Win64 LLP64 ABI support with cross-compiler tooling by xdqi · Pull Request #630 · lkl/linux

xdqi · 2026-05-04T13:42:34Z

lkl: Win64 LLP64 ABI support with cross-compiler tooling

Summary

This PR enables LKL to be compiled for Win64 (PE) targets using a hybrid LP64/LLP64 toolchain. The core challenge is that Win64 follows the LLP64 data model (long=32-bit) while the LKL kernel internally uses LP64 (long=64-bit). These patches bridge that gap with compile-time typedef remapping, cross-compiler shims, and a custom setjmp/longjmp for the MS ABI.

Changes

1. `__lkl_long_t` typedef for cross-ABI compatibility

New header arch/lkl/include/uapi/asm/lkl_long.h defines __lkl_long_t / __lkl_ulong_t:
- LLP64 (MinGW-w64 x64): expands to long long (64-bit)
- LP64 (kernel, Linux, Cygwin, macOS): expands to long (unchanged)
headers_install.py auto-replaces long with __lkl_long_t in installed UAPI headers and inserts the required include
All host_ops implementations, syscall parameters, ioctl casts, and tests updated accordingly

2. Cross-compiler wrapper scripts (`tools/lkl/bin/`)

x86_64-w64-mingw32-gcc: routes kernel code (-D__KERNEL__) to x86_64-pc-cygwin-gcc (LP64), user-space to real MinGW-w64 gcc (LLP64), with generated specs to a patched linker
x86_64-w64-mingw32-cc: symlink to the gcc wrapper
Patched x86_64-w64-mingw32-ld: supports PE --image-base=0x10000
Patched x86_64-w64-mingw32-objcopy: PE format support

3. Custom setjmp/longjmp for x86_64 MS ABI

Replaces libc setjmp.h with raw register save/restore inline assembly
Bypasses Windows SEH RtlUnwindEx which cannot handle cross-stack jumps used by LKL's cooperative context switching
Falls back to libc setjmp/longjmp on non-Win64 platforms

4. Patched binutils: fix weak externals in PE COFF after `ld -r`

When ld -r creates a relocatable object (e.g. vmlinux), the tagndx field in PE COFF weak external aux entries is not relocated — it retains stale symbol indices from the original object. All 352 C_NT_WEAK symbols in the LKL vmlinux have incorrect tagndx values: some point to non-code sections (e.g. .data), others to completely unrelated code symbols (e.g. bpf_arena_get_user_vm_start → cmd_line_append).

The existing binutils code only fell back to name-based lookup when tagndx pointed to a non-code section, missing the case where it points to a valid-but-wrong code symbol. This caused objcopy -G to resolve weak externals to incorrect addresses, leading to runtime crashes (e.g. ktime_get dereferencing NULL through tk_core->clock).

The fix in coff_nt_weak_to_local() (lkl-binutils@64f5155) always searches for the .weak.<name>.<tag> implementation by name first — PE COFF weak external naming is deterministic and reliable. Only falls back to tagndx when no name match is found.

Repositoriy: xdqi/lkl-binutils (lkl branch, based on binutils-2.25.1)

Toolchain

The build environment is Debian 13 (trixie):

Component	Source
MinGW-w64 cross-compiler	Debian 13 system packages (`gcc-mingw-w64-x86-64`, `binutils-mingw-w64-x86-64`)
Cygwin cross-compiler	deb-cygwin snapshot — `x86_64-pc-cygwin-gcc` produces LP64 PE/COFF output directly
Patched binutils	Custom ld/objcopy with PE `--image-base` support (lkl-binutils)

The kernel is compiled with Cygwin GCC (LP64, direct PE output), while user-space libs and tests use MinGW-w64 GCC (LLP64). The __lkl_long_t typedef bridges the ABI between these two worlds.

Disclaimer

This pull request is mostly done by Claude Code, but I audited every line of code :).

Cross-compiler wrapper (tools/lkl/bin/): - x86_64-w64-mingw32-gcc: routes kernel code (-D__KERNEL__) to x86_64-pc-cygwin-gcc (LP64), user-space to real mingw-gcc (LLP64) with a specs file to override linker path - x86_64-w64-mingw32-cc: symlink to gcc wrapper - x86_64-w64-mingw32-ld: patched ld supporting PE --image-base - x86_64-w64-mingw32-objcopy: patched objcopy for PE format Custom runtime: - lib/jmp_buf.c: setjmp/longjmp for x86_64 MS ABI (10 callee-saved XMM registers + different GPR set vs SysV)

PE64 default image base is 0x140000000. Weak symbols resolve to 0, causing rel32 relocation overflow (displacement > 2GB). Setting --image-base=0x10000 keeps all addresses in the low 32-bit range.

On Win64 (MinGW-w64 LLP64), 'long' is 32-bit while the LKL kernel (compiled with Cygwin LP64 GCC) uses 64-bit 'long'. This causes ABI mismatches in syscall parameters, UAPI struct layouts, and pointer casts between user-space and kernel. Introduce __lkl_long_t / __lkl_ulong_t conditional typedefs (arch/lkl/include/uapi/asm/lkl_long.h): - LLP64 (MinGW-w64 x64): expands to 'long long' (64-bit) - LP64 (Linux/Cygwin/macOS): expands to 'long' (no change) headers_install.py replaces 'long' with __lkl_long_t in installed UAPI headers and auto-inserts #include <lkl/asm/lkl_long.h>. Source files updated to use __lkl_long_t for syscall parameter arrays, ioctl pointer casts, and return values. Also fix case-sensitive includes in virtio_net_wintap.c for Linux cross-compilation.

tavip

Thanks for the PR @xdqi ! Could you please expand a little bit about the benefits of switching to LLP64?

tavip · 2026-05-04T18:19:10Z


 define nt_host
  $(call set_autoconf_var,NT,y)
+  $(call set_kernel_config,INIT_STACK_ALL_ZERO,n)


Why is this needed?

It's used to make my old Cygwin GCC 11 happy. We can remove it if we maintain our own newer Cygwin cross compiler.

tavip · 2026-05-04T18:44:07Z

Thanks for the weak externals fix in xdqi/lkl-binutils@64f5155!

I think it is time to move away from comitting binaries. We could use a script that downloads, patches and builds the tools similar with tools/lkl/scripts/dpdk-sdk-build.sh.

When I worked on this, I wondered why we are stuck on Binutils 2.25.
MSYS2 Binutils had already removed the patches years ago.
The {i686,x86_64}-w64-ming32-{ld,objcopy} on Debian 13 doesn't work so I have to use this old version.

xdqi · 2026-05-05T11:57:19Z

Just to make the LKL run under vanilla 64-bit Win32 environment instaed of Cygwin.
I used the Cygwin compiler to compile kernel because kernel have to work in LP64.
But the Win32 is LLP64, so the bridging code (LKL API for user) must use a special LKL long type.

Add missing SPDX-License-Identifier to the cross-compiler wrapper script and fix missing blank line after declaration in jmp_buf.c. Signed-off-by: Sheldon Qi <3365420+xdqi@users.noreply.github.com>

Remove leading double underscores from the cross-ABI typedefs. These types bridge the LP64/LLP64 gap between kernel and host userspace and do not need reserved-namespace identifiers. Signed-off-by: Sheldon Qi <3365420+xdqi@users.noreply.github.com>

lkl.h uses lkl_long_t / lkl_ulong_t but does not include the header that defines them. Add the missing include so native Linux builds can find these typedefs. Signed-off-by: Sheldon Qi <3365420+xdqi@users.noreply.github.com>

Replace the i686 cross-binutils (as, ld, objcopy) with freshly built versions from binutils-gdb configured with --target=i686-w64-mingw32. These properly handle 32-bit PE format and fix the page fault crash during kernel boot on mingw32. Signed-off-by: Sheldon Qi <3365420+xdqi@users.noreply.github.com>

xdqi added 3 commits May 4, 2026 17:52

lkl: add --image-base=0x10000 for Win64 PE builds

baaf3c6

PE64 default image base is 0x140000000. Weak symbols resolve to 0, causing rel32 relocation overflow (displacement > 2GB). Setting --image-base=0x10000 keeps all addresses in the low 32-bit range.

tavip reviewed May 4, 2026

View reviewed changes

tavip requested a review from thehajime May 4, 2026 18:44

xdqi added 4 commits May 6, 2026 13:43

lkl: add SPDX tag to gcc wrapper and fix coding style

b951be6

Add missing SPDX-License-Identifier to the cross-compiler wrapper script and fix missing blank line after declaration in jmp_buf.c. Signed-off-by: Sheldon Qi <3365420+xdqi@users.noreply.github.com>

lkl: include lkl_long.h in lkl.h for lkl_long_t definition

2a90ef7

lkl.h uses lkl_long_t / lkl_ulong_t but does not include the header that defines them. Add the missing include so native Linux builds can find these typedefs. Signed-off-by: Sheldon Qi <3365420+xdqi@users.noreply.github.com>

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Win64 LLP64 ABI support with cross-compiler tooling#630

Win64 LLP64 ABI support with cross-compiler tooling#630
xdqi wants to merge 7 commits intolkl:masterfrom
xdqi:lkl-win64

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xdqi commented May 4, 2026

lkl: Win64 LLP64 ABI support with cross-compiler tooling

Summary

Changes

1. __lkl_long_t typedef for cross-ABI compatibility

2. Cross-compiler wrapper scripts (tools/lkl/bin/)

3. Custom setjmp/longjmp for x86_64 MS ABI

4. Patched binutils: fix weak externals in PE COFF after ld -r

Toolchain

Disclaimer

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1. `__lkl_long_t` typedef for cross-ABI compatibility

2. Cross-compiler wrapper scripts (`tools/lkl/bin/`)

4. Patched binutils: fix weak externals in PE COFF after `ld -r`