plugins: enable linking with clang/lld

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.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Tested-by: Stefan Weil <sw@weilnetz.de>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20250110203401.178532-4-pierrick.bouvier@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20250116160306.1709518-26-alex.bennee@linaro.org>

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'),

meson.build

@@ -377,6 +377,11 @@ 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)

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(

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',