text: #ifndef LINUX_VIRTIO_H
  #define LINUX_VIRTIO_H
  #include <linux/scatterlist.h>
  #include <linux/kernel.h>
  
  /* TODO: empty stubs for now. Broken but enough for virtio_ring.c */
  #define list_add_tail(a, b) do {} while (0)
  #define list_del(a) do {} while (0)
  
  #define BIT_WORD(nr)		((nr) / BITS_PER_LONG)
  #define BITS_PER_BYTE		8
  #define BITS_PER_LONG (sizeof(long) * BITS_PER_BYTE)
  #define BIT_MASK(nr)		(1UL << ((nr) % BITS_PER_LONG))
  
  /* TODO: Not atomic as it should be:
   * we don't use this for anything important. */
  static inline void clear_bit(int nr, volatile unsigned long *addr)
  {
  	unsigned long mask = BIT_MASK(nr);
  	unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
  
  	*p &= ~mask;
  }
  
  static inline int test_bit(int nr, const volatile unsigned long *addr)
  {
          return 1UL & (addr[BIT_WORD(nr)] >> (nr & (BITS_PER_LONG-1)));
  }
  /* end of stubs */
  
  struct virtio_device {
  	void *dev;
  	unsigned long features[1];
  };
  
  struct virtqueue {
  	/* TODO: commented as list macros are empty stubs for now.
  	 * Broken but enough for virtio_ring.c
  	 * 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;
  };
  
  #define MODULE_LICENSE(__MODULE_LICENSE_value) \
  	const char *__MODULE_LICENSE_name = __MODULE_LICENSE_value
  
  /* Interfaces exported by virtio_ring. */
  int virtqueue_add_sgs(struct virtqueue *vq,
  		      struct scatterlist *sgs[],
  		      unsigned int out_sgs,
  		      unsigned int in_sgs,
  		      void *data,
  		      gfp_t gfp);
  
  int virtqueue_add_outbuf(struct virtqueue *vq,
  			 struct scatterlist sg[], unsigned int num,
  			 void *data,
  			 gfp_t gfp);
  
  int virtqueue_add_inbuf(struct virtqueue *vq,
  			struct scatterlist sg[], unsigned int num,
  			void *data,
  			gfp_t gfp);
  
  void virtqueue_kick(struct virtqueue *vq);
  
  void *virtqueue_get_buf(struct virtqueue *vq, unsigned int *len);
  
  void virtqueue_disable_cb(struct virtqueue *vq);
  
  bool virtqueue_enable_cb(struct virtqueue *vq);
  bool virtqueue_enable_cb_delayed(struct virtqueue *vq);
  
  void *virtqueue_detach_unused_buf(struct virtqueue *vq);
  struct virtqueue *vring_new_virtqueue(unsigned int index,
  				      unsigned int num,
  				      unsigned int vring_align,
  				      struct virtio_device *vdev,
  				      bool weak_barriers,
  				      void *pages,
  				      void (*notify)(struct virtqueue *vq),
  				      void (*callback)(struct virtqueue *vq),
  				      const char *name);
  void vring_del_virtqueue(struct virtqueue *vq);
  
  #endif
  