virtio_ring.h

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#ifndef _VIRTIO_RING_H
#define _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 <stdint.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). */
    uint64_t addr;
    /* Length. */
    uint32_t len;
    /* The flags as indicated above. */
    uint16_t flags;
    /* We chain unused descriptors via this, too */
    uint16_t next;
};

struct vring_avail {
    uint16_t flags;
    uint16_t idx;
    uint16_t ring[];
};

/* u32 is used here for ids for padding reasons. */
struct vring_used_elem {
    /* Index of start of used descriptor chain. */
    uint32_t id;
    /* Total length of the descriptor chain which was used (written to) */
    uint32_t len;
};

struct vring_used {
    uint16_t flags;
    uint16_t idx;
    struct vring_used_elem ring[];
};

struct vring {
    unsigned int num;

    struct vring_desc *desc;

    struct vring_avail *avail;

    struct vring_used *used;
};

/* 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.
 *      uint16_t avail_flags;
 *      uint16_t avail_idx;
 *      uint16_t available[num];
 *      uint16_t used_event_idx;
 *
 *      // Padding to the next align boundary.
 *      char pad[];
 *
 *      // A ring of used descriptor heads with free-running index.
 *      uint16_t used_flags;
 *      uint16_t used_idx;
 *      struct vring_used_elem used[num];
 *      uint16_t 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) (*(uint16_t *)&(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 = (struct vring_desc *)p;
    vr->avail = (struct vring_avail *)((uint8_t*)p + num*sizeof(struct vring_desc));
    vr->used = (struct vring_used *)(((unsigned long)&vr->avail->ring[num] + sizeof(uint16_t)
                         + align-1) & ~(align - 1));
}

static inline unsigned vring_size(unsigned int num, unsigned long align)
{
    return ((sizeof(struct vring_desc) * num + sizeof(uint16_t) * (3 + num)
             + align - 1) & ~(align - 1))
        + sizeof(uint16_t) * 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(uint16_t event_idx, uint16_t new_idx, uint16_t 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 (uint16_t)(new_idx - event_idx - 1) < (uint16_t)(new_idx - old);
}

#endif /* _VIRTIO_RING_H */