static int coroutine_fn bdrv_co_do_copy_on_readv(BlockDriverState *bs, int64_t sector_num, int nb_sectors, QEMUIOVector *qiov) { /* Perform I/O through a temporary buffer so that users who scribble over * their read buffer while the operation is in progress do not end up * modifying the image file. This is critical for zero-copy guest I/O * where anything might happen inside guest memory. */ void *bounce_buffer; BlockDriver *drv = bs->drv; struct iovec iov; QEMUIOVector bounce_qiov; int64_t cluster_sector_num; int cluster_nb_sectors; size_t skip_bytes; int ret; /* Cover entire cluster so no additional backing file I/O is required when * allocating cluster in the image file. */ bdrv_round_to_clusters(bs, sector_num, nb_sectors, &cluster_sector_num, &cluster_nb_sectors); trace_bdrv_co_do_copy_on_readv(bs, sector_num, nb_sectors, cluster_sector_num, cluster_nb_sectors); iov.iov_len = cluster_nb_sectors * BDRV_SECTOR_SIZE; iov.iov_base = bounce_buffer = qemu_try_blockalign(bs, iov.iov_len); if (bounce_buffer == NULL) { ret = -ENOMEM; goto err; } qemu_iovec_init_external(&bounce_qiov, &iov, 1); ret = drv->bdrv_co_readv(bs, cluster_sector_num, cluster_nb_sectors, &bounce_qiov); if (ret < 0) { goto err; } if (drv->bdrv_co_write_zeroes && buffer_is_zero(bounce_buffer, iov.iov_len)) { ret = bdrv_co_do_write_zeroes(bs, cluster_sector_num, cluster_nb_sectors, 0); } else { /* This does not change the data on the disk, it is not necessary * to flush even in cache=writethrough mode. */ ret = drv->bdrv_co_writev(bs, cluster_sector_num, cluster_nb_sectors, &bounce_qiov); } if (ret < 0) { /* It might be okay to ignore write errors for guest requests. If this * is a deliberate copy-on-read then we don't want to ignore the error. * Simply report it in all cases. */ goto err; } skip_bytes = (sector_num - cluster_sector_num) * BDRV_SECTOR_SIZE; qemu_iovec_from_buf(qiov, 0, bounce_buffer + skip_bytes, nb_sectors * BDRV_SECTOR_SIZE); err: qemu_vfree(bounce_buffer); return ret; }