[RFC PATCH 0/5] Add dead syscalls elimination support

[Zhangjin Wu <falcon@tinylab.org>]

Hi, all

CONFIG_HAVE_LD_DEAD_CODE_DATA_ELIMINATION allows to eliminate dead code
and data, this patchset allows to further eliminate dead syscalls which
are not used in target system.

It includes 5 patches:

- syscall: Allow configure used system calls

  This adds a new CONFIG_SYSCALLS_USED option to allow users or tools to
  tell kernel what syscalls are used in target system. A list of
  used syscalls can be passed to it.

- MIPS: Add dead syscalls elimination support

  Add CONFIG_SYSCALLS_USED support for mips, it simply adds a 'used'
  variant for the syscall*.tbl and accordingly updates the kernel apis
  and eventually the sys_call_table. The unused ones in the table are
  replaced by sys_ni_syscall and therefore if they are also not used by
  kernel itself, they will be eliminated by gc-sections.

  The old architectures use syscall*.tbl, so, they can use this method.

- RISC-V: Enable dead code elimination

  Select HAVE_LD_DEAD_CODE_DATA_ELIMINATION for riscv.

- RISC-V: Add dead syscalls elimination support

  Add CONFIG_SYSCALLS_USED support for riscv, it simply adds a 'used'
  variant for the *syscall_table.c and eventually the sys_call_table.

  The new architectures use generic unistd.h, they can use this method.

- nolibc: Record used syscalls in their own sections

  This allows to record syscalls used by a nolibc based application. It
  is possible to eliminate dead syscalls automatically while building
  the monolithic kernel+nolibc software.

Testing shows, on both mips and riscv, with a small config, gc-sections
shrinks ~10% and syscalls_used shrinks another ~5%.

This patchset is only a prototype implementation, welcome your feedback
and suggestion, Thanks.

Related emails:

- Re: Re: Kernel-only deployments
  https://lore.kernel.org/lkml/20230216130935.37976-1-falcon@tinylab.org/

- Re: Re: RISC-V: Enable dead code elimination
  https://lore.kernel.org/linux-riscv/Y+qSBu3YZH0JPY4I@spud/T/#t

Best Regards,
- Zhangjin Wu

---

Zhangjin Wu (5):
  syscall: Allow configure used system calls
  MIPS: Add dead syscalls elimination support
  RISC-V: Enable dead code elimination
  RISC-V: Add dead syscalls elimination support
  nolibc: Record used syscalls in their own sections

 arch/mips/Kconfig                   |  1 +
 arch/mips/kernel/syscalls/Makefile  | 24 ++++++++-
 arch/riscv/Kconfig                  |  2 +
 arch/riscv/kernel/Makefile          |  5 +-
 arch/riscv/kernel/syscalls/Makefile | 38 ++++++++++++++
 arch/riscv/kernel/vmlinux.lds.S     |  2 +-
 init/Kconfig                        | 22 +++++++++
 tools/include/nolibc/Makefile       |  2 +-
 tools/include/nolibc/arch-aarch64.h | 17 ++++---
 tools/include/nolibc/arch-arm.h     | 15 +++---
 tools/include/nolibc/arch-i386.h    | 17 ++++---
 tools/include/nolibc/arch-mips.h    | 15 +++---
 tools/include/nolibc/arch-riscv.h   | 17 ++++---
 tools/include/nolibc/arch-x86_64.h  | 17 ++++---
 tools/include/nolibc/arch.h         |  2 +
 tools/include/nolibc/record.h       | 77 +++++++++++++++++++++++++++++
 16 files changed, 226 insertions(+), 47 deletions(-)
 create mode 100644 arch/riscv/kernel/syscalls/Makefile
 create mode 100644 tools/include/nolibc/record.h

-- 
2.25.1