text: #ifndef _LINUX_VIRTIO_RING_H
  #define _LINUX_VIRTIO_RING_H
  /* An interface for efficient virtio implementation, currently for use by KVM
   * and lguest, but hopefully others soon.  Do NOT change this since it will
   * break existing servers and clients.
   *
   * This header is BSD licensed so anyone can use the definitions to implement
   * compatible drivers/servers.
   *
   * Redistribution and use in source and binary forms, with or without
   * modification, are permitted provided that the following conditions
   * are met:
   * 1. Redistributions of source code must retain the above copyright
   *    notice, this list of conditions and the following disclaimer.
   * 2. Redistributions in binary form must reproduce the above copyright
   *    notice, this list of conditions and the following disclaimer in the
   *    documentation and/or other materials provided with the distribution.
   * 3. Neither the name of IBM nor the names of its contributors
   *    may be used to endorse or promote products derived from this software
   *    without specific prior written permission.
   * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS ``AS IS'' AND
   * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   * ARE DISCLAIMED.  IN NO EVENT SHALL IBM OR CONTRIBUTORS BE LIABLE
   * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   * SUCH DAMAGE.
   *
   * Copyright Rusty Russell IBM Corporation 2007. */
  #include <linux/types.h>
  #include <linux/virtio_types.h>
  
  /* This marks a buffer as continuing via the next field. */
  #define VRING_DESC_F_NEXT	1
  /* This marks a buffer as write-only (otherwise read-only). */
  #define VRING_DESC_F_WRITE	2
  /* This means the buffer contains a list of buffer descriptors. */
  #define VRING_DESC_F_INDIRECT	4
  
  /* The Host uses this in used->flags to advise the Guest: don't kick me when
   * you add a buffer.  It's unreliable, so it's simply an optimization.  Guest
   * will still kick if it's out of buffers. */
  #define VRING_USED_F_NO_NOTIFY	1
  /* The Guest uses this in avail->flags to advise the Host: don't interrupt me
   * when you consume a buffer.  It's unreliable, so it's simply an
   * optimization.  */
  #define VRING_AVAIL_F_NO_INTERRUPT	1
  
  /* We support indirect buffer descriptors */
  #define VIRTIO_RING_F_INDIRECT_DESC	28
  
  /* The Guest publishes the used index for which it expects an interrupt
   * at the end of the avail ring. Host should ignore the avail->flags field. */
  /* The Host publishes the avail index for which it expects a kick
   * at the end of the used ring. Guest should ignore the used->flags field. */
  #define VIRTIO_RING_F_EVENT_IDX		29
  
  /* Virtio ring descriptors: 16 bytes.  These can chain together via "next". */
  struct vring_desc {
  	/* Address (guest-physical). */
  	__virtio64 addr;
  	/* Length. */
  	__virtio32 len;
  	/* The flags as indicated above. */
  	__virtio16 flags;
  	/* We chain unused descriptors via this, too */
  	__virtio16 next;
  };
  
  struct vring_avail {
  	__virtio16 flags;
  	__virtio16 idx;
  	__virtio16 ring[];
  };
  
  /* u32 is used here for ids for padding reasons. */
  struct vring_used_elem {
  	/* Index of start of used descriptor chain. */
  	__virtio32 id;
  	/* Total length of the descriptor chain which was used (written to) */
  	__virtio32 len;
  };
  
  struct vring_used {
  	__virtio16 flags;
  	__virtio16 idx;
  	struct vring_used_elem ring[];
  };
  
  struct vring {
  	unsigned int num;
  
  	struct vring_desc *desc;
  
  	struct vring_avail *avail;
  
  	struct vring_used *used;
  };
  
  /* Alignment requirements for vring elements.
   * When using pre-virtio 1.0 layout, these fall out naturally.
   */
  #define VRING_AVAIL_ALIGN_SIZE 2
  #define VRING_USED_ALIGN_SIZE 4
  #define VRING_DESC_ALIGN_SIZE 16
  
  /* The standard layout for the ring is a continuous chunk of memory which looks
   * like this.  We assume num is a power of 2.
   *
   * struct vring
   * {
   *	// The actual descriptors (16 bytes each)
   *	struct vring_desc desc[num];
   *
   *	// A ring of available descriptor heads with free-running index.
   *	__virtio16 avail_flags;
   *	__virtio16 avail_idx;
   *	__virtio16 available[num];
   *	__virtio16 used_event_idx;
   *
   *	// Padding to the next align boundary.
   *	char pad[];
   *
   *	// A ring of used descriptor heads with free-running index.
   *	__virtio16 used_flags;
   *	__virtio16 used_idx;
   *	struct vring_used_elem used[num];
   *	__virtio16 avail_event_idx;
   * };
   */
  /* We publish the used event index at the end of the available ring, and vice
   * versa. They are at the end for backwards compatibility. */
  #define vring_used_event(vr) ((vr)->avail->ring[(vr)->num])
  #define vring_avail_event(vr) (*(__virtio16 *)&(vr)->used->ring[(vr)->num])
  
  static __inline__ void vring_init(struct vring *vr, unsigned int num, void *p,
  			      unsigned long align)
  {
  	vr->num = num;
  	vr->desc = p;
  	vr->avail = p + num*sizeof(struct vring_desc);
  	vr->used = (void *)(((unsigned long)&vr->avail->ring[num] + sizeof(__virtio16)
  		+ align-1) & ~(align - 1));
  }
  
  static __inline__ unsigned vring_size(unsigned int num, unsigned long align)
  {
  	return ((sizeof(struct vring_desc) * num + sizeof(__virtio16) * (3 + num)
  		 + align - 1) & ~(align - 1))
  		+ sizeof(__virtio16) * 3 + sizeof(struct vring_used_elem) * num;
  }
  
  /* The following is used with USED_EVENT_IDX and AVAIL_EVENT_IDX */
  /* Assuming a given event_idx value from the other size, if
   * we have just incremented index from old to new_idx,
   * should we trigger an event? */
  static __inline__ int vring_need_event(__u16 event_idx, __u16 new_idx, __u16 old)
  {
  	/* Note: Xen has similar logic for notification hold-off
  	 * in include/xen/interface/io/ring.h with req_event and req_prod
  	 * corresponding to event_idx + 1 and new_idx respectively.
  	 * Note also that req_event and req_prod in Xen start at 1,
  	 * event indexes in virtio start at 0. */
  	return (__u16)(new_idx - event_idx - 1) < (__u16)(new_idx - old);
  }
  
  #endif /* _LINUX_VIRTIO_RING_H */
  