static void scsi_do_read(void *opaque, int ret)
{
    SCSIDiskReq *r = opaque;
    SCSIDiskState *s = DO_UPCAST(SCSIDiskState, qdev, r->req.dev);
    uint32_t n;
    if (r->req.aiocb != NULL) {
        r->req.aiocb = NULL;
        bdrv_acct_done(s->qdev.conf.bs, &r->acct);
    if (ret < 0) {
        if (scsi_handle_rw_error(r, -ret)) {
            goto done;
    if (r->req.sg) {
        dma_acct_start(s->qdev.conf.bs, &r->acct, r->req.sg, BDRV_ACCT_READ);
        r->req.resid -= r->req.sg->size;
        r->req.aiocb = dma_bdrv_read(s->qdev.conf.bs, r->req.sg, r->sector,
                                     scsi_dma_complete, r);
    } else {
        n = scsi_init_iovec(r, SCSI_DMA_BUF_SIZE);
        bdrv_acct_start(s->qdev.conf.bs, &r->acct, n * BDRV_SECTOR_SIZE, BDRV_ACCT_READ);
        r->req.aiocb = bdrv_aio_readv(s->qdev.conf.bs, r->sector, &r->qiov, n,
                                      scsi_read_complete, r);
done:
    if (!r->req.io_canceled) {
        scsi_req_unref(&r->req);