virtio: Replace one cpu_mfence() by cpu_lfence(), and one by cpu_sfence().

[dragonfly.git] / sys / dev / virtual / virtio / virtio / virtqueue.c

/*-
 * Copyright (c) 2011, Bryan Venteicher <bryanv@daemoninthecloset.org>
 * All rights reserved.
 *
 * 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 unmodified, 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.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``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 THE AUTHOR 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.
 *
 * $FreeBSD: src/sys/dev/virtio/virtqueue.c,v 1.2 2012/04/14 05:48:04 grehan Exp $
 */

/*
 * Implements the virtqueue interface as basically described
 * in the original VirtIO paper.
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/malloc.h>
#include <sys/sglist.h>
#include <sys/serialize.h>
#include <vm/vm.h>
#include <vm/pmap.h>

#include <machine/cpu.h>
#include <machine/atomic.h>
#include <sys/bus.h>
#include <sys/rman.h>

#include "virtio.h"
#include "virtqueue.h"
#include "virtio_ring.h"

#include "virtio_bus_if.h"

struct virtqueue {
        device_t                 vq_dev;
        char                     vq_name[VIRTQUEUE_MAX_NAME_SZ];
        uint16_t                 vq_queue_index;
        uint16_t                 vq_nentries;
        uint32_t                 vq_flags;

#define VIRTQUEUE_FLAG_INDIRECT  0x0001
#define VIRTQUEUE_FLAG_EVENT_IDX 0x0002

        int                      vq_alignment;
        int                      vq_ring_size;
        void                    *vq_ring_mem;
        int                      vq_max_indirect_size;
        int                      vq_indirect_mem_size;

        virtqueue_intr_t        *vq_intrhand;
        void                    *vq_intrhand_arg;

        struct vring             vq_ring;
        uint16_t                 vq_free_cnt;
        uint16_t                 vq_queued_cnt;
        /*
         * Head of the free chain in the descriptor table. If
         * there are no free descriptors, this will be set to
         * VQ_RING_DESC_CHAIN_END.
         */
        uint16_t                 vq_desc_head_idx;
        /*
         * Last consumed descriptor in the used table,
         * trails vq_ring.used->idx.
         */
        uint16_t                 vq_used_cons_idx;

        struct vq_desc_extra {
                void              *cookie;
                struct vring_desc *indirect;
                vm_paddr_t         indirect_paddr;
                uint16_t           ndescs;
        } vq_descx[0];
};

/*
 * The maximum virtqueue size is 2^15. Use that value as the end of
 * descriptor chain terminator since it will never be a valid index
 * in the descriptor table. This is used to verify we are correctly
 * handling vq_free_cnt.
 */
#define VQ_RING_DESC_CHAIN_END 32768

#define VQASSERT(_vq, _exp, _msg, ...)                          \
    KASSERT((_exp),("%s: %s - "_msg, __func__, (_vq)->vq_name,  \
        ##__VA_ARGS__))

#define VQ_RING_ASSERT_VALID_IDX(_vq, _idx)                     \
    VQASSERT((_vq), (_idx) < (_vq)->vq_nentries,                \
        "invalid ring index: %d, max: %d", (_idx),              \
        (_vq)->vq_nentries)

#define VQ_RING_ASSERT_CHAIN_TERM(_vq)                          \
    VQASSERT((_vq), (_vq)->vq_desc_head_idx ==                  \
        VQ_RING_DESC_CHAIN_END, "full ring terminated "         \
        "incorrectly: head idx: %d", (_vq)->vq_desc_head_idx)

static int      virtqueue_init_indirect(struct virtqueue *vq, int);
static void     virtqueue_free_indirect(struct virtqueue *vq);
static void     virtqueue_init_indirect_list(struct virtqueue *,
                    struct vring_desc *);

static void     vq_ring_init(struct virtqueue *);
static void     vq_ring_update_avail(struct virtqueue *, uint16_t);
static uint16_t vq_ring_enqueue_segments(struct virtqueue *,
                    struct vring_desc *, uint16_t, struct sglist *, int, int);
static int      vq_ring_use_indirect(struct virtqueue *, int);
static void     vq_ring_enqueue_indirect(struct virtqueue *, void *,
                    struct sglist *, int, int);
static int      vq_ring_must_notify_host(struct virtqueue *);
static void     vq_ring_notify_host(struct virtqueue *);
static void     vq_ring_free_chain(struct virtqueue *, uint16_t);

uint64_t
virtqueue_filter_features(uint64_t features)
{
        uint64_t mask;

        mask = (1 << VIRTIO_TRANSPORT_F_START) - 1;
        mask |= VIRTIO_RING_F_INDIRECT_DESC;
        mask |= VIRTIO_RING_F_EVENT_IDX;

        return (features & mask);
}

int
virtqueue_alloc(device_t dev, uint16_t queue, uint16_t size, int align,
    vm_paddr_t highaddr, struct vq_alloc_info *info, struct virtqueue **vqp)
{
        struct virtqueue *vq;
        int error;

        *vqp = NULL;
        error = 0;

        if (size == 0) {
                device_printf(dev,
                    "virtqueue %d (%s) does not exist (size is zero)\n",
                    queue, info->vqai_name);
                return (ENODEV);
        } else if (!powerof2(size)) {
                device_printf(dev,
                    "virtqueue %d (%s) size is not a power of 2: %d\n",
                    queue, info->vqai_name, size);
                return (ENXIO);
        } else if (info->vqai_maxindirsz > VIRTIO_MAX_INDIRECT) {
                device_printf(dev, "virtqueue %d (%s) requested too many "
                    "indirect descriptors: %d, max %d\n",
                    queue, info->vqai_name, info->vqai_maxindirsz,
                    VIRTIO_MAX_INDIRECT);
                return (EINVAL);
        }

        vq = kmalloc(sizeof(struct virtqueue) +
            size * sizeof(struct vq_desc_extra), M_DEVBUF, M_INTWAIT | M_ZERO);
        if (vq == NULL) {
                device_printf(dev, "cannot allocate virtqueue\n");
                return (ENOMEM);
        }

        vq->vq_dev = dev;
        strlcpy(vq->vq_name, info->vqai_name, sizeof(vq->vq_name));
        vq->vq_queue_index = queue;
        vq->vq_alignment = align;
        vq->vq_nentries = size;
        vq->vq_free_cnt = size;
        vq->vq_intrhand = info->vqai_intr;
        vq->vq_intrhand_arg = info->vqai_intr_arg;

        if (VIRTIO_BUS_WITH_FEATURE(dev, VIRTIO_RING_F_EVENT_IDX) != 0)
                vq->vq_flags |= VIRTQUEUE_FLAG_EVENT_IDX;

        if (info->vqai_maxindirsz > 1) {
                error = virtqueue_init_indirect(vq, info->vqai_maxindirsz);
                if (error)
                        goto fail;
        }

        vq->vq_ring_size = round_page(vring_size(size, align));
        vq->vq_ring_mem = contigmalloc(vq->vq_ring_size, M_DEVBUF,
            M_WAITOK | M_ZERO, 0, highaddr, PAGE_SIZE, 0);
        if (vq->vq_ring_mem == NULL) {
                device_printf(dev,
                    "cannot allocate memory for virtqueue ring\n");
                error = ENOMEM;
                goto fail;
        }

        vq_ring_init(vq);
        virtqueue_disable_intr(vq);

        *vqp = vq;

fail:
        if (error)
                virtqueue_free(vq);

        return (error);
}

static int
virtqueue_init_indirect(struct virtqueue *vq, int indirect_size)
{
        device_t dev;
        struct vq_desc_extra *dxp;
        int i, size;

        dev = vq->vq_dev;

        if (VIRTIO_BUS_WITH_FEATURE(dev, VIRTIO_RING_F_INDIRECT_DESC) == 0) {
                /*
                 * Indirect descriptors requested by the driver but not
                 * negotiated. Return zero to keep the initialization
                 * going: we'll run fine without.
                 */
                if (bootverbose)
                        device_printf(dev, "virtqueue %d (%s) requested "
                            "indirect descriptors but not negotiated\n",
                            vq->vq_queue_index, vq->vq_name);
                return (0);
        }

        size = indirect_size * sizeof(struct vring_desc);
        vq->vq_max_indirect_size = indirect_size;
        vq->vq_indirect_mem_size = size;
        vq->vq_flags |= VIRTQUEUE_FLAG_INDIRECT;

        for (i = 0; i < vq->vq_nentries; i++) {
                dxp = &vq->vq_descx[i];

                dxp->indirect = contigmalloc(size, M_DEVBUF, M_WAITOK,
                    0, BUS_SPACE_MAXADDR, 16, 0);
                if (dxp->indirect == NULL) {
                        device_printf(dev, "cannot allocate indirect list\n");
                        return (ENOMEM);
                }

                dxp->indirect_paddr = vtophys(dxp->indirect);
                virtqueue_init_indirect_list(vq, dxp->indirect);
        }

        return (0);
}

static void
virtqueue_free_indirect(struct virtqueue *vq)
{
        struct vq_desc_extra *dxp;
        int i;

        for (i = 0; i < vq->vq_nentries; i++) {
                dxp = &vq->vq_descx[i];

                if (dxp->indirect == NULL)
                        break;

                contigfree(dxp->indirect, vq->vq_indirect_mem_size, M_DEVBUF);
                dxp->indirect = NULL;
                dxp->indirect_paddr = 0;
        }

        vq->vq_flags &= ~VIRTQUEUE_FLAG_INDIRECT;
        vq->vq_indirect_mem_size = 0;
}

static void
virtqueue_init_indirect_list(struct virtqueue *vq,
    struct vring_desc *indirect)
{
        int i;

        bzero(indirect, vq->vq_indirect_mem_size);

        for (i = 0; i < vq->vq_max_indirect_size - 1; i++)
                indirect[i].next = i + 1;
        indirect[i].next = VQ_RING_DESC_CHAIN_END;
}

int
virtqueue_reinit(struct virtqueue *vq, uint16_t size)
{
        struct vq_desc_extra *dxp;
        int i;

        if (vq->vq_nentries != size) {
                device_printf(vq->vq_dev,
                    "%s: '%s' changed size; old=%hu, new=%hu\n",
                    __func__, vq->vq_name, vq->vq_nentries, size);
                return (EINVAL);
        }

        /* Warn if the virtqueue was not properly cleaned up. */
        if (vq->vq_free_cnt != vq->vq_nentries) {
                device_printf(vq->vq_dev,
                    "%s: warning, '%s' virtqueue not empty, "
                    "leaking %d entries\n", __func__, vq->vq_name,
                    vq->vq_nentries - vq->vq_free_cnt);
        }

        vq->vq_desc_head_idx = 0;
        vq->vq_used_cons_idx = 0;
        vq->vq_queued_cnt = 0;
        vq->vq_free_cnt = vq->vq_nentries;

        /* To be safe, reset all our allocated memory. */
        bzero(vq->vq_ring_mem, vq->vq_ring_size);
        for (i = 0; i < vq->vq_nentries; i++) {
                dxp = &vq->vq_descx[i];
                dxp->cookie = NULL;
                dxp->ndescs = 0;
                if (vq->vq_flags & VIRTQUEUE_FLAG_INDIRECT)
                        virtqueue_init_indirect_list(vq, dxp->indirect);
        }

        vq_ring_init(vq);
        virtqueue_disable_intr(vq);

        return (0);
}

void
virtqueue_free(struct virtqueue *vq)
{

        if (vq->vq_free_cnt != vq->vq_nentries) {
                device_printf(vq->vq_dev, "%s: freeing non-empty virtqueue, "
                    "leaking %d entries\n", vq->vq_name,
                    vq->vq_nentries - vq->vq_free_cnt);
        }

        if (vq->vq_flags & VIRTQUEUE_FLAG_INDIRECT)
                virtqueue_free_indirect(vq);

        if (vq->vq_ring_mem != NULL) {
                contigfree(vq->vq_ring_mem, vq->vq_ring_size, M_DEVBUF);
                vq->vq_ring_size = 0;
                vq->vq_ring_mem = NULL;
        }

        kfree(vq, M_DEVBUF);
}

vm_paddr_t
virtqueue_paddr(struct virtqueue *vq)
{
        return (vtophys(vq->vq_ring_mem));
}

int
virtqueue_size(struct virtqueue *vq)
{
        return (vq->vq_nentries);
}

int
virtqueue_empty(struct virtqueue *vq)
{

        return (vq->vq_nentries == vq->vq_free_cnt);
}

int
virtqueue_full(struct virtqueue *vq)
{

        return (vq->vq_free_cnt == 0);
}

void
virtqueue_notify(struct virtqueue *vq, lwkt_serialize_t interlock)
{
        /* Ensure updated avail->idx is visible to host. */
        cpu_mfence();

        if (vq_ring_must_notify_host(vq)) {
                if (interlock != NULL)
                        lwkt_serialize_exit(interlock);
                vq_ring_notify_host(vq);
                if (interlock != NULL)
                        lwkt_serialize_enter(interlock);
        }
        vq->vq_queued_cnt = 0;
}

int
virtqueue_nused(struct virtqueue *vq)
{
        uint16_t used_idx, nused;

        used_idx = vq->vq_ring.used->idx;
        nused = (uint16_t)(used_idx - vq->vq_used_cons_idx);
        VQASSERT(vq, nused <= vq->vq_nentries, "used more than available");

        return (nused);
}

int
virtqueue_intr(struct virtqueue *vq)
{

        if (vq->vq_intrhand == NULL ||
            vq->vq_used_cons_idx == vq->vq_ring.used->idx)
                return (0);

        vq->vq_intrhand(vq->vq_intrhand_arg);

        return (1);
}

/*
 * Enable interrupts on a given virtqueue. Returns 1 if there are
 * additional entries to process on the virtqueue after we return.
 */
int
virtqueue_enable_intr(struct virtqueue *vq)
{
        /*
         * Enable interrupts, making sure we get the latest
         * index of what's already been consumed.
         */
        vq->vq_ring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
        if (vq->vq_flags & VIRTQUEUE_FLAG_EVENT_IDX) {
                vring_used_event(&vq->vq_ring) = vq->vq_used_cons_idx;
        } else {
               vq->vq_ring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
        }

        cpu_mfence();

        /*
         * Additional items may have been consumed in the time between
         * since we last checked and enabled interrupts above. Let our
         * caller know so it processes the new entries.
         */
        if (vq->vq_used_cons_idx != vq->vq_ring.used->idx)
                return (1);

        return (0);
}

int
virtqueue_postpone_intr(struct virtqueue *vq)
{
        uint16_t ndesc;

        /*
         * Postpone until at least half of the available descriptors
         * have been consumed.
         *
         * XXX Adaptive factor? (Linux uses 3/4)
         */
        ndesc = (uint16_t)(vq->vq_ring.avail->idx - vq->vq_used_cons_idx) / 2;

        if (vq->vq_flags & VIRTQUEUE_FLAG_EVENT_IDX)
                vring_used_event(&vq->vq_ring) = vq->vq_used_cons_idx + ndesc;
        else
                vq->vq_ring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;

        cpu_mfence();

        /*
         * Enough items may have already been consumed to meet our
         * threshold since we last checked. Let our caller know so
         * it processes the new entries.
         */
        if (virtqueue_nused(vq) > ndesc)
                return (1);

        return (0);
}

void
virtqueue_disable_intr(struct virtqueue *vq)
{
        /*
         * Note this is only considered a hint to the host.
         */
        if ((vq->vq_flags & VIRTQUEUE_FLAG_EVENT_IDX) == 0)
                vq->vq_ring.avail->flags |= VRING_AVAIL_F_NO_INTERRUPT;
}

int
virtqueue_enqueue(struct virtqueue *vq, void *cookie, struct sglist *sg,
    int readable, int writable)
{
        struct vq_desc_extra *dxp;
        int needed;
        uint16_t head_idx, idx;

        needed = readable + writable;

        VQASSERT(vq, cookie != NULL, "enqueuing with no cookie");
        VQASSERT(vq, needed == sg->sg_nseg,
            "segment count mismatch, %d, %d", needed, sg->sg_nseg);
        VQASSERT(vq,
            needed <= vq->vq_nentries || needed <= vq->vq_max_indirect_size,
            "too many segments to enqueue: %d, %d/%d", needed,
            vq->vq_nentries, vq->vq_max_indirect_size);

        if (needed < 1)
                return (EINVAL);
        if (vq->vq_free_cnt == 0)
                return (ENOSPC);

        if (vq_ring_use_indirect(vq, needed)) {
                vq_ring_enqueue_indirect(vq, cookie, sg, readable, writable);
                return (0);
        } else if (vq->vq_free_cnt < needed)
                return (EMSGSIZE);

        head_idx = vq->vq_desc_head_idx;
        VQ_RING_ASSERT_VALID_IDX(vq, head_idx);
        dxp = &vq->vq_descx[head_idx];

        VQASSERT(vq, dxp->cookie == NULL,
            "cookie already exists for index %d", head_idx);
        dxp->cookie = cookie;
        dxp->ndescs = needed;

        idx = vq_ring_enqueue_segments(vq, vq->vq_ring.desc, head_idx,
            sg, readable, writable);

        vq->vq_desc_head_idx = idx;
        vq->vq_free_cnt -= needed;
        if (vq->vq_free_cnt == 0)
                VQ_RING_ASSERT_CHAIN_TERM(vq);
        else
                VQ_RING_ASSERT_VALID_IDX(vq, idx);

        vq_ring_update_avail(vq, head_idx);

        return (0);
}

void *
virtqueue_dequeue(struct virtqueue *vq, uint32_t *len)
{
        struct vring_used_elem *uep;
        void *cookie;
        uint16_t used_idx, desc_idx;

        if (vq->vq_used_cons_idx == vq->vq_ring.used->idx)
                return (NULL);

        used_idx = vq->vq_used_cons_idx++ & (vq->vq_nentries - 1);
        uep = &vq->vq_ring.used->ring[used_idx];

        cpu_lfence();
        desc_idx = (uint16_t) uep->id;
        if (len != NULL)
                *len = uep->len;

        vq_ring_free_chain(vq, desc_idx);

        cookie = vq->vq_descx[desc_idx].cookie;
        VQASSERT(vq, cookie != NULL, "no cookie for index %d", desc_idx);
        vq->vq_descx[desc_idx].cookie = NULL;

        return (cookie);
}

void *
virtqueue_poll(struct virtqueue *vq, uint32_t *len)
{
        void *cookie;

        /* We only poll the virtqueue when dumping to virtio-blk */
        while ((cookie = virtqueue_dequeue(vq, len)) == NULL)
                ;       

        return (cookie);
}

void *
virtqueue_drain(struct virtqueue *vq, int *last)
{
        void *cookie;
        int idx;

        cookie = NULL;
        idx = *last;

        while (idx < vq->vq_nentries && cookie == NULL) {
                if ((cookie = vq->vq_descx[idx].cookie) != NULL) {
                        vq->vq_descx[idx].cookie = NULL;
                        /* Free chain to keep free count consistent. */
                        vq_ring_free_chain(vq, idx);
                }
                idx++;
        }

        *last = idx;

        return (cookie);
}

void
virtqueue_dump(struct virtqueue *vq)
{

        if (vq == NULL)
                return;

        kprintf("VQ: %s - size=%d; free=%d; used=%d; queued=%d; "
            "desc_head_idx=%d; avail.idx=%d; used_cons_idx=%d; "
            "used.idx=%d; avail.flags=0x%x; used.flags=0x%x\n",
            vq->vq_name, vq->vq_nentries, vq->vq_free_cnt,
            virtqueue_nused(vq), vq->vq_queued_cnt, vq->vq_desc_head_idx,
            vq->vq_ring.avail->idx, vq->vq_used_cons_idx,
            vq->vq_ring.used->idx, vq->vq_ring.avail->flags,
            vq->vq_ring.used->flags);
}

static void
vq_ring_init(struct virtqueue *vq)
{
        struct vring *vr;
        char *ring_mem;
        int i, size;

        ring_mem = vq->vq_ring_mem;
        size = vq->vq_nentries;
        vr = &vq->vq_ring;

        vring_init(vr, size, ring_mem, vq->vq_alignment);

        for (i = 0; i < size - 1; i++)
                vr->desc[i].next = i + 1;
        vr->desc[i].next = VQ_RING_DESC_CHAIN_END;
}

static void
vq_ring_update_avail(struct virtqueue *vq, uint16_t desc_idx)
{
        uint16_t avail_idx;

        /*
         * Place the head of the descriptor chain into the next slot and make
         * it usable to the host. The chain is made available now rather than
         * deferring to virtqueue_notify() in the hopes that if the host is
         * currently running on another CPU, we can keep it processing the new
         * descriptor.
         */
        avail_idx = vq->vq_ring.avail->idx & (vq->vq_nentries - 1);
        vq->vq_ring.avail->ring[avail_idx] = desc_idx;

        cpu_sfence();
        vq->vq_ring.avail->idx++;

        /* Keep pending count until virtqueue_notify() for debugging. */
        vq->vq_queued_cnt++;
}

static uint16_t
vq_ring_enqueue_segments(struct virtqueue *vq, struct vring_desc *desc,
    uint16_t head_idx, struct sglist *sg, int readable, int writable)
{
        struct sglist_seg *seg;
        struct vring_desc *dp;
        int i, needed;
        uint16_t idx;

        needed = readable + writable;

        for (i = 0, idx = head_idx, seg = sg->sg_segs;
             i < needed;
             i++, idx = dp->next, seg++) {
                VQASSERT(vq, idx != VQ_RING_DESC_CHAIN_END,
                    "premature end of free desc chain");

                dp = &desc[idx];
                dp->addr = seg->ss_paddr;
                dp->len = seg->ss_len;
                dp->flags = 0;

                if (i < needed - 1)
                        dp->flags |= VRING_DESC_F_NEXT;
                if (i >= readable)
                        dp->flags |= VRING_DESC_F_WRITE;
        }

        return (idx);
}

static int
vq_ring_use_indirect(struct virtqueue *vq, int needed)
{

        if ((vq->vq_flags & VIRTQUEUE_FLAG_INDIRECT) == 0)
                return (0);

        if (vq->vq_max_indirect_size < needed)
                return (0);

        if (needed < 2)
                return (0);

        return (1);
}

static void
vq_ring_enqueue_indirect(struct virtqueue *vq, void *cookie,
    struct sglist *sg, int readable, int writable)
{
        struct vring_desc *dp;
        struct vq_desc_extra *dxp;
        int needed;
        uint16_t head_idx;

        needed = readable + writable;
        VQASSERT(vq, needed <= vq->vq_max_indirect_size,
            "enqueuing too many indirect descriptors");

        head_idx = vq->vq_desc_head_idx;
        VQ_RING_ASSERT_VALID_IDX(vq, head_idx);
        dp = &vq->vq_ring.desc[head_idx];
        dxp = &vq->vq_descx[head_idx];

        VQASSERT(vq, dxp->cookie == NULL,
            "cookie already exists for index %d", head_idx);
        dxp->cookie = cookie;
        dxp->ndescs = 1;

        dp->addr = dxp->indirect_paddr;
        dp->len = needed * sizeof(struct vring_desc);
        dp->flags = VRING_DESC_F_INDIRECT;

        vq_ring_enqueue_segments(vq, dxp->indirect, 0,
            sg, readable, writable);

        vq->vq_desc_head_idx = dp->next;
        vq->vq_free_cnt--;
        if (vq->vq_free_cnt == 0)
                VQ_RING_ASSERT_CHAIN_TERM(vq);
        else
                VQ_RING_ASSERT_VALID_IDX(vq, vq->vq_desc_head_idx);

        vq_ring_update_avail(vq, head_idx);
}

static int
vq_ring_must_notify_host(struct virtqueue *vq)
{
        uint16_t new_idx, prev_idx, event_idx;

        if (vq->vq_flags & VIRTQUEUE_FLAG_EVENT_IDX) {
                new_idx = vq->vq_ring.avail->idx;
                prev_idx = new_idx - vq->vq_queued_cnt;
                event_idx = vring_avail_event(&vq->vq_ring);

                return (vring_need_event(event_idx, new_idx, prev_idx) != 0);
        }

        return ((vq->vq_ring.used->flags & VRING_USED_F_NO_NOTIFY) == 0);
}

static void
vq_ring_notify_host(struct virtqueue *vq)
{
        VIRTIO_BUS_NOTIFY_VQ(vq->vq_dev, vq->vq_queue_index);
}

static void
vq_ring_free_chain(struct virtqueue *vq, uint16_t desc_idx)
{
        struct vring_desc *dp;
        struct vq_desc_extra *dxp;

        VQ_RING_ASSERT_VALID_IDX(vq, desc_idx);
        dp = &vq->vq_ring.desc[desc_idx];
        dxp = &vq->vq_descx[desc_idx];

        if (vq->vq_free_cnt == 0)
                VQ_RING_ASSERT_CHAIN_TERM(vq);

        vq->vq_free_cnt += dxp->ndescs;
        dxp->ndescs--;

        if ((dp->flags & VRING_DESC_F_INDIRECT) == 0) {
                while (dp->flags & VRING_DESC_F_NEXT) {
                        VQ_RING_ASSERT_VALID_IDX(vq, dp->next);
                        dp = &vq->vq_ring.desc[dp->next];
                        dxp->ndescs--;
                }
        }
        VQASSERT(vq, dxp->ndescs == 0, "failed to free entire desc chain");

        /*
         * We must append the existing free chain, if any, to the end of
         * newly freed chain. If the virtqueue was completely used, then
         * head would be VQ_RING_DESC_CHAIN_END (ASSERTed above).
         */
        dp->next = vq->vq_desc_head_idx;
        vq->vq_desc_head_idx = desc_idx;
}