bug: introduce ASSERT_STRUCT_OFFSET

ASSERT_STRUCT_OFFSET allows to assert during the build of
the kernel that a field in a struct have an expected offset.

KVM used to have such macro, but there is almost nothing KVM specific
in it so move it to build_bug.h, so that it can be used in other
places in KVM.

Signed-off-by: Maxim Levitsky <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Paolo Bonzini <[e-mail]>

arch/x86/kvm/vmx/vmcs12.h

···
 /*
  * For save/restore compatibility, the vmcs12 field offsets must not change.
  */
-#define CHECK_OFFSET(field, loc)				\
-	BUILD_BUG_ON_MSG(offsetof(struct vmcs12, field) != (loc),	\
-		"Offset of " #field " in struct vmcs12 has changed.")
+#define CHECK_OFFSET(field, loc) \
+	ASSERT_STRUCT_OFFSET(struct vmcs12, field, loc)
 
 static inline void vmx_check_vmcs12_offsets(void)
 {

include/linux/build_bug.h

···
 #define static_assert(expr, ...) __static_assert(expr, ##__VA_ARGS__, #expr)
 #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
 
+
+/*
+ * Compile time check that field has an expected offset
+ */
+#define ASSERT_STRUCT_OFFSET(type, field, expected_offset)	\
+	BUILD_BUG_ON_MSG(offsetof(type, field) != (expected_offset),	\
+		"Offset of " #field " in " #type " has changed.")
+
+
 #endif	/* _LINUX_BUILD_BUG_H */