[PATCH liburing 0/3] Add x86 32-bit support for the nolibc build

[Ammar Faizi <ammarfaizi2@gnuweeb.org>]

Hi,

This series adds nolibc support for x86 32-bit. There are 3 patches in
this series:

1) Use `__NR_mmap2` instead of `__NR_mmap` for x86 32-bit.
2) Provide `get_page_size()` function for x86 32-bit.
3) Add x86 32-bit native syscall support.

The most noticeable changes is the patch 3. Unlike x86-64, only
use native syscall from the __do_syscall macros when CONFIG_NOLIBC is
enabled for 32-bit build. The reason is because the libc syscall
wrapper can do better in 32-bit. The libc syscall wrapper can dispatch
the best syscall instruction that the environment is supported, there
are at least two variants syscall instruction for x86 32-bit, they are:
`int $0x80` and `sysenter`. The `int $0x80` instruction is always
available, but `sysenter` is not, it relies on VDSO. liburing always
uses `int $0x80` for syscall if it's compiled with CONFIG_NOLIBC,
otherwise it uses whatever the libc provides.

Extra notes for __do_syscall6() macro:
On i386, the 6th argument of syscall goes in %ebp. However, both Clang
and GCC cannot use %ebp in the clobber list and in the "r" constraint
without using -fomit-frame-pointer. To make it always available for
any kind of compilation, the below workaround is implemented:

  1) Push the 6-th argument.
  2) Push %ebp.
  3) Load the 6-th argument from 4(%esp) to %ebp.
  4) Do the syscall (int $0x80).
  5) Pop %ebp (restore the old value of %ebp).
  6) Add %esp by 4 (undo the stack pointer).

WARNING:
  Don't use register variables for __do_syscall6(), there is a known
  GCC bug that results in an endless loop.

BugLink: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105032


===== How is this tested? =====

This has been tested on x86-64 Linux (compiled with 32-bit bin support)
with the following commands:

  sudo apt-get install gcc-i686-linux-gnu g++-i686-linux-gnu -y;
  ./configure --cc=i686-linux-gnu-gcc --cxx=i686-linux-gnu-g++ --nolibc;
  sudo make -j8 runtests;


Signed-off-by: Alviro Iskandar Setiawan <alviro.iskandar@gnuweeb.org>
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
---

Ammar Faizi (3):
  arch/syscall-defs: Use `__NR_mmap2` instead of `__NR_mmap` for x86 32-bit
  arch/x86/lib: Provide `get_page_size()` function for x86 32-bit
  arch/x86/syscall: Add x86 32-bit native syscall support

 src/arch/syscall-defs.h |  11 ++-
 src/arch/x86/lib.h      |  17 -----
 src/arch/x86/syscall.h  | 150 +++++++++++++++++++++++++++++++++++++++-
 3 files changed, 157 insertions(+), 21 deletions(-)


base-commit: c47b44afb686d794f8ed62feb77e9c42431c8444
-- 
Ammar Faizi