Add terminology and boundaries RFC

text/0000-terminology-and-boundaries.md

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+- Feature Name: Terminology and Boundary Definition
+- Start Date: 2018-10-11
+- RFC PR: (leave this empty)
+- Nebulet Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+The Nebulet operating system, being so different from popular operating systems
+of today, will require new terminology to refer to abstraction boundaries.
+Terms such as "userspace" and "kernelspace" will carry too much baggage from
+existing OSes, which is likely to confuse potential contributors. This RFC
+proposes we define the following terms to refer to parts of our OS, adapted
+from [EROS](https://web.archive.org/web/20160412201504/http://www.coyotos.org/docs/misc/eros-structure.pdf):
+the _kernel_, which implements threading and inter-proces communication (IPC)
+as a wasm module, the _nucleus_, which implements a small wasm JIT natively,
+and the _firmament_, which specifies IPC interfaces for drivers to implement.
+Applications will be implemented atop these abstractions.
+
+# Motivation
+[motivation]: #motivation
+
+Today's common operating systems generally use hardware-based isolation
+approaches, such as protection rings on x86 or processor modes on ARM, to
+protect the kernel's memory from tampering. In colloquial operating systems
+discussion, the phrases "kernelspace" and "ring 0" have become synonymous,
+as have the phrases "userspace" and "ring 3". As Nebulet uses software-based
+isolation and not protection rings, these terms will be unfortunately
+overloaded, confusing potential contributors. The world of microkernel-based
+research OSes has provided some alternative terminology, some of which is
+potentially useful here.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+The Nebulet operating system architecture is defined in terms of four
+components:
+
+- The _kernel_, which implements the core functionality of a microkernel OS:
+  thread/process scheduling and isolation, and inter-process communication
+  (IPC). These two pieces of functionality will be further specified in future
+  RFCs.
+- The _nucleus_, which implements a small WebAssembly runtime in native code. A
+  nucleus will implement the minimal set of hardware interfaces required by the
+  kernel and will likely resemble an exokernel on its surface. A nucleus need not
+  be implemented in "ring 0", it could be implemented as a Linux userspace
+  program.
+- The _firmament_, which specifies interfaces for hardware drivers. Drivers in
+  Nebulet, like in any microkernel OS, are just processes, which means that a
+  sufficiently flexible operating system will require standardized driver
+  interfaces so as to make applications hardware-independent.
+- _Applications_ are software compiled to WebAssembly that the user runs.
+
+**Example 1:** a [UTM](https://en.wikipedia.org/wiki/Unified_threat_management)
+appliance running on an Atheros SoC based on Nebulet might use:
+
+- A nucleus compiled for ARM, possibly making use of the SoC's secure boot chip
+  to implement a tamper-resistant boot.
+- The official Nebulet kernel.
+- Ethernet drivers that implement the Data Plane firmament specification.
+- A cryptographic accelerator driver that uses a custom cryptography chip on
+  the SoC, that implements the Cryptography firmament specification.
+- A firewall implemented as a Nebulet application that uses the Data Plane
+  firmament IPC interfaces.
+- A VPN implemented as a Nebulet application that uses both the Cryptography and
+  Data Plane firmament IPC interfaces.
+
+**Example 2:** an x86-64 laptop based on Nebulet might use:
+
+- A nucleus compiled for x86-64, with some integration with UEFI Secure Boot.
+- The official Nebulet kernel.
+- Graphics drivers that implement the GPU firmament specification.
+- Wi-Fi drivers that implement the Data Plane firmament specification.
+- An implementation of Layer 3+ networking, standardized as the Sockets
+  firmament specification.
+- A Vulkan library that uses the GPU firmament specification.
+- A desktop environment implemented as a Nebulet application that statically
+  links the Vulkan library into its wasm module. The desktop environment
+  provides an IPC protocol for opening windows and drawing widgets. This
+  particular protocol is not part of the firmament, as it is not relevant to a
+  hardware driver.
+- Firefox, which uses the desktop environment's IPC interface to create a window
+  onscreen, statically links to the Vulkan library for rendering, and uses the
+  Sockets firmament specification.
+
+**Example 3:** a cluster of [TALOS II 2U servers](https://www.raptorcs.com/content/TL2SV1/intro.html)
+being used as a machine learning laboratory might use:
+
+- A nucleus compiled for POWER9, that integrates with OpenBMC.
+- The official Nebulet kernel.
+- NVIDIA graphics drivers that implement the GPGPU firmament specification.
+- Ethernet drivers that implement the Data Plane firmament specification.
+- An implementation of the Sockets firmament.
+- TensorFlow for clusters ported to a Nebulet application that uses the GPGPU
+  and Socket firmaments.
+- Jupyter and the entire SciPy stack (lol) ported to Nebulet, using the Sockets
+  firmament and many ad-hoc application-layer IPC interfaces.
+
+Note that in all examples, we can re-use the Nebulet kernel and firmament
+interfaces.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+The nucleus must provide a thin set of abstractions over the hardware, in
+particular the CPU. These abstractions must include:
+- Access and control over CPU power states, if applicable.
+- Access and control over CPU interrupts, if applicable.
+- Access and control of the MMU, if applicable.
+- Access and control over CPU registers if necessary for common drivers on
+  the target hardware.
+
+The nucleus can also implement part of a verified boot chain if such a feature
+is desirable. In this case, the nucleus will be verified by the bootloader and
+it will verify the Nebulet kernel and other startup data, such as initial
+ramdisk or commandline.
+
+The Nebulet kernel will use these functions to implement the core functionality
+of threading, scheduling, process isolation, and inter-process communication.
+
+A firmament specification will describe the IPC interfaces and datatypes that
+an application using that piece of hardware will interact with. The Data Plane
+firmament specification used in the above examples might be defined as follows
+(using Rust-like syntax):
+
+```rust
+// The supporting structs and enums are assumed to have
+// functions that can be called easily from the receiver side.
+// We'll need to flesh out the IPC system for this to be in any
+// way realistic.
+struct MacAddr([u8; 6]);
+enum Ethertype {
+    Arp,
+    Ipv4,
+    Ipv6,
+}
+
+struct EthernetPacket {
+    data: Bytes,
+}
+
+impl EthernetPacket {
+    pub fn dst(&self) -> &MacAddr;
+    pub fn src(&self) -> &MacAddr;
+    pub fn ethertype(&self) -> &Ethertype;
+    pub fn next_layer(&self) -> &L3Packet;
+}
+
+interface EthernetNic {
+    fn send(&mut self, pkt: EthernetPacket) -> Result<()>;
+    fn send_inplace(&mut self, func: impl Fn(&mut EthernetPacket) -> Result<()>) -> Result<()>;
+    fn recv(&mut self) -> Future<Result<EthernetPacket>>;
+}
+```
+
+An interface is implemented by the driver side and used by the application.
+Firmament interfaces could be provided to the application on initialization,
+like in [CloudABI](https://cloudabi.org), or they could be dynamically queried,
+depending on the security model, which is yet to be specified.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Implementing all drivers as WebAssembly applications may not be able to provide
+maximum performance along the critical path of an application, in particular,
+networking- and GPU-intensive applications may find such performance limits to
+be problematic.
+
+Specifying all classes of hardware as firmament drivers will be a tall order,
+depending on how wide interest in Nebulet is. The RFC process should allow for
+upstreaming the most useful ones in the community, though, as Nebulet becomes
+used in domains the original designers did not anticipate.
+
+The terminology chosen is rather alien, and not typically used in the context of
+operating systems research and development. This may confuse seasoned operating
+systems developers who could be valuable assets to this project.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+- **Why is this design the best in the space of possible designs?** This design
+  introduces architectural boundaries that are sensible for a unique operating
+  system such as Nebulet.
+- **What other designs have been considered and what is the rationale for not
+  choosing them?**
+  - A _monolithic kernel_ design was not chosen because this niche is already
+    filled extremely well by existing OSes such as Linux. Running WebAssembly
+    in kernel space on Linux is actively being pursued through the
+    [WasmJIT](https://github.com/rianhunter/wasmjit) project.
+- **What is the impact of not doing this?** Architectural boundaries would be
+  allowed to emerge organically. This will likely result in an overspecialized
+  operating system based on its initial applications, or a poorly-documented
+  set of semi-specified, semi-organic interfaces, both limiting code re-use and
+  diluting the value proposition of Nebulet.
+
+# Prior art
+[prior-art]: #prior-art
+
+This terminology and boundary specification was mostly inspired by
+[EROS](https://web.archive.org/web/20160412201504/http://www.coyotos.org/docs/misc/eros-structure.pdf).
+Some other microkernel operating systems, such as [Robigalia](https://robigalia.org),
+have chosen to adopt similar terminology.
+
+The concept of an operating system kernel making use of a JIT-compiled environment
+dates back to the mainframe days, with IBM mainframes including
+hardware-assisted JIT compilation, which enabled fast, architecture-independent
+applications. More recently, Microsoft Research has experimented with
+Singularity, an operating system using the .NET virtual machine. Likewise, Sun
+Microsystems developed a Java-based operating system in the late 90s.
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+- How do we handle cases where pure WebAssembly drivers aren't fast enough?
+  - Should we allow native drivers to be loaded into the nucleus?
+  - Should we allow native drivers to be statically compiled into the nucleus?
+    If so, does the nucleus also implement the firmament interfaces?
+  - Can we offer (possibly privileged) nucleus interfaces that mitigate this in
+    common cases?
+- What should the process of standardizing firmament interfaces look like? Can
+  we re-use the RFC process? Can we make templates for firmament RFCs?
+- How does this interact with our IPC system and security model? Both must be
+  specified independently of this RFC.
+- How do applications acquire handles to firmament functions?