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Document that modules must be WASI modules (#329) #402

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15 changes: 15 additions & 0 deletions docs/WASI.md
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SLEdge only implemented a subset of the WASI syscall interface

## Module Requirements and Assumptions

SLEdge assumes that every module it loads is a **WASI module** compiled by aWsm and linked against `libsledge`. aWsm can also emit WebAssembly outside the WASI spec (for example, modules that do not use linear memory), but such modules are **not** supported by SLEdge.

Concretely, a module must satisfy the following to run:

- **WASI, compiled by aWsm.** A module is produced by running aWsm over a `*.wasm` file to emit `*.bc`, then linking that `*.bc` with `libsledge` into a `*.wasm.so` (see `applications/Makefile`). The runtime `dlopen`s the resulting `*.so` and resolves the aWsm-generated `sledge_abi__*` symbols (see `runtime/include/sledge_abi_symbols.h`).
- **Linear memory is mandatory.** SLEdge's entire I/O model lives in the module's linear memory: the body of an inbound HTTP request is copied into the module's view of stdin, and stdout/stderr are read back out of linear memory to form the response (see "File System" below). A module that does not use linear memory cannot exchange data with the runtime and is rejected at load time, because its aWsm-generated memory symbols (`sledge_abi__init_mem`, `sledge_abi__wasm_memory_starting_pages`, `sledge_abi__wasm_memory_max_pages`) are absent.

Note that the presence of the `wasi_snapshot_preview1_*` import shims is *not* a useful signal of WASI-ness: those symbols are provided by `libsledge` and are unconditionally exported from every `*.so` via `--whole-archive --export-dynamic`, regardless of which WASI calls the module actually imports. Validation therefore keys off the aWsm-generated module symbols, not the WASI import shims.

### `--runtime-globals`

The `sledge_abi__init_globals` symbol is only emitted when aWsm is run with the `--runtime-globals` flag. SLEdge tolerates its absence (globals are then inlined as constants by aWsm), so this symbol is resolved but not required. The "correct" configuration for SLEdge has not been settled; `applications/Makefile` currently builds with `--runtime-globals`.

## Arguments

The WASI calls `args_sizes_get` and `args_get` are supported. HTTP query parameters are captured and passed as arguments.
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