docs: align with scripts/syscall.tbl migration

Update the documentation to reflect the migration of the following
architectures to the centralized syscall table format:

  arc, arm64, csky, hexagon, loongarch, nios2, openrisc, riscv

As of commit 3db80c999d ("riscv: convert to generic syscall table"),
these architectures no longer rely on include/uapi/asm-generic/unistd.h.
Instead, syscall table headers (syscall_table_{32,64}.h) are generated by
scripts/syscalltbl.sh based on entries in scripts/syscall.tbl, with ABIs
specified in arch/*/kernel/Makefile.syscalls.

For the convenience of developers working with older kernel versions, the
original documentation is fully retained, with new sections added to
cover the scripts/syscall.tbl approach.

Verified with `make htmldocs`.

Signed-off-by: Jesung Yang <y.j3ms.n@gmail.com>
Link: https://lore.kernel.org/lkml/20240704143611.2979589-1-arnd@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20250506194841.1567737-1-y.j3ms.n@gmail.com>

Documentation/process/adding-syscalls.rst

@@ -248,6 +248,52 @@ To summarize, you need a commit that includes:
  - fallback stub in ``kernel/sys_ni.c``
 
 
+.. _syscall_generic_6_11:
+
+Since 6.11
+~~~~~~~~~~
+
+Starting with kernel version 6.11, general system call implementation for the
+following architectures no longer requires modifications to
+``include/uapi/asm-generic/unistd.h``:
+
+ - arc
+ - arm64
+ - csky
+ - hexagon
+ - loongarch
+ - nios2
+ - openrisc
+ - riscv
+
+Instead, you need to update ``scripts/syscall.tbl`` and, if applicable, adjust
+``arch/*/kernel/Makefile.syscalls``.
+
+As ``scripts/syscall.tbl`` serves as a common syscall table across multiple
+architectures, a new entry is required in this table::
+
+    468   common   xyzzy     sys_xyzzy
+
+Note that adding an entry to ``scripts/syscall.tbl`` with the "common" ABI
+also affects all architectures that share this table. For more limited or
+architecture-specific changes, consider using an architecture-specific ABI or
+defining a new one.
+
+If a new ABI, say ``xyz``, is introduced, the corresponding updates should be
+made to ``arch/*/kernel/Makefile.syscalls`` as well::
+
+    syscall_abis_{32,64} += xyz (...)
+
+To summarize, you need a commit that includes:
+
+ - ``CONFIG`` option for the new function, normally in ``init/Kconfig``
+ - ``SYSCALL_DEFINEn(xyzzy, ...)`` for the entry point
+ - corresponding prototype in ``include/linux/syscalls.h``
+ - new entry in ``scripts/syscall.tbl``
+ - (if needed) Makefile updates in ``arch/*/kernel/Makefile.syscalls``
+ - fallback stub in ``kernel/sys_ni.c``
+
+
 x86 System Call Implementation
 ------------------------------
 
@@ -353,6 +399,41 @@ To summarize, you need:
    ``include/uapi/asm-generic/unistd.h``
 
 
+Since 6.11
+~~~~~~~~~~
+
+This applies to all the architectures listed in :ref:`Since 6.11<syscall_generic_6_11>`
+under "Generic System Call Implementation", except arm64. See
+:ref:`Compatibility System Calls (arm64)<compat_arm64>` for more information.
+
+You need to extend the entry in ``scripts/syscall.tbl`` with an extra column
+to indicate that a 32-bit userspace program running on a 64-bit kernel should
+hit the compat entry point::
+
+    468   common     xyzzy     sys_xyzzy    compat_sys_xyzzy
+
+To summarize, you need:
+
+ - ``COMPAT_SYSCALL_DEFINEn(xyzzy, ...)`` for the compat entry point
+ - corresponding prototype in ``include/linux/compat.h``
+ - modification of the entry in ``scripts/syscall.tbl`` to include an extra
+   "compat" column
+ - (if needed) 32-bit mapping struct in ``include/linux/compat.h``
+
+
+.. _compat_arm64:
+
+Compatibility System Calls (arm64)
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+On arm64, there is a dedicated syscall table for compatibility system calls
+targeting 32-bit (AArch32) userspace: ``arch/arm64/tools/syscall_32.tbl``.
+You need to add an additional line to this table specifying the compat
+entry point::
+
+    468   common     xyzzy     sys_xyzzy    compat_sys_xyzzy
+
+
 Compatibility System Calls (x86)
 --------------------------------
 
@@ -575,3 +656,6 @@ References and Sources
  - Recommendation from Linus Torvalds that x32 system calls should prefer
    compatibility with 64-bit versions rather than 32-bit versions:
    https://lore.kernel.org/r/CA+55aFxfmwfB7jbbrXxa=K7VBYPfAvmu3XOkGrLbB1UFjX1+Ew@mail.gmail.com
+ - Patch series revising system call table infrastructure to use
+   scripts/syscall.tbl across multiple architectures:
+   https://lore.kernel.org/lkml/20240704143611.2979589-1-arnd@kernel.org