Re: [PATCH v3 3/3] plugins: enable linking with clang/lld

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

+Akihiko & Yonggang for
https://lore.kernel.org/qemu-devel/20201006120900.1579-1-luoyonggang@gmail.com/

On 28/11/24 21:15, Pierrick Bouvier wrote:
> Windows uses a special mechanism to enable plugins to work (DLL delay
> loading). Option for lld is different than ld.
> 
> MSYS2 clang based environment use lld by default, so restricting to this
> config on Windows is safe, and will avoid false bug reports.
> 
> Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>

> ---
>   meson.build                   |  5 +++++
>   contrib/plugins/meson.build   |  2 +-
>   plugins/meson.build           | 24 ++++++++++++++++++++----
>   tests/tcg/plugins/meson.build |  3 +--
>   4 files changed, 27 insertions(+), 7 deletions(-)
> 
> diff --git a/meson.build b/meson.build
> index 97cefb7cdd7..f286fb4f4a0 100644
> --- a/meson.build
> +++ b/meson.build
> @@ -354,6 +354,11 @@ elif host_os == 'sunos'
>     qemu_common_flags += '-D__EXTENSIONS__'
>   elif host_os == 'haiku'
>     qemu_common_flags += ['-DB_USE_POSITIVE_POSIX_ERRORS', '-D_BSD_SOURCE', '-fPIC']
> +elif host_os == 'windows'
> +  # plugins use delaylib, and clang needs to be used with lld to make it work.
> +  if compiler.get_id() == 'clang' and compiler.get_linker_id() != 'ld.lld'
> +    error('On windows, you need to use lld with clang - use msys2 clang64/clangarm64 env')
> +  endif
>   endif
>   
>   # Choose instruction set (currently x86-only)
> diff --git a/contrib/plugins/meson.build b/contrib/plugins/meson.build
> index 63a32c2b4f0..484b9a808c8 100644
> --- a/contrib/plugins/meson.build
> +++ b/contrib/plugins/meson.build
> @@ -12,7 +12,7 @@ if get_option('plugins')
>         t += shared_module(i, files(i + '.c') + 'win32_linker.c',
>                           include_directories: '../../include/qemu',
>                           link_depends: [win32_qemu_plugin_api_lib],
> -                        link_args: ['-Lplugins', '-lqemu_plugin_api'],
> +                        link_args: win32_qemu_plugin_api_link_flags,
>                           dependencies: glib)
>       else
>         t += shared_module(i, files(i + '.c'),
> diff --git a/plugins/meson.build b/plugins/meson.build
> index 98542e926f8..d60be2a4d6d 100644
> --- a/plugins/meson.build
> +++ b/plugins/meson.build
> @@ -17,14 +17,15 @@ if not enable_modules
>         capture: true,
>         command: ['sed', '-ne', 's/^[[:space:]]*\\(qemu_.*\\);/_\\1/p', '@INPUT@'])
>       emulator_link_args += ['-Wl,-exported_symbols_list,plugins/qemu-plugins-ld64.symbols']
> +  elif host_os == 'windows' and meson.get_compiler('c').get_id() == 'clang'
> +    # LLVM/lld does not support exporting specific symbols. However, it works
> +    # out of the box with dllexport/dllimport attribute we set in the code.
>     else
>       emulator_link_args += ['-Xlinker', '--dynamic-list=' + qemu_plugin_symbols.full_path()]
>     endif
>   endif
>   
>   if host_os == 'windows'
> -  dlltool = find_program('dlltool', required: true)
> -
>     # Generate a .lib file for plugins to link against.
>     # First, create a .def file listing all the symbols a plugin should expect to have
>     # available in qemu
> @@ -33,12 +34,27 @@ if host_os == 'windows'
>       output: 'qemu_plugin_api.def',
>       capture: true,
>       command: ['sed', '-e', '0,/^/s//EXPORTS/; s/[{};]//g', '@INPUT@'])
> +
>     # then use dlltool to assemble a delaylib.
> +  # The delaylib will have an "imaginary" name (qemu.exe), that is used by the
> +  # linker file we add with plugins (win32_linker.c) to identify that we want
> +  # to find missing symbols in current program.
> +  win32_qemu_plugin_api_link_flags = ['-Lplugins', '-lqemu_plugin_api']
> +  if meson.get_compiler('c').get_id() == 'clang'
> +    # With LLVM/lld, delaylib is specified at link time (-delayload)
> +    dlltool = find_program('llvm-dlltool', required: true)
> +    dlltool_cmd = [dlltool, '-d', '@INPUT@', '-l', '@OUTPUT@', '-D', 'qemu.exe']
> +    win32_qemu_plugin_api_link_flags += ['-Wl,-delayload=qemu.exe']
> +  else
> +    # With gcc/ld, delay lib is built with a specific delay parameter.
> +    dlltool = find_program('dlltool', required: true)
> +    dlltool_cmd = [dlltool, '--input-def', '@INPUT@',
> +                   '--output-delaylib', '@OUTPUT@', '--dllname', 'qemu.exe']
> +  endif
>     win32_qemu_plugin_api_lib = configure_file(
>       input: win32_plugin_def,
>       output: 'libqemu_plugin_api.a',
> -    command: [dlltool, '--input-def', '@INPUT@',
> -              '--output-delaylib', '@OUTPUT@', '--dllname', 'qemu.exe']
> +    command: dlltool_cmd
>     )
>   endif
>   specific_ss.add(files(
> diff --git a/tests/tcg/plugins/meson.build b/tests/tcg/plugins/meson.build
> index f847849b1b7..87a17d67bd4 100644
> --- a/tests/tcg/plugins/meson.build
> +++ b/tests/tcg/plugins/meson.build
> @@ -5,9 +5,8 @@ if get_option('plugins')
>         t += shared_module(i, files(i + '.c') + '../../../contrib/plugins/win32_linker.c',
>                           include_directories: '../../../include/qemu',
>                           link_depends: [win32_qemu_plugin_api_lib],
> -                        link_args: ['-Lplugins', '-lqemu_plugin_api'],
> +                        link_args: win32_qemu_plugin_api_link_flags,
>                           dependencies: glib)
> -
>       else
>         t += shared_module(i, files(i + '.c'),
>                           include_directories: '../../../include/qemu',