QEMU源码全解析29 —— QOM介绍（18）

于 2023-08-07 18:13:36 发布
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分类专栏： QEMU/KVM KVM 文章标签： QEMU KVM
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本文深入讲解QEMU中QOM（Quantum Object Model）的类属性设置及获取接口，以str类型的属性为例，探讨object_class_property_add_str函数及其内部使用的property_set_str和property_get_str函数。通过分析machine_class_init函数中的代码，展示如何在QEMU中实际操作类属性。
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接前一篇文章： QEMU源码全解析28 —— QOM介绍（17）

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！

前文讲解了类属性的添加接口。本文讲解类属性的设置及获取接口。

每一种类属性都有自己的add函数，如布尔（bool）、字符串（str）、enum（枚举）等。它们都在qom/object.c中，逐个来看： 

  1. ObjectProperty *
  2. object_class_property_add_bool(ObjectClass *klass, const char *name,
  3.                                     bool (*get)(Object *, Error **),
  4.                                     void (*set)(Object *, bool, Error **))
  5. {
  6.     BoolProperty *prop = g_malloc0(sizeof(*prop));
  7.  
  8.     prop->get = get;
  9.     prop->set = set;
  10.  
  11.     return object_class_property_add(klass, name, "bool",
  12.                                      get ? property_get_bool : NULL,
  13.                                      set ? property_set_bool : NULL,
  14.                                      NULL,
  15.                                      prop);
  16. }
  1. ObjectProperty *
  2. object_class_property_add_enum(ObjectClass *klass, const char *name,
  3.                                     const char *typename,
  4.                                     const QEnumLookup *lookup,
  5.                                     int (*get)(Object *, Error **),
  6.                                     void (*set)(Object *, int, Error **))
  7. {
  8.     EnumProperty *prop = g_malloc(sizeof(*prop));
  9.  
  10.     prop->lookup = lookup;
  11.     prop->get = get;
  12.     prop->set = set;
  13.  
  14.     return object_class_property_add(klass, name, typename,
  15.                                      get ? property_get_enum : NULL,
  16.                                      set ? property_set_enum : NULL,
  17.                                      NULL,
  18.                                      prop);
  19. }
  1. ObjectProperty *
  2. object_class_property_add_str(ObjectClass *klass, const char *name,
  3.                                    char *(*get)(Object *, Error **),
  4.                                    void (*set)(Object *, const char *,
  5.                                                Error **))
  6. {
  7.     StringProperty *prop = g_malloc0(sizeof(*prop));
  8.  
  9.     prop->get = get;
  10.     prop->set = set;
  11.  
  12.     return object_class_property_add(klass, name, "string",
  13.                                      get ? property_get_str : NULL,
  14.                                      set ? property_set_str : NULL,
  15.                                      NULL,
  16.                                      prop);
  17. }

这次以str为例进行讲解。上一回已经讲过，类属性的添加是通过object_class_property_add接口完成的。而str类型属性的添加object_class_property_add_str函数则是调用了此接口。为了便于理解，再次贴出object_class_property_add函数代码，在qom/object.c中，如下： 

  1. ObjectProperty *
  2. object_class_property_add(ObjectClass *klass,
  3.                           const char *name,
  4.                           const char *type,
  5.                           ObjectPropertyAccessor *get,
  6.                           ObjectPropertyAccessor *set,
  7.                           ObjectPropertyRelease *release,
  8.                           void *opaque)
  9. {
  10.     ObjectProperty *prop;
  11.  
  12.     assert(!object_class_property_find(klass, name));
  13.  
  14.     prop = g_malloc0(sizeof(*prop));
  15.  
  16.     prop->name = g_strdup(name);
  17.     prop->type = g_strdup(type);
  18.  
  19.     prop->get = get;
  20.     prop->set = set;
  21.     prop->release = release;
  22.     prop->opaque = opaque;
  23.  
  24.     g_hash_table_insert(klass->properties, prop->name, prop);
  25.  
  26.     return prop;
  27. }

property_set_str和property_get_str函数在前文都已给出过，这里再贴一下，在同文件（qom/object.c）中： 

  1. static void property_set_str(Object *obj, Visitor *v, const char *name,
  2.                              void *opaque, Error **errp)
  3. {
  4.     StringProperty *prop = opaque;
  5.     char *value;
  6.  
  7.     if (!visit_type_str(v, name, &value, errp)) {
  8.         return;
  9.     }
  10.  
  11.     prop->set(obj, value, errp);
  12.     g_free(value);
  13. }
  1. static void property_get_str(Object *obj, Visitor *v, const char *name,
  2.                              void *opaque, Error **errp)
  3. {
  4.     StringProperty *prop = opaque;
  5.     char *value;
  6.     Error *err = NULL;
  7.  
  8.     value = prop->get(obj, &err);
  9.     if (err) {
  10.         error_propagate(errp, err);
  11.         return;
  12.     }
  13.  
  14.     visit_type_str(v, name, &value, errp);
  15.     g_free(value);
  16. }

从这两个函数中就可以看到，StringProperty结构的set和get成员函数为调用object_class_property_add_str函数时传入的参数，形参分别为：void (*set)(Object *, const char *, Error **))和char *(*get)(Object *, Error **)。在object_property_add_str函数中使用了上边的property_set_str和property_get_str函数。在property_set_str函数中最终调用了形参void (*set)(Object *, const char *, Error **)对应的实参，即真正类型属性的设置函数；在property_get_str函数中最终调用了形参char *(*get)(Object *, Error **)对应的实参，即真正的类型属性的获取函数。

仍以machine为例，hw/core/machine.c的machine_class_init函数代码如下： 

  1. static void machine_class_init(ObjectClass *oc, void *data)
  2. {
  3.     MachineClass *mc = MACHINE_CLASS(oc);
  4.  
  5.     /* Default 128 MB as guest ram size */
  6.     mc->default_ram_size = 128 * MiB;
  7.     mc->rom_file_has_mr = true;
  8.  
  9.     /* numa node memory size aligned on 8MB by default.
  10.      * On Linux, each node's border has to be 8MB aligned
  11.      */
  12.     mc->numa_mem_align_shift = 23;
  13.  
  14.     object_class_property_add_str(oc, "kernel",
  15.         machine_get_kernel, machine_set_kernel);
  16.     object_class_property_set_description(oc, "kernel",
  17.         "Linux kernel image file");
  18.  
  19.     object_class_property_add_str(oc, "initrd",
  20.         machine_get_initrd, machine_set_initrd);
  21.     object_class_property_set_description(oc, "initrd",
  22.         "Linux initial ramdisk file");
  23.  
  24.     object_class_property_add_str(oc, "append",
  25.         machine_get_append, machine_set_append);
  26.     object_class_property_set_description(oc, "append",
  27.         "Linux kernel command line");
  28.  
  29.     object_class_property_add_str(oc, "dtb",
  30.         machine_get_dtb, machine_set_dtb);
  31.     object_class_property_set_description(oc, "dtb",
  32.         "Linux kernel device tree file");
  33.  
  34.     object_class_property_add_str(oc, "dumpdtb",
  35.         machine_get_dumpdtb, machine_set_dumpdtb);
  36.     object_class_property_set_description(oc, "dumpdtb",
  37.         "Dump current dtb to a file and quit");
  38.  
  39.     object_class_property_add(oc, "boot", "BootConfiguration",
  40.         machine_get_boot, machine_set_boot,
  41.         NULL, NULL);
  42.     object_class_property_set_description(oc, "boot",
  43.         "Boot configuration");
  44.  
  45.     object_class_property_add(oc, "smp", "SMPConfiguration",
  46.         machine_get_smp, machine_set_smp,
  47.         NULL, NULL);
  48.     object_class_property_set_description(oc, "smp",
  49.         "CPU topology");
  50.  
  51.     object_class_property_add(oc, "phandle-start", "int",
  52.         machine_get_phandle_start, machine_set_phandle_start,
  53.         NULL, NULL);
  54.     object_class_property_set_description(oc, "phandle-start",
  55.         "The first phandle ID we may generate dynamically");
  56.  
  57.     object_class_property_add_str(oc, "dt-compatible",
  58.         machine_get_dt_compatible, machine_set_dt_compatible);
  59.     object_class_property_set_description(oc, "dt-compatible",
  60.         "Overrides the \"compatible\" property of the dt root node");
  61.  
  62.     object_class_property_add_bool(oc, "dump-guest-core",
  63.         machine_get_dump_guest_core, machine_set_dump_guest_core);
  64.     object_class_property_set_description(oc, "dump-guest-core",
  65.         "Include guest memory in a core dump");
  66.  
  67.     object_class_property_add_bool(oc, "mem-merge",
  68.         machine_get_mem_merge, machine_set_mem_merge);
  69.     object_class_property_set_description(oc, "mem-merge",
  70.         "Enable/disable memory merge support");
  71.  
  72.     object_class_property_add_bool(oc, "usb",
  73.         machine_get_usb, machine_set_usb);
  74.     object_class_property_set_description(oc, "usb",
  75.         "Set on/off to enable/disable usb");
  76.  
  77.     object_class_property_add_bool(oc, "graphics",
  78.         machine_get_graphics, machine_set_graphics);
  79.     object_class_property_set_description(oc, "graphics",
  80.         "Set on/off to enable/disable graphics emulation");
  81.  
  82.     object_class_property_add_str(oc, "firmware",
  83.         machine_get_firmware, machine_set_firmware);
  84.     object_class_property_set_description(oc, "firmware",
  85.         "Firmware image");
  86.  
  87.     object_class_property_add_bool(oc, "suppress-vmdesc",
  88.         machine_get_suppress_vmdesc, machine_set_suppress_vmdesc);
  89.     object_class_property_set_description(oc, "suppress-vmdesc",
  90.         "Set on to disable self-describing migration");
  91.  
  92.     object_class_property_add_link(oc, "confidential-guest-support",
  93.                                    TYPE_CONFIDENTIAL_GUEST_SUPPORT,
  94.                                    offsetof(MachineState, cgs),
  95.                                    machine_check_confidential_guest_support,
  96.                                    OBJ_PROP_LINK_STRONG);
  97.     object_class_property_set_description(oc, "confidential-guest-support",
  98.                                           "Set confidential guest scheme to support");
  99.  
  100.     /* For compatibility */
  101.     object_class_property_add_str(oc, "memory-encryption",
  102.         machine_get_memory_encryption, machine_set_memory_encryption);
  103.     object_class_property_set_description(oc, "memory-encryption",
  104.         "Set memory encryption object to use");
  105.  
  106.     object_class_property_add_link(oc, "memory-backend", TYPE_MEMORY_BACKEND,
  107.                                    offsetof(MachineState, memdev), object_property_allow_set_link,
  108.                                    OBJ_PROP_LINK_STRONG);
  109.     object_class_property_set_description(oc, "memory-backend",
  110.                                           "Set RAM backend"
  111.                                           "Valid value is ID of hostmem based backend");
  112.  
  113.     object_class_property_add(oc, "memory", "MemorySizeConfiguration",
  114.         machine_get_mem, machine_set_mem,
  115.         NULL, NULL);
  116.     object_class_property_set_description(oc, "memory",
  117.         "Memory size configuration");
  118. }

此函数中很多地方调用了object_class_property_add_str函数，以其中之一为例： 

  1. object_class_property_add_str(oc, "kernel",
  2.         machine_get_kernel, machine_set_kernel);

代码中的machine_set_kernel和machine_get_kernel函数就是传入object_class_property_add_str函数的与形参对应的实参，也即对于machine类的属性进行设置和获取时实际调用的函数。

至此， QEMU 中的QOM相关的内容就简要介绍完了。欲知后事如何，且看下回分解。

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