static ssize_t virtio_net_receive(NetClientState *nc, const uint8_t *buf, size_t size)
{
    VirtIONet *n = qemu_get_nic_opaque(nc);
    VirtIONetQueue *q = virtio_net_get_subqueue(nc);
    VirtIODevice *vdev = VIRTIO_DEVICE(n);
    struct iovec mhdr_sg[VIRTQUEUE_MAX_SIZE];
    struct virtio_net_hdr_mrg_rxbuf mhdr;
    unsigned mhdr_cnt = 0;
    size_t offset, i, guest_offset;

    if (!virtio_net_can_receive(nc)) {
        return -1;
    }

    /* hdr_len refers to the header we supply to the guest */
    if (!virtio_net_has_buffers(q, size + n->guest_hdr_len - n->host_hdr_len)) {
        return 0;
    }

    if (!receive_filter(n, buf, size))
        return size;

    offset = i = 0;

    while (offset < size) {
        VirtQueueElement elem;
        int len, total;
        const struct iovec *sg = elem.in_sg;

        total = 0;

        if (virtqueue_pop(q->rx_vq, &elem) == 0) {
            if (i == 0)
                return -1;
            error_report("virtio-net unexpected empty queue: "
                    "i %zd mergeable %d offset %zd, size %zd, "
                    "guest hdr len %zd, host hdr len %zd guest features 0x%x",
                    i, n->mergeable_rx_bufs, offset, size,
                    n->guest_hdr_len, n->host_hdr_len, vdev->guest_features);
            exit(1);
        }

        if (elem.in_num < 1) {
            error_report("virtio-net receive queue contains no in buffers");
            exit(1);
        }

        if (i == 0) {
            assert(offset == 0);
            if (n->mergeable_rx_bufs) {
                mhdr_cnt = iov_copy(mhdr_sg, ARRAY_SIZE(mhdr_sg),
                                    sg, elem.in_num,
                                    offsetof(typeof(mhdr), num_buffers),
                                    sizeof(mhdr.num_buffers));
            }

            receive_header(n, sg, elem.in_num, buf, size);
            offset = n->host_hdr_len;
            total += n->guest_hdr_len;
            guest_offset = n->guest_hdr_len;
        } else {
            guest_offset = 0;
        }

        /* copy in packet.  ugh */
        len = iov_from_buf(sg, elem.in_num, guest_offset,
                           buf + offset, size - offset);
        total += len;
        offset += len;
        /* If buffers can't be merged, at this point we
         * must have consumed the complete packet.
         * Otherwise, drop it. */
        if (!n->mergeable_rx_bufs && offset < size) {
#if 0
            error_report("virtio-net truncated non-mergeable packet: "
                         "i %zd mergeable %d offset %zd, size %zd, "
                         "guest hdr len %zd, host hdr len %zd",
                         i, n->mergeable_rx_bufs,
                         offset, size, n->guest_hdr_len, n->host_hdr_len);
#endif
            return size;
        }

        /* signal other side */
        virtqueue_fill(q->rx_vq, &elem, total, i++);
    }

    if (mhdr_cnt) {
        stw_p(&mhdr.num_buffers, i);
        iov_from_buf(mhdr_sg, mhdr_cnt,
                     0,
                     &mhdr.num_buffers, sizeof mhdr.num_buffers);
    }

    virtqueue_flush(q->rx_vq, i);
    virtio_notify(vdev, q->rx_vq);

    return size;
}
static ssize_t virtio_net_receive(NetClientState *nc, const uint8_t *buf, size_t size)
{
    VirtIONet *n = qemu_get_nic_opaque(nc);
    VirtIONetQueue *q = virtio_net_get_subqueue(nc);
    VirtIODevice *vdev = VIRTIO_DEVICE(n);
    struct iovec mhdr_sg[VIRTQUEUE_MAX_SIZE];
    struct virtio_net_hdr_mrg_rxbuf mhdr;
    unsigned mhdr_cnt = 0;
    size_t offset, i, guest_offset;

    if (!virtio_net_can_receive(nc)) {
        return -1;
    }

     
    if (!virtio_net_has_buffers(q, size + n->guest_hdr_len - n->host_hdr_len)) {
        return 0;
    }

    if (!receive_filter(n, buf, size))
        return size;

    offset = i = 0;

    while (offset < size) {
        VirtQueueElement elem;
        int len, total;
        const struct iovec *sg = elem.in_sg;

        total = 0;

        if (virtqueue_pop(q->rx_vq, &elem) == 0) {
            if (i == 0)
                return -1;
            error_report("virtio-net unexpected empty queue: "
                    "i %zd mergeable %d offset %zd, size %zd, "
                    "guest hdr len %zd, host hdr len %zd guest features 0x%x",
                    i, n->mergeable_rx_bufs, offset, size,
                    n->guest_hdr_len, n->host_hdr_len, vdev->guest_features);
            exit(1);
        }

        if (elem.in_num < 1) {
            error_report("virtio-net receive queue contains no in buffers");
            exit(1);
        }

        if (i == 0) {
            assert(offset == 0);
            if (n->mergeable_rx_bufs) {
                mhdr_cnt = iov_copy(mhdr_sg, ARRAY_SIZE(mhdr_sg),
                                    sg, elem.in_num,
                                    offsetof(typeof(mhdr), num_buffers),
                                    sizeof(mhdr.num_buffers));
            }

            receive_header(n, sg, elem.in_num, buf, size);
            offset = n->host_hdr_len;
            total += n->guest_hdr_len;
            guest_offset = n->guest_hdr_len;
        } else {
            guest_offset = 0;
        }

         
        len = iov_from_buf(sg, elem.in_num, guest_offset,
                           buf + offset, size - offset);
        total += len;
        offset += len;
         
        if (!n->mergeable_rx_bufs && offset < size) {
#if 0
            error_report("virtio-net truncated non-mergeable packet: "
                         "i %zd mergeable %d offset %zd, size %zd, "
                         "guest hdr len %zd, host hdr len %zd",
                         i, n->mergeable_rx_bufs,
                         offset, size, n->guest_hdr_len, n->host_hdr_len);
#endif
            return size;
        }

         
        virtqueue_fill(q->rx_vq, &elem, total, i++);
    }

    if (mhdr_cnt) {
        stw_p(&mhdr.num_buffers, i);
        iov_from_buf(mhdr_sg, mhdr_cnt,
                     0,
                     &mhdr.num_buffers, sizeof mhdr.num_buffers);
    }

    virtqueue_flush(q->rx_vq, i);
    virtio_notify(vdev, q->rx_vq);

    return size;
}