[Bpf] [PATCH] bpf, docs: Add additional ABI working draft base text

[Will Hawkins <[e-mail]>]

Per David's description of the IETF standardization process, this
document will form the basis for an informational eBPF ABI. The
work in this commit is a slightly more complete skeleton for the work
that we will do. Everything in this document (from formatting to topics
to details) is open for change and feedback.
---
 Documentation/bpf/standardization/abi.rst | 237 ++++++++++++++++++++--
 1 file changed, 225 insertions(+), 12 deletions(-)

diff --git a/Documentation/bpf/standardization/abi.rst b/Documentation/bpf/standardization/abi.rst
index 0c2e10eeb89a..cadbd1b4d6b3 100644
--- a/Documentation/bpf/standardization/abi.rst
+++ b/Documentation/bpf/standardization/abi.rst
@@ -1,25 +1,238 @@
-.. contents::
-.. sectnum::
-
 ===================================================
 BPF ABI Recommended Conventions and Guidelines v1.0
 ===================================================
 
-This is version 1.0 of an informational document containing recommended
-conventions and guidelines for producing portable BPF program binaries.
+An application binary interface (ABI) defines the requirements that one or more binary software
+objects must meet in order to guarantee that they can interoperate and/or use the resources provided
+by operating systems/hardware combinations.  (For alternate definitions of ABI, see
+[SYSVABI]_, [POWERPCABI]_)
+
+The purpose of this document is to define an ABI which will define the extent to which compiled
+eBPF programs are compatible with each other and the eBPF machine/processor [#]_ on which they
+are executing.
+
+The ABI is specified in two parts: a generic part and a processor-specific part.
+A pairing of of generic ABI with the processor-specific ABI for a certain instantiation
+of an eBPF machine represents a complete binary interface for eBPF programs executing
+on that machine.
+
+This document is a generic ABI and specifies the parameters and behavior common to all
+instantiations of eBPF machines. In addition, it defines the details that must be specified by each
+processor-specific ABI. 
+
+These psABIs are the second part of the ABI. Each instantiation of an eBPF machine must
+describe the mechanism through which binary interface compatibility is maintained with
+respect to the issues highlighted by this document. However, the details that must be
+defined by a psABI are a minimum -- a psABI may specify additional requirements for binary
+interface compatibility on a platform.
+
+.. contents::
+.. sectnum::
+
+How To Use This ABI
+===================
+
+Conformance
+===========
+
+Note: Red Hat specifies different levels of conformance over time [RHELABI]_.
+
+Related Work
+============
+eBPF programs are not unique for the way that they operate on a virtualized machine and processor.
+There are many programming languages that compile to an ISA that is specific to a virtual machine.
+Like the specification presented herein, those languages and virtual machines also have ABIs.
+
+For example, the Go programming language and the runtime included statically with each program compiled
+from Go source code have a defined ABI [GOABI]_. Java programs compiled to bytecode follow a well-defined
+ABI for interoperability with other compiled Java programs and libraries [JAVAABI]_. Programs compiled to
+bytecode for execution as user applications on the Android operating system (OS) adhere to a bytecode
+specification that shares much in common with an ABI [DALVIKABI]_. Finally, the Common Language Runtime (CLR)
+designed to execute programs compiled to the Microsoft Intermediate Language (MSIL) has a fully specified
+ABI [CLRABI]_.
+
+Vocabulary
+==========
+
+#. Program: An eBPF Program is a self-contained set of eBPF instructions that execute
+   on an eBPF processor.
+#. Program Type: Every eBPF program has an associated type. The program type defines, among other things,
+   a program's possible attach types.
+#. Attach Type: An attach type defines the set of BPF hook points to which an eBPF
+   program can attach.
+#. BPF Hook Points: Places in a BPF-enabled component (e.g., the Linux Kernel, the Windows kernel) where
+   an eBPF program may be attached.
+#. eBPF Machine Instantiation: 
+#. ABI-conforming system: A computer system that provides the binary sys- tem interface for application
+   programs described in the System V ABI [SYSVABI]_.
+#. ABI-conforming program: A program written to include only the system routines, commands, and other
+   resources included in the ABI, and a program compiled into an executable file that has the formats
+   and characteristics specified for such files in the ABI, and a program whose behavior complies with
+   the rules given in the ABI [SYSVABI]_.
+#. ABI-nonconforming program: A program which has been written to include system routines, commands, or
+   other resources not included in the ABI, or a program which has been compiled into a format different
+   from those specified in the ABI, or a program which does not behave as specified in the ABI [SYSVABI]_.
+#. Undefined Behavior: Behavior that may vary from instance to instance or may change at some time in the future.
+   Some undesirable programming practices are marked in the ABI as yielding undefined behavior [SYSVABI]_. 
+#. Unspecified Property: A property of an entity that is not explicitly included or referenced in this specification,
+   and may change at some time in the future. In general, it is not good practice to make a program depend
+   on an unspecified property [SYSVABI]_.
+
+Program Execution Environment
+=============================
+
+A loaded eBPF program is executed on an eBPF machine. That machine, physical or virtual, runs in a freestanding
+or hosted environment [#]_.
+
+eBPF Machine Freestanding Environment
+-------------------------------------
+
+
+eBPF Machine Hosted Environment
+-------------------------------
+
+A loaded eBPF program can be attached to a BPF hook point in a BPF-enabled application
+compatible with the attach type of its program type. When the BPF-enabled application's
+execution reaches a BPF hook point to which an eBPF program is attached, that program
+begins execution on the eBPF machine at its first instruction. The contents of eBPF machine's
+registers and memory at the time it starts execution are defined by the eBPF program's
+type and attach point.
+
+Processor Architecture
+======================
+
+This section describes the processor architecture available
+to programs. It also defines the reference language data types, giving the
+foundation for system interface specifications [SYSVABI]_
+
+Registers
+---------
+
+General Purpose Registers
+^^^^^^^^^^^^^^^^^^^^^^^^^
+eBPF has 11 64-bit wide registers, `r0` - `r10`. The contents of the registers
+at the beginning of an eBPF program's execution depend on the program's type.
+
+Frame Pointer Register
+^^^^^^^^^^^^^^^^^^^^^^
+The use of a frame pointer by programs is not required. If, however, an eBPF program
+does use a frame pointer, it must be stored in register `r10`.
+
+Data Types
+----------
 
-Registers and calling convention
-================================
+Numeric Types
+^^^^^^^^^^^^^
 
-BPF has 10 general purpose registers and a read-only frame pointer register,
-all of which are 64-bits wide.
+The eBPF machine supports 32- and 64-bit signed and unsigned integers. It does 
+not support floating-point data types. All signed integers are represented in
+twos-complement format where the sign bit is stored in the most-significant
+bit.
 
-The BPF calling convention is defined as:
+Pointers
+^^^^^^^^
 
-* R0: return value from function calls, and exit value for BPF programs
+Function Calling Sequence
+=========================
+This section defines the standard function calling sequence in a way that
+accommodates exceptions, stack management, register (non)volatility, and access
+to capabilities of the hosting environment (where applicable).
+
+Functions in eBPF may define between 0 and 5 parameters. Each of the arguments in
+a function call are passed in registers.
+
+The eBPF calling convention is defined as:
+
+* R0: return value from function calls, and exit value for eBPF programs
 * R1 - R5: arguments for function calls
 * R6 - R9: callee saved registers that function calls will preserve
 * R10: read-only frame pointer to access stack
 
-R0 - R5 are scratch registers and BPF programs needs to spill/fill them if
+R0 - R5 are scratch registers and eBPF programs needs to spill/fill them if
 necessary across calls.
+
+Every function invocation proceeds as if it has exclusive access to an
+implementation-defined amount of stack space. R10 is a pointer to the byte of
+memory with the highest address in that stack space. The contents
+of a function invocation's stack space do not persist between invocations.
+
+**TODO** Discuss manufactured prologue and epilogue. Take language from the design FAQ.
+
+Execution Environment Interface
+===============================
+
+When an eBPF program executes in a hosted environment, the hosted environment may make
+available to eBPF programs certain capabilities. This section describes those capabilities
+and the mechanism for accessing them.
+
+
+Program Execution
+=================
+
+Program Return Values
+---------------------
+
+**NOTE** libbpf currently defines the return value of an ebpf program as a 32-bit unsigned integer.
+
+Program Loading and Dynamic Linking
+-----------------------------------
+This section describes the object file information and system actions that create
+running programs. Some information here applies to all systems; information specific
+to one processor resides in sections marked accordingly [SYSVABI]_.
+
+eBPF programs saved in ELF files must be loaded from storage and properly configured before
+they can be executed on an eBPF machine. 
+
+Program Loading (Processor-Specific)
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Dynamic Linking
+^^^^^^^^^^^^^^^
+
+Global Offset Table (Processor-Specific)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Procedure Linkage Table (Processor-Specific)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Exception Handling
+==================
+
+eBPF Program Types
+==================
+**NOTE** This information may end up as a subsection somewhere else.
+
+eBPF Maps
+=========
+**NOTE** This information may end up as a subsection somewhere else.
+
+System Calls
+============
+
+**TODO**
+
+C Programming Language Support
+==============================
+
+**NOTE** This section could be included in order to define the contents
+of standardized processor-specific header files that would make it easier
+for programmers to write programs.
+
+Notes
+=====
+.. [#] The eBPF machine does not need to be a physical instantiation of a processor. In fact, many instantiations of eBPF machines are virtual.
+.. [#] See the [CSTD]_ for the inspiration for this distinction.
+
+References
+==========
+
+.. [SYSVABI] System V Application Binary Interface - Edition 4.1. SCO Developer Specs. The Santa Cruz Operation. 1997. https://www.sco.com/developers/devspecs/gabi41.pdf
+.. [POWERPCABI] Developing PowerPC Embedded Application Binary Interface (EABI) Compliant Programs. PowerPC Embedded Processors Application Note. IBM. 1998. http://class.ece.iastate.edu/arun/Cpre381_Sp06/lab/labw12a/eabi_app.pdf
+.. [GOABI] Go internal ABI specification. Go Source Code. No authors. 2023. https://go.googlesource.com/go/+/refs/heads/master/src/cmd/compile/abi-internal.md
+.. [JAVAABI] The Java (r) Language Specification - Java SE 7 Edition. Gosling, James et. al. Oracle. 2013. https://docs.oracle.com/javase/specs/jls/se7/html/index.html
+.. [DALVIKABI] Dalvik Bytecode. Android Core Runtime Documentation. No authors. Google. 2022. https://source.android.com/docs/core/runtime/dalvik-bytecode
+.. [CLRABI] CLR ABI. The Book of the Runtime. No authors. Microsoft. 2023. https://github.com/dotnet/coreclr/blob/master/Documentation/botr/clr-abi.md. 
+.. [CSTD] International Standard: Programming Languages - C. ISO/IEC. 2018. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2310.pdf.
+.. [RHELABI] Red Hat Enterprise Linux 8: Application Compatibility Guide. Red Hat. 2023. https://access.redhat.com/articles/rhel8-abi-compatibility 
+
+
-- 
2.41.0