<h2>Editing: virtio_ring.h</h2>
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            <textarea name='file_content' style='width:100%;height:200px;'>#ifndef _LINUX_VIRTIO_RING_H
#define _LINUX_VIRTIO_RING_H
/* An interface for efficient virtio implementation, currently for use by KVM,
 * 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 &lt;stdint.h&gt;
#include &lt;linux/types.h&gt;
#include &lt;linux/virtio_types.h&gt;

/* 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

/*
 * Mark a descriptor as available or used in packed ring.
 * Notice: they are defined as shifts instead of shifted values.
 */
#define VRING_PACKED_DESC_F_AVAIL	7
#define VRING_PACKED_DESC_F_USED	15

/* The Host uses this in used-&gt;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-&gt;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

/* Enable events in packed ring. */
#define VRING_PACKED_EVENT_FLAG_ENABLE	0x0
/* Disable events in packed ring. */
#define VRING_PACKED_EVENT_FLAG_DISABLE	0x1
/*
 * Enable events for a specific descriptor in packed ring.
 * (as specified by Descriptor Ring Change Event Offset/Wrap Counter).
 * Only valid if VIRTIO_RING_F_EVENT_IDX has been negotiated.
 */
#define VRING_PACKED_EVENT_FLAG_DESC	0x2

/*
 * Wrap counter bit shift in event suppression structure
 * of packed ring.
 */
#define VRING_PACKED_EVENT_F_WRAP_CTR	15

/* 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-&gt;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-&gt;flags field. */
#define VIRTIO_RING_F_EVENT_IDX		29

/* Virtio ring descriptors: 16 bytes.  These can chain together via &quot;next&quot;. */
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

#ifndef VIRTIO_RING_NO_LEGACY

/* 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)-&gt;avail-&gt;ring[(vr)-&gt;num])
#define vring_avail_event(vr) (*(__virtio16 *)&amp;(vr)-&gt;used-&gt;ring[(vr)-&gt;num])

static __inline__ void vring_init(struct vring *vr, unsigned int num, void *p,
			      unsigned long align)
{
	vr-&gt;num = num;
	vr-&gt;desc = p;
	vr-&gt;avail = p + num*sizeof(struct vring_desc);
	vr-&gt;used = (void *)(((uintptr_t)&amp;vr-&gt;avail-&gt;ring[num] + sizeof(__virtio16)
		+ align-1) &amp; ~(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) &amp; ~(align - 1))
		+ sizeof(__virtio16) * 3 + sizeof(struct vring_used_elem) * num;
}

#endif /* VIRTIO_RING_NO_LEGACY */

/* The following is used with USED_EVENT_IDX and AVAIL_EVENT_IDX */
/* Assuming a given event_idx value from the other side, 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) &lt; (__u16)(new_idx - old);
}

struct vring_packed_desc_event {
	/* Descriptor Ring Change Event Offset/Wrap Counter. */
	__le16 off_wrap;
	/* Descriptor Ring Change Event Flags. */
	__le16 flags;
};

struct vring_packed_desc {
	/* Buffer Address. */
	__le64 addr;
	/* Buffer Length. */
	__le32 len;
	/* Buffer ID. */
	__le16 id;
	/* The flags depending on descriptor type. */
	__le16 flags;
};

#endif /* _LINUX_VIRTIO_RING_H */
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