First cut of rumprun unikernel configuration specification

This is mostly extracted from current librumprun_base/config.c and
app-tools/rumprun source. Work in progress.

diff --git a/doc/config.md b/doc/config.md
new file mode 100644 (file)
index 0000000..747d16c
--- /dev/null
+++ b/doc/config.md
@@ -0,0 +1,144 @@
+# Rumprun unikernel configuration
+
+This document specifies the JSON format used for configuring a rumprun unikernel
+and the platform-dependent methods for passing configuration to a rumprun
+unikernel.
+
+*This is a work in progress, the described format and interfaces are not yet
+stable! The document may not reflect the current implementation. Users are
+advised to continue using the `rumprun` tool to launch rumprun unikernels until
+further notice.*
+
+Configuration interfaces and/or behaviours not documented here are considered
+unofficial and experimental, and may be removed without warning.
+
+## Configuration format
+
+The configuration format is a valid JSON expression:
+
+    { <JSON expression> }
+
+Unless otherwise stated, all keys are optional.
+
+### cmdline, rc: Program command line(s)
+
+The `cmdline` key specifies the command line for unikernels containing a single
+program:
+
+    "cmdline": "arg0 arg1 arg2 ... argN"
+
+_argN_: Corresponds to the `argv[N]` passed to the program.
+
+Unikernels which contain more than one program (using multibake) MUST use
+the `rc` key to specify command lines for each baked-in program:
+
+    "rc": [
+         {
+             "bin" : "arg0",
+             "argv" : [ "arg1", "arg2", ..., "argN" ],
+             "runmode" : "& OR |" (optional)
+         },
+         ...
+    ]
+
+Each element of `rc` describes a single program, in the order in which they
+are baked into the unikernel image.
+
+_argN_: Corresponds to the `argv[N]` passed to the program.
+_runmode_:
+ * `&`: run this program in background
+ * `|`: pipe output of current program to next defined program
+ * (default): run this program in foreground and wait for it to successfully
+   exit before running further programs
+
+### env: Environment variables
+
+    "env": "NAME=VALUE"
+    ...
+
+Sets the environment variable `NAME` to `VALUE`.
+
+_FIXME_: Relies on specifying multiple `env` keys, which is not valid JSON.
+Proposed specification as an array follows:
+
+    "env": [
+        "NAME=VALUE",
+        ...
+    ]
+
+Sets the environment variable `NAME` to `VALUE`.
+
+### hostname: Kernel hostname setting
+
+    "hostname": "hostname"
+
+_hostname_: The kernel hostname.
+
+### net: Network interfaces
+
+    "net": {
+        "if": "if",
+        "cloner": "cloner",
+        "type": "type",
+        "method": "method",
+        ...
+    }
+    ...
+
+_if_: The kernel name of the network interface. (eg. `vioif0`, `xenif0`)
+_cloner_: Create the interface? (boolean, Xen only)
+_type_, _method_: Network interface type and configuration method.
+
+_TODO_: Document methods.
+
+_FIXME_: Relies on specifying multiple `net` keys, which is not valid JSON.
+Should be change to use an array instead.
+
+### blk: Block devices and file systems
+
+    "blk": {
+        "source": "dev",
+        "path": "/dev/ld0a",
+        "fstype": "blk",
+        "mountpoint": "/etc",
+    }
+    ...
+
+_source_: `dev`, `vnd` or `etfs`
+_path_: The path to the block device/etfs key. 
+_fstype_: `blk` or `kernfs`
+_mountpoint_: The filesystem mount point.
+
+_TODO_: Incomplete. Document semantics of various sources, and differences
+between Xen and hw.
+
+_FIXME_: Relies on specifying multiple `blk` keys, which is not valid JSON.
+Should be change to use an array instead.
+
+## Passing configuration to the unikernel
+
+### hw platform on x86
+
+On x86 bare metal (this includes QEMU, KVM or other hypervisors using HVM)
+rumprun uses the multiboot protocol.
+
+Configuration may be passed either directly on the kernel command line, or
+loaded as a multiboot module. If a multiboot module containing configuration is
+found, it is used in preference to the kernel command line.
+
+_TODO_: Provide examples. Also, can the `ROOTFSCFG=` hack go away now?
+
+_TODO_: Make it explicit what a "multiboot module containing configuration"
+means. Ignore modules which we don't understand (e.g. does not start with `{`).
+
+### hw platform on ARM
+
+TBD.
+
+### xen platform on x86
+
+On x86 paravirtualized Xen, the rumprun configuration must be written to the
+domain-specific Xenstore key `rumprun/cfg` before the domU is started.
+
+_TODO_: Examples. Do we officially want to support passing config on Xen via
+cmdline (I think not).