +illegal exceptions mandatory: binary interoperability becomes unachievable and +the result is irrevocable damage to POWER's reputation.) + +We also advocate to consider reserving some bits as a "countdown" where the new +mode will be enabled only for a certain number of instructions. This avoids an +explicit need to "flip back", reducing binary code size. Note that it is not a +good idea to let the counter cross a branch or other change in PC (and to throw +illegal instruction trap if attempted). However traps and exceptions themselves +will need to save (and restore) the countdown, just as the rest of the PCR and +other modeswitching bits need to be saved. + +Instructions that we need to add, which are a normal part of GPUs, include +ATAN2, LOG, NORMALISE, YUV2RGB, Khronos Compliance FP mode (different from both +IEEE754 and "NI" mode), and many more. Many of these may turn out to be useful +in a wider context: they however need to be fully isolated behind +"mode-setting" before being in any way considered for Standards-track formal +adoption. + +Some mode-setting instructions are privileged, i.e can only be set by the +kernel (e.g 32 or 64 bit mode). Most of the escape sequences that we propose +will be (have to be) usable without the need for an expensive system call +overhead (because some of the instructions needed will be in extremely tight +inner loops). + +\subsection{About Libre-SOC Commercial Project} + +The Libre-SOC Commercial Product is a hybrid GPU-GPU-VPU intended for +mass-volume production. There is no separate GPU, because the CPU is the GPU. +There is no separate VPU, because the CPU is the GPU. There is not even a +separate pipeline: the CPU pipelines are the GPU and VPU pipelines. + +Closest equivalents include the ARC core (which has VPU extensions and 3D +extensions in the form of Broadcom's VideoCore IV) and the ICubeCorp IC3128. +Both are considered "hybrid" CPU-GPU-VPU processors. + +"Normal" Commercial GPUs are entirely separate processors. The development cost +and complexity purely in terms of Software Drivers alone is immense. We reject +that approach (and as a small team we do not have the resources anyway). + +With the project being Libre - not proprietary and secretive and never to be +published, ever - it is no good having the extensions as "custom" because +"custom" is specifically for the cases where the augmented toolchain is never, +under any circumstances, published and made public by the proprietary company +(and would never be accepted upstream anyway). For business commercial reasons, +Libre-SOC is the total opposite of this proprietary, secretive approach. + +Therefore, to meet our business objectives: + +\begin{itemize} +\parskip 0pt +\itemsep 1pt + +\item + + As shown from Nyuzi and Larrabee, although ideally suited to high + performance compute tasks, a "traditional" general-purpose full + IEEE754-compliant Vector ISA (such as that in POWER9) is not an adequate + basis for a commercially competitive GPU. Nyuzi's conclusion is that using + such general-purpose Vector ISAs results in reaching only 25% performance + (or requiring 4-fold increase in power consumption) to achieve par with + current commercial-grade GPUs. + +\item + + We are not going the "traditional" (separate custom GPU) route because it + is not practical for a new team to design hardware and spend 8+ man-years + on massively complex inter-processor driver development as well + +\item + + We cannot meet our objectives with a "custom extension" because the + financial burden on our team to maintain a total hard fork of not just + toolchains, but also entire GNU/Linux Distros, is highly undesirable, and + completely impractical (we know for certain that Redhat would strongly + object to any efforts to hard-fork Fedora) + +\item + + We could invent our own custom GPU instruction set (or use and extend an + existing one, to save a man-decade on toolchain development) however even + to switch over to that "Dual ISA" GPU instruction set in the next clock + cycle still requires a PCR modeswitch bit in order to avoid needing a full + Inter-Processor Bus Architecture like on "traditional" GPUs. + +\item + + If extending any instruction set, rather than have a Dual ISA (which needs + the PCR modeswitch bit to access it) we would rather extend POWER. + +\item + + We cannot "go ahead anyway" because to do so would be highly irresponsible + and cause massive disruption to the POWER community. + +\end{itemize} + +With all impractical options eliminated the only remaining responsible option +is to extend the POWER ISA in an atomically-managed (IANA-style) formal +fashion, whilst (critically and absolutely essentially) always providing a PCR +compatibility mode that is fully POWER compliant, including all illegal +instruction traps. + + diff --git a/powerpc-add/src/power-spec.bib b/powerpc-add/src/power-spec.bib new file mode 100644 index 0000000..cbdab86 --- /dev/null +++ b/powerpc-add/src/power-spec.bib @@ -0,0 +1,6 @@ +@Misc{foo-bar, + title = "A dummy bib entry to keep LaTeX happy", + publisher = {"Libre-SOC"}, + year = 2020 +} + diff --git a/powerpc-add/src/preface.tex b/powerpc-add/src/preface.tex new file mode 100644 index 0000000..baa330d --- /dev/null +++ b/powerpc-add/src/preface.tex @@ -0,0 +1,5 @@ +\chapter{Preface} + +This document describes the Libre-SOC ISAMUX additions to the PowerPC architecture. + +\cite{foo-bar}