QEMU源码全解析39 —— Machine（9）

本文链接： https://blog.csdn.net/phmatthaus/article/details/132312389

本文深入解析QEMU源码中的Machine结构，通过引用《趣谈Linux操作系统》和《QEMU/KVM》相关资料，探讨了MACHINE函数的定义、OBJECT_CHECK宏的使用，以及object_dynamic_cast_assert函数的实现。文章介绍了Object作为所有对象祖先的定义，并阐述了Machine类型检查的重要性。下一部分将结合实际应用进一步展开分析。

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接前一篇文章： QEMU源码全解析38 —— Machine（8）

本文内容参考：

《趣谈 Linux操作系统》 —— 刘超，极客时间

《 QEMU /KVM》源码解析与应用 —— 李强，机械工业出版社

特此致谢！

上一回经过层层展开探索，终于看到了MACHINE的定义，实际上它是一个函数，代码如下（include/hw/boards.h中）：


static inline G_GNUC_UNUSED MachineState *MACHINE(const void *obj)
{
    return OBJECT_CHECK(MachineState, obj, TYPE_MACHINE);
}

OBJECT_CHECK是一个宏，在include/qom/object.h中定义，代码如下：


/**
 * OBJECT_CHECK:
 * @type: The C type to use for the return value.
 * @obj: A derivative of @type to cast.
 * @name: The QOM typename of @type
 *
 * A type safe version of @object_dynamic_cast_assert.  Typically each class
 * will define a macro based on this type to perform type safe dynamic_casts to
 * this object type.
 *
 * If an invalid object is passed to this function, a run time assert will be
 * generated.
 */
#define OBJECT_CHECK(type, obj, name) \
    ((type *)object_dynamic_cast_assert(OBJECT(obj), (name), \
                                        __FILE__, __LINE__, __func__))

因此，代入宏定义后，MACHINE函数的最终代码为：


static inline G_GNUC_UNUSED MachineState *MACHINE(const void *obj)
{
    return ((MachineState*)object_dynamic_cast_assert(OBJECT(obj), ("machine"), __FILE__, __LINE__, __func__));
}

object_dynamic_cast_assert函数在qom/object.c中，代码如下：


Object *object_dynamic_cast_assert(Object *obj, const char *typename,
                                   const char *file, int line, const char *func)
{
    trace_object_dynamic_cast_assert(obj ? obj->class->type->name : "(null)",
                                     typename, file, line, func);
 
#ifdef CONFIG_QOM_CAST_DEBUG
    int i;
    Object *inst;
 
    for (i = 0; obj && i < OBJECT_CLASS_CAST_CACHE; i++) {
        if (qatomic_read(&obj->class->object_cast_cache[i]) == typename) {
            goto out;
        }
    }
 
    inst = object_dynamic_cast(obj, typename);
 
    if (!inst && obj) {
        fprintf(stderr, "%s:%d:%s: Object %p is not an instance of type %s\n",
                file, line, func, obj, typename);
        abort();
    }
 
    assert(obj == inst);
 
    if (obj && obj == inst) {
        for (i = 1; i < OBJECT_CLASS_CAST_CACHE; i++) {
            qatomic_set(&obj->class->object_cast_cache[i - 1],
                       qatomic_read(&obj->class->object_cast_cache[i]));
        }
        qatomic_set(&obj->class->object_cast_cache[i - 1], typename);
    }
 
out:
#endif
    return obj;
}

Object前文已介绍过，是所有对象的祖先。这里为了便于理解，再给出其代码。在include/ qemu /typedefs.h中定义，如下：

typedef struct Object Object;

struct Object在中定义，代码如下：


/**
 * struct Object:
 *
 * The base for all objects.  The first member of this object is a pointer to
 * a #ObjectClass.  Since C guarantees that the first member of a structure
 * always begins at byte 0 of that structure, as long as any sub-object places
 * its parent as the first member, we can cast directly to a #Object.
 *
 * As a result, #Object contains a reference to the objects type as its
 * first member.  This allows identification of the real type of the object at
 * run time.
 */
struct Object
{
    /* private: */
    ObjectClass *class;
    ObjectFree *free;
    GHashTable *properties;
    uint32_t ref;
    Object *parent;
};

实际上就是类型进行检查，确保传入的对象类型是machine类型。

本着“一张一弛”的原则，本回就到这里。下一回将结合实际调用MACHINE宏的地方，继续深入展开分析。