virtio解析 - 1
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virtio是一种IO（块设备和网络设备等）虚拟化技术，它是一种半虚拟化（paravirtualization）解决方案，由guest应用和hypervisor之间的一些IO通信协议组成，这也意味着设备驱动是专门为虚拟化设计的。同全虚拟化方案相比，半虚拟化方案大大减少了在guest和host中交互需要的cpu cycles，因为性能更加优秀。
virtio可以分为这么几个部分：前端驱动层，后端驱动层和传输层
前端virtio驱动，在guestos中一般作为一个设备驱动存在
后端virtio驱动，在hypervisor中实现，接收前端virtio驱动的io 请求，在具体的物理设备上执行io请求
传输层，前端virtio驱动和后端virtio驱动的通信通过一个称为virtqueue的队列来实现
上图中的virtio-blk virtio-net virtio-pci virtio-balloon virtio-console都是前端virtio驱动。
这五种virtio设备可以抽象为virtio_device，这样就可以通过virtqueue来与后端驱动通信。
这些数据类型定义在 linux/include/linux/virtio.h里。
/**
* virtqueue - a queue to register buffers for sending or receiving.
* @list: the chain of virtqueues for this device
* @callback: the function to call when buffers are consumed (can be NULL).
* @name: the name of this virtqueue (mainly for debugging)
* @vdev: the virtio device this queue was created for.
* @priv: a pointer for the virtqueue implementation to use.
* @index: the zero-based ordinal number for this queue.
* @num_free: number of elements we expect to be able to fit.
*
* A note on @num_free: with indirect buffers, each buffer needs one
* element in the queue, otherwise a buffer will need one element per
* sg element.
*/
struct virtqueue {
struct list_head list;
void (*callback)(struct virtqueue *vq);
const char *name;
struct virtio_device *vdev;
unsigned int index;
unsigned int num_free;
void *priv;
};
/**
* virtio_device - representation of a device using virtio
* @index: unique position on the virtio bus
* @failed: saved value for VIRTIO_CONFIG_S_FAILED bit (for restore)
* @config_enabled: configuration change reporting enabled
* @config_change_pending: configuration change reported while disabled
* @config_lock: protects configuration change reporting
* @dev: underlying device.
* @id: the device type identification (used to match it with a driver).
* @config: the configuration ops for this device.
* @vringh_config: configuration ops for host vrings.
* @vqs: the list of virtqueues for this device.
* @features: the features supported by both driver and device.
* @priv: private pointer for the driver's use.
*/
struct virtio_device {
int index;
bool failed;
bool config_enabled;
bool config_change_pending;
spinlock_t config_lock;
struct device dev;
struct virtio_device_id id;
const struct virtio_config_ops *config;
const struct vringh_config_ops *vringh_config;
struct list_head vqs;
u64 features;
void *priv;
};
/**
* virtio_driver - operations for a virtio I/O driver
* @driver: underlying device driver (populate name and owner).
* @id_table: the ids serviced by this driver.
* @feature_table: an array of feature numbers supported by this driver.
* @feature_table_size: number of entries in the feature table array.
* @feature_table_legacy: same as feature_table but when working in legacy mode.
* @feature_table_size_legacy: number of entries in feature table legacy array.
* @probe: the function to call when a device is found. Returns 0 or -errno.
* @scan: optional function to call after successful probe; intended
* for virtio-scsi to invoke a scan.
* @remove: the function to call when a device is removed.
* @config_changed: optional function to call when the device configuration
* changes; may be called in interrupt context.
* @freeze: optional function to call during suspend/hibernation.
* @restore: optional function to call on resume.
*/
struct virtio_driver {
struct device_driver driver;
const struct virtio_device_id *id_table;
const unsigned int *feature_table;
unsigned int feature_table_size;
const unsigned int *feature_table_legacy;
unsigned int feature_table_size_legacy;
int (*validate)(struct virtio_device *dev);
int (*probe)(struct virtio_device *dev);
void (*scan)(struct virtio_device *dev);
void (*remove)(struct virtio_device *dev);
void (*config_changed)(struct virtio_device *dev);
#ifdef CONFIG_PM
int (*freeze)(struct virtio_device *dev);
int (*restore)(struct virtio_device *dev);
#endif
};