QEMU源码全解析 —— virtio（22）

已于 2024-02-22 10:42:01 修改
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本文继续探讨QEMU源码中的virtio驱动，重点在于virtio驱动的初始化，介绍了virtio_pci_config_ops及其初始化过程。通过分析Linux内核源码，解释了get_status和set_status等关键函数的功能，它们直接操作设备状态并涉及QEMU中的MemoryRegionOps。
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接前一篇文章： QEMU源码全解析 —— virtio（21）

前几回讲解了 virtio 驱动的加载。本回开始讲解virtio驱动的初始化。

在讲解virtio驱动的初始化之前，先要介绍virtio配置的函数集合变量virtio_pci_config_ops。实际上前文书也有提到，如下图的右上角：

virtio_pci_config_ops的初始化有两处，分别在 Linux 内核源码/drivers/virtio/virtio_pci_legacy.c和Linux内核源码/drivers/virtio/virtio_pci_modern.c中。代码分别如下：

  • legacy 
  1. static const struct virtio_config_ops virtio_pci_config_ops = {
  2. 	.get		= vp_get,
  3. 	.set		= vp_set,
  4. 	.get_status	= vp_get_status,
  5. 	.set_status	= vp_set_status,
  6. 	.reset		= vp_reset,
  7. 	.find_vqs	= vp_find_vqs,
  8. 	.del_vqs	= vp_del_vqs,
  9. 	.synchronize_cbs = vp_synchronize_vectors,
  10. 	.get_features	= vp_get_features,
  11. 	.finalize_features = vp_finalize_features,
  12. 	.bus_name	= vp_bus_name,
  13. 	.set_vq_affinity = vp_set_vq_affinity,
  14. 	.get_vq_affinity = vp_get_vq_affinity,
  15. };
  • modern 
  1. static const struct virtio_config_ops virtio_pci_config_ops = {
  2. 	.get		= vp_get,
  3. 	.set		= vp_set,
  4. 	.generation	= vp_generation,
  5. 	.get_status	= vp_get_status,
  6. 	.set_status	= vp_set_status,
  7. 	.reset		= vp_reset,
  8. 	.find_vqs	= vp_modern_find_vqs,
  9. 	.del_vqs	= vp_del_vqs,
  10. 	.synchronize_cbs = vp_synchronize_vectors,
  11. 	.get_features	= vp_get_features,
  12. 	.finalize_features = vp_finalize_features,
  13. 	.bus_name	= vp_bus_name,
  14. 	.set_vq_affinity = vp_set_vq_affinity,
  15. 	.get_vq_affinity = vp_get_vq_affinity,
  16. 	.get_shm_region  = vp_get_shm_region,
  17. 	.disable_vq_and_reset = vp_modern_disable_vq_and_reset,
  18. 	.enable_vq_after_reset = vp_modern_enable_vq_after_reset,
  19. };

在此以Linux内核源码/drivers/virtio/virtio_pci_modern.c中的virtio_pci_config_ops为例进行讲解。

在前文书讲到的virtio_pci_modern_probe函数（Linux内核源码/drivers/virtio/virtio_pci_modern.c）中： 

  1. /* the PCI probing function */
  2. int virtio_pci_modern_probe(struct virtio_pci_device *vp_dev)
  3. {
  4. 	struct virtio_pci_modern_device *mdev = &vp_dev->mdev;
  5. 	struct pci_dev *pci_dev = vp_dev->pci_dev;
  6. 	int err;
  7.  
  8. 	mdev->pci_dev = pci_dev;
  9.  
  10. 	err = vp_modern_probe(mdev);
  11. 	if (err)
  12. 		return err;
  13.  
  14. 	if (mdev->device)
  15. 		vp_dev->vdev.config = &virtio_pci_config_ops;
  16. 	else
  17. 		vp_dev->vdev.config = &virtio_pci_config_nodev_ops;
  18.  
  19. 	vp_dev->config_vector = vp_config_vector;
  20. 	vp_dev->setup_vq = setup_vq;
  21. 	vp_dev->del_vq = del_vq;
  22. 	vp_dev->isr = mdev->isr;
  23. 	vp_dev->vdev.id = mdev->id;
  24.  
  25. 	return 0;
  26. }

virtio_pci_config_ops变量被赋值给了virtio_device结构的config成员。struct virtio_device的定义在Linux内核源码/include/linux/virtio.h中，代码如下： 

  1. /**
  2.  * struct virtio_device - representation of a device using virtio
  3.  * @index: unique position on the virtio bus
  4.  * @failed: saved value for VIRTIO_CONFIG_S_FAILED bit (for restore)
  5.  * @config_enabled: configuration change reporting enabled
  6.  * @config_change_pending: configuration change reported while disabled
  7.  * @config_lock: protects configuration change reporting
  8.  * @vqs_list_lock: protects @vqs.
  9.  * @dev: underlying device.
  10.  * @id: the device type identification (used to match it with a driver).
  11.  * @config: the configuration ops for this device.
  12.  * @vringh_config: configuration ops for host vrings.
  13.  * @vqs: the list of virtqueues for this device.
  14.  * @features: the features supported by both driver and device.
  15.  * @priv: private pointer for the driver's use.
  16.  */
  17. struct virtio_device {
  18. 	int index;
  19. 	bool failed;
  20. 	bool config_enabled;
  21. 	bool config_change_pending;
  22. 	spinlock_t config_lock;
  23. 	spinlock_t vqs_list_lock;
  24. 	struct device dev;
  25. 	struct virtio_device_id id;
  26. 	const struct virtio_config_ops *config;
  27. 	const struct vringh_config_ops *vringh_config;
  28. 	struct list_head vqs;
  29. 	u64 features;
  30. 	void *priv;
  31. };

其中的struct virtio_config_ops的定义在Linux内核源码/include/linux/virtio_config.h中，代码如下： 

  1. /**
  2.  * struct virtio_config_ops - operations for configuring a virtio device
  3.  * Note: Do not assume that a transport implements all of the operations
  4.  *       getting/setting a value as a simple read/write! Generally speaking,
  5.  *       any of @get/@set, @get_status/@set_status, or @get_features/
  6.  *       @finalize_features are NOT safe to be called from an atomic
  7.  *       context.
  8.  * @get: read the value of a configuration field
  9.  *	vdev: the virtio_device
  10.  *	offset: the offset of the configuration field
  11.  *	buf: the buffer to write the field value into.
  12.  *	len: the length of the buffer
  13.  * @set: write the value of a configuration field
  14.  *	vdev: the virtio_device
  15.  *	offset: the offset of the configuration field
  16.  *	buf: the buffer to read the field value from.
  17.  *	len: the length of the buffer
  18.  * @generation: config generation counter (optional)
  19.  *	vdev: the virtio_device
  20.  *	Returns the config generation counter
  21.  * @get_status: read the status byte
  22.  *	vdev: the virtio_device
  23.  *	Returns the status byte
  24.  * @set_status: write the status byte
  25.  *	vdev: the virtio_device
  26.  *	status: the new status byte
  27.  * @reset: reset the device
  28.  *	vdev: the virtio device
  29.  *	After this, status and feature negotiation must be done again
  30.  *	Device must not be reset from its vq/config callbacks, or in
  31.  *	parallel with being added/removed.
  32.  * @find_vqs: find virtqueues and instantiate them.
  33.  *	vdev: the virtio_device
  34.  *	nvqs: the number of virtqueues to find
  35.  *	vqs: on success, includes new virtqueues
  36.  *	callbacks: array of callbacks, for each virtqueue
  37.  *		include a NULL entry for vqs that do not need a callback
  38.  *	names: array of virtqueue names (mainly for debugging)
  39.  *		include a NULL entry for vqs unused by driver
  40.  *	Returns 0 on success or error status
  41.  * @del_vqs: free virtqueues found by find_vqs().
  42.  * @synchronize_cbs: synchronize with the virtqueue callbacks (optional)
  43.  *      The function guarantees that all memory operations on the
  44.  *      queue before it are visible to the vring_interrupt() that is
  45.  *      called after it.
  46.  *      vdev: the virtio_device
  47.  * @get_features: get the array of feature bits for this device.
  48.  *	vdev: the virtio_device
  49.  *	Returns the first 64 feature bits (all we currently need).
  50.  * @finalize_features: confirm what device features we'll be using.
  51.  *	vdev: the virtio_device
  52.  *	This sends the driver feature bits to the device: it can change
  53.  *	the dev->feature bits if it wants.
  54.  *	Note that despite the name this	can be called any number of
  55.  *	times.
  56.  *	Returns 0 on success or error status
  57.  * @bus_name: return the bus name associated with the device (optional)
  58.  *	vdev: the virtio_device
  59.  *      This returns a pointer to the bus name a la pci_name from which
  60.  *      the caller can then copy.
  61.  * @set_vq_affinity: set the affinity for a virtqueue (optional).
  62.  * @get_vq_affinity: get the affinity for a virtqueue (optional).
  63.  * @get_shm_region: get a shared memory region based on the index.
  64.  * @disable_vq_and_reset: reset a queue individually (optional).
  65.  *	vq: the virtqueue
  66.  *	Returns 0 on success or error status
  67.  *	disable_vq_and_reset will guarantee that the callbacks are disabled and
  68.  *	synchronized.
  69.  *	Except for the callback, the caller should guarantee that the vring is
  70.  *	not accessed by any functions of virtqueue.
  71.  * @enable_vq_after_reset: enable a reset queue
  72.  *	vq: the virtqueue
  73.  *	Returns 0 on success or error status
  74.  *	If disable_vq_and_reset is set, then enable_vq_after_reset must also be
  75.  *	set.
  76.  */
  77. struct virtio_config_ops {
  78. 	void (*get)(struct virtio_device *vdev, unsigned offset,
  79. 		    void *buf, unsigned len);
  80. 	void (*set)(struct virtio_device *vdev, unsigned offset,
  81. 		    const void *buf, unsigned len);
  82. 	u32 (*generation)(struct virtio_device *vdev);
  83. 	u8 (*get_status)(struct virtio_device *vdev);
  84. 	void (*set_status)(struct virtio_device *vdev, u8 status);
  85. 	void (*reset)(struct virtio_device *vdev);
  86. 	int (*find_vqs)(struct virtio_device *, unsigned nvqs,
  87. 			struct virtqueue *vqs[], vq_callback_t *callbacks[],
  88. 			const char * const names[], const bool *ctx,
  89. 			struct irq_affinity *desc);
  90. 	void (*del_vqs)(struct virtio_device *);
  91. 	void (*synchronize_cbs)(struct virtio_device *);
  92. 	u64 (*get_features)(struct virtio_device *vdev);
  93. 	int (*finalize_features)(struct virtio_device *vdev);
  94. 	const char *(*bus_name)(struct virtio_device *vdev);
  95. 	int (*set_vq_affinity)(struct virtqueue *vq,
  96. 			       const struct cpumask *cpu_mask);
  97. 	const struct cpumask *(*get_vq_affinity)(struct virtio_device *vdev,
  98. 			int index);
  99. 	bool (*get_shm_region)(struct virtio_device *vdev,
  100. 			       struct virtio_shm_region *region, u8 id);
  101. 	int (*disable_vq_and_reset)(struct virtqueue *vq);
  102. 	int (*enable_vq_after_reset)(struct virtqueue *vq);
  103. };

再回过头来看一下Linux内核源码/drivers/virtio/virtio_pci_modern.c中的virtio_pci_config_ops，对照着上边 struct virtio_config_ops的定义。 

  1. static const struct virtio_config_ops virtio_pci_config_ops = {
  2. 	.get		= vp_get,
  3. 	.set		= vp_set,
  4. 	.generation	= vp_generation,
  5. 	.get_status	= vp_get_status,
  6. 	.set_status	= vp_set_status,
  7. 	.reset		= vp_reset,
  8. 	.find_vqs	= vp_modern_find_vqs,
  9. 	.del_vqs	= vp_del_vqs,
  10. 	.synchronize_cbs = vp_synchronize_vectors,
  11. 	.get_features	= vp_get_features,
  12. 	.finalize_features = vp_finalize_features,
  13. 	.bus_name	= vp_bus_name,
  14. 	.set_vq_affinity = vp_set_vq_affinity,
  15. 	.get_vq_affinity = vp_get_vq_affinity,
  16. 	.get_shm_region  = vp_get_shm_region,
  17. 	.disable_vq_and_reset = vp_modern_disable_vq_and_reset,
  18. 	.enable_vq_after_reset = vp_modern_enable_vq_after_reset,
  19. };

virtio_pci_config_ops结构中的成员函数通常是virtio PCI代理设备的IO操作，包括读写virtio PCI代理设备的PIO和MMIO，如get_status和set_status成员对应的vp_get_status函数和vp_set_status函数。分别来看：

  • get_status

根据struct virtio_config_ops中的说明：

@get_status: read the status byte
*    vdev: the virtio_device
*    Returns the status byte

get_status的作用是读取状态字节。有一个参数vdev，代表了virtio device。返回值为读取到的状态字节。

get_status所指向的vp_get_status函数也在Linux内核源码/drivers/virtio/virtio_pci_modern.c中，代码如下： 

  1. /* config->{get,set}_status() implementations */
  2. static u8 vp_get_status(struct virtio_device *vdev)
  3. {
  4. 	struct virtio_pci_device *vp_dev = to_vp_device(vdev);
  5.  
  6. 	return vp_modern_get_status(&vp_dev->mdev);
  7. }

vp_modern_get_status函数在Linux内核源码/drivers/virtio/virtio_pci_modern_dev.c中，代码如下： 

  1. /*
  2.  * vp_modern_get_status - get the device status
  3.  * @mdev: the modern virtio-pci device
  4.  *
  5.  * Returns the status read from device
  6.  */
  7. u8 vp_modern_get_status(struct virtio_pci_modern_device *mdev)
  8. {
  9. 	struct virtio_pci_common_cfg __iomem *cfg = mdev->common;
  10.  
  11. 	return vp_ioread8(&cfg->device_status);
  12. }
  13. EXPORT_SYMBOL_GPL(vp_modern_get_status);
  • set_status

@set_status: write the status byte
*    vdev: the virtio_device
*    status: the new status byte

set_status的作用是写入状态字节。有两个参数：vdev代表了virtio device；status为新的要写入的状态字节。

set_status所指向的vp_set_status函数也在Linux内核源码/drivers/virtio/virtio_pci_modern.c中，代码如下： 

  1. static void vp_set_status(struct virtio_device *vdev, u8 status)
  2. {
  3. 	struct virtio_pci_device *vp_dev = to_vp_device(vdev);
  4.  
  5. 	/* We should never be setting status to 0. */
  6. 	BUG_ON(status == 0);
  7. 	vp_modern_set_status(&vp_dev->mdev, status);
  8. }

vp_modern_set_status函数在Linux内核源码/drivers/virtio/virtio_pci_modern_dev.c中，代码如下： 

  1. /*
  2.  * vp_modern_set_status - set status to device
  3.  * @mdev: the modern virtio-pci device
  4.  * @status: the status set to device
  5.  */
  6. void vp_modern_set_status(struct virtio_pci_modern_device *mdev,
  7. 				 u8 status)
  8. {
  9. 	struct virtio_pci_common_cfg __iomem *cfg = mdev->common;
  10.  
  11. 	/*
  12. 	 * Per memory-barriers.txt, wmb() is not needed to guarantee
  13. 	 * that the cache coherent memory writes have completed
  14. 	 * before writing to the MMIO region.
  15. 	 */
  16. 	vp_iowrite8(status, &cfg->device_status);
  17. }
  18. EXPORT_SYMBOL_GPL(vp_modern_set_status);

vp_modern_get_status和vp_modern_set_status函数直接读写vp_dev->mdev->common->device_status。从前文书（ QEMU源码全解析 —— virtio（14） ）的讲解可知，vp_dev->common对应的是virtio PCI代理设备第四个BAR表示的地址中的一段空间。

vp_dev->mdev->common的类型为struct virtio_pci_common_cfg，该结构的定义在Linux内核源码/include/uapi/linux/virtio_pci.h中，代码如下： 

  1. /* Fields in VIRTIO_PCI_CAP_COMMON_CFG: */
  2. struct virtio_pci_common_cfg {
  3. 	/* About the whole device. */
  4. 	__le32 device_feature_select;	/* read-write */
  5. 	__le32 device_feature;		/* read-only */
  6. 	__le32 guest_feature_select;	/* read-write */
  7. 	__le32 guest_feature;		/* read-write */
  8. 	__le16 msix_config;		/* read-write */
  9. 	__le16 num_queues;		/* read-only */
  10. 	__u8 device_status;		/* read-write */
  11. 	__u8 config_generation;		/* read-only */
  12.  
  13. 	/* About a specific virtqueue. */
  14. 	__le16 queue_select;		/* read-write */
  15. 	__le16 queue_size;		/* read-write, power of 2. */
  16. 	__le16 queue_msix_vector;	/* read-write */
  17. 	__le16 queue_enable;		/* read-write */
  18. 	__le16 queue_notify_off;	/* read-only */
  19. 	__le32 queue_desc_lo;		/* read-write */
  20. 	__le32 queue_desc_hi;		/* read-write */
  21. 	__le32 queue_avail_lo;		/* read-write */
  22. 	__le32 queue_avail_hi;		/* read-write */
  23. 	__le32 queue_used_lo;		/* read-write */
  24. 	__le32 queue_used_hi;		/* read-write */
  25. };

struct virtio_pci_common_cfg的每一个成员都表示一个virtio PCI代理设备modern MMIO地址空间中对应的值，读写这写成员都会陷入到 QEMU 中。比如上面的读取或者设置设备状态的device_status成员，其地址从virtio_pci_common_cfg结构开始的偏移20字节处（4+4+4+4+2+2=20），所以读写该地址的时候会陷入到QEMU中，并且地址是virtio设备的common MemoryRegion偏移20字节处。该MemoryRegion对应的回调操作结构是common_ops，类型为MemoryRegionOps。

common_ops在hw/virtio/virtio-pci.c中初始化，代码如下： 

  1. static void virtio_pci_modern_regions_init(VirtIOPCIProxy *proxy,
  2.                                            const char *vdev_name)
  3. {
  4.     static const MemoryRegionOps common_ops = {
  5.         .read = virtio_pci_common_read,
  6.         .write = virtio_pci_common_write,
  7.         .impl = {
  8.             .min_access_size = 1,
  9.             .max_access_size = 4,
  10.         },
  11.         .endianness = DEVICE_LITTLE_ENDIAN,
  12.     };
  13.     ……
  14. }

回到struct_pci_config_ops。 

  1. static const struct virtio_config_ops virtio_pci_config_ops = {
  2. 	.get		= vp_get,
  3. 	.set		= vp_set,
  4. 	.generation	= vp_generation,
  5. 	.get_status	= vp_get_status,
  6. 	.set_status	= vp_set_status,
  7. 	.reset		= vp_reset,
  8. 	.find_vqs	= vp_modern_find_vqs,
  9. 	.del_vqs	= vp_del_vqs,
  10. 	.synchronize_cbs = vp_synchronize_vectors,
  11. 	.get_features	= vp_get_features,
  12. 	.finalize_features = vp_finalize_features,
  13. 	.bus_name	= vp_bus_name,
  14. 	.set_vq_affinity = vp_set_vq_affinity,
  15. 	.get_vq_affinity = vp_get_vq_affinity,
  16. 	.get_shm_region  = vp_get_shm_region,
  17. 	.disable_vq_and_reset = vp_modern_disable_vq_and_reset,
  18. 	.enable_vq_after_reset = vp_modern_enable_vq_after_reset,
  19. };

virtio_pci_config_ops的各个函数封装了这些I/O操作，不仅是MMO操作，还有PIO操作。virtio设备可以通过此结构中的各个回调函数来驱动设备。

本回就讲到这里。下一回以virtio balloon设备的初始化过程为例，分析virtio设备的初始化过程，即上一回讲到的virtio驱动初始化设备的过程中的“执行设备相关的 初始化操作 ”一步。

欲知后事如何，且看下回分解。

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