plugins: make test/example plugins work on windows

Generate a qemu_plugin_api.lib delay import lib on windows, for
windows qemu plugins to link against.

Implement an example dll load fail hook to link up the API functions
correctly when a plugin is loaded on windows.

Update the build scripts for the test and example plugins to use these
things.

Signed-off-by: Greg Manning <gmanning@rapitasystems.com>
Acked-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231102172053.17692-3-gmanning@rapitasystems.com>
[AJB: use find_program for dlltool, s/Windows/windows/]
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231106185112.2755262-15-alex.bennee@linaro.org>

2 files changed, 50 insertions, 4 deletions

diff --git a/contrib/plugins/Makefile b/contrib/plugins/Makefile
index 8ba78c7a32..751fa38619 100644
--- a/contrib/plugins/Makefile
+++ b/contrib/plugins/Makefile
@@ -22,7 +22,14 @@ NAMES += hwprofile
 NAMES += cache
 NAMES += drcov
 
-SONAMES := $(addsuffix .so,$(addprefix lib,$(NAMES)))
+ifeq ($(CONFIG_WIN32),y)
+SO_SUFFIX := .dll
+LDLIBS += $(shell $(PKG_CONFIG) --libs glib-2.0)
+else
+SO_SUFFIX := .so
+endif
+
+SONAMES := $(addsuffix $(SO_SUFFIX),$(addprefix lib,$(NAMES)))
 
 # The main QEMU uses Glib extensively so it's perfectly fine to use it
 # in plugins (which many example do).
@@ -35,15 +42,20 @@ all: $(SONAMES)
 %.o: %.c
 	$(CC) $(CFLAGS) $(PLUGIN_CFLAGS) -c -o $@ $<
 
-lib%.so: %.o
-ifeq ($(CONFIG_DARWIN),y)
+ifeq ($(CONFIG_WIN32),y)
+lib%$(SO_SUFFIX): %.o win32_linker.o ../../plugins/qemu_plugin_api.lib
+	$(CC) -shared -o $@ $^ $(LDLIBS)
+else ifeq ($(CONFIG_DARWIN),y)
+lib%$(SO_SUFFIX): %.o
 	$(CC) -bundle -Wl,-undefined,dynamic_lookup -o $@ $^ $(LDLIBS)
 else
+lib%$(SO_SUFFIX): %.o
 	$(CC) -shared -o $@ $^ $(LDLIBS)
 endif
 
+
 clean:
-	rm -f *.o *.so *.d
+	rm -f *.o *$(SO_SUFFIX) *.d
 	rm -Rf .libs
 
 .PHONY: all clean
diff --git a/contrib/plugins/win32_linker.c b/contrib/plugins/win32_linker.c
new file mode 100644
index 0000000000..7534b2b8bf
--- /dev/null
+++ b/contrib/plugins/win32_linker.c
@@ -0,0 +1,34 @@
+/*
+ * Copyright (C) 2023, Greg Manning <gmanning@rapitasystems.com>
+ *
+ * This hook, __pfnDliFailureHook2, is documented in the microsoft documentation here:
+ * https://learn.microsoft.com/en-us/cpp/build/reference/error-handling-and-notification
+ * It gets called when a delay-loaded DLL encounters various errors.
+ * We handle the specific case of a DLL looking for a "qemu.exe",
+ * and give it the running executable (regardless of what it is named).
+ *
+ * This work is licensed under the terms of the GNU LGPL, version 2 or later.
+ * See the COPYING.LIB file in the top-level directory.
+ */
+
+#include <windows.h>
+#include <delayimp.h>
+
+FARPROC WINAPI dll_failure_hook(unsigned dliNotify, PDelayLoadInfo pdli);
+
+
+PfnDliHook __pfnDliFailureHook2 = dll_failure_hook;
+
+FARPROC WINAPI dll_failure_hook(unsigned dliNotify, PDelayLoadInfo pdli) {
+    if (dliNotify == dliFailLoadLib) {
+        /* If the failing request was for qemu.exe, ... */
+        if (strcmp(pdli->szDll, "qemu.exe") == 0) {
+            /* Then pass back a pointer to the top level module. */
+            HMODULE top = GetModuleHandle(NULL);
+            return (FARPROC) top;
+        }
+    }
+    /* Otherwise we can't do anything special. */
+    return 0;
+}
+