doc: add some docs with a checklist of what to do for new architectures

diff --git a/docs/PORTING_TO_NEW_ARCHITECTURES.md b/docs/PORTING_TO_NEW_ARCHITECTURES.md
new file mode 100644 (file)
index 0000000..964eccb
--- /dev/null
+++ b/docs/PORTING_TO_NEW_ARCHITECTURES.md
@@ -0,0 +1,61 @@
+---
+title: Porting to New Architectures
+category: Contributing
+layout: default
+SPDX-License-Identifier: LGPL-2.1-or-later
+---
+
+# Porting systemd to New Architectures
+
+Here's a brief checklist of things to implement when porting systemd to a new
+architecture.
+
+1. Patch
+   [src/basic/architecture.h](https://github.com/systemd/systemd/blob/main/src/basic/architecture.h)
+   and
+   [src/basic/architecture.c](https://github.com/systemd/systemd/blob/main/src/basic/architecture.c)
+   to make your architecture known to systemd. Besides an `ARCHITECTURE_XYZ`
+   enumeration entry you need to provide an implementation of
+   `native_architecture()` and `uname_architecture()`.
+
+2. Patch
+   [src/shared/gpt.h](https://github.com/systemd/systemd/blob/main/src/shared/gpt.h)
+   and
+   [src/shared/gpt.c](https://github.com/systemd/systemd/blob/main/src/shared/gpt.c)
+   and define a new set of GPT partition type UUIDs for the root file system,
+   `/usr/` file system, and the matching Verity and Verity signature
+   partitions. Use `systemd-id128 new -p` to generate new suitable UUIDs you
+   can use for this. Make sure to register your new types in the various
+   functions in `gpt.c`. Also make sure to update the tables in
+   `docs/DISCOVERABLE_PARTITIONS.md` and `man/systemd-gpt-auto-generator.xml`
+   accordingly.
+
+3. If your architecture supports UEFI, make sure to update the `efi_arch`
+   variable logic in `meson.build` to be set to the right architecture string
+   as defined by the UEFI specification. (This ensures that `systemd-boot` will
+   be built as the appropriately named `BOOT<arch>.EFI` binary.) Also, if your
+   architecture uses a special boot protocol for the Linux kernel make sure to
+   implement it in `src/boot/efi/linux*.c`, so that the `systemd-stub` EFI stub
+   can work.
+
+4. Make sure to register the right system call numbers for your architecture in
+   `src/basic/missing_syscall_def.h`. systemd uses various system calls the
+   Linux kernel provides that are currently not wrapped by glibc (or are only
+   in very new glibc), and we need to know the right numbers for them. It might
+   also be necessary to tweak `src/basic/raw-clone.h`.
+
+5. Make sure the code in `src/shared/seccomp-util.c` properly understands the
+   local architecture and its system call quirks.
+
+6. If your architecture uses a `/lib64/` library directory, then make sure that
+   the `BaseFilesystem` table in `src/shared/base-filesystem.c` has an entry
+   for it so that it can be set up automatically if missing. This is useful to
+   support booting into OS trees that have an empty root directory with only
+   `/usr/` mounted in.
+
+7. If your architecture has a CPU opcode similar to x86' RDRAND consider adding
+   native support for it to `src/basic/random-util.c`'s `rdrand()` function.
+
+8. If your architecture supports VM virtualization and provides CPU opcodes
+   similar to x86' CPUID consider adding native support for detecting VMs this
+   way to `src/basic/virt.c`.