static int coroutine_fn bdrv_co_do_copy_on_readv(BdrvChild *child, int64_t offset, unsigned int bytes, QEMUIOVector *qiov) { BlockDriverState *bs = child->bs; /* 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_offset; unsigned int cluster_bytes; size_t skip_bytes; int ret; /* FIXME We cannot require callers to have write permissions when all they * are doing is a read request. If we did things right, write permissions * would be obtained anyway, but internally by the copy-on-read code. As * long as it is implemented here rather than in a separate filter driver, * the copy-on-read code doesn't have its own BdrvChild, however, for which * it could request permissions. Therefore we have to bypass the permission * system for the moment. */ // assert(child->perm & (BLK_PERM_WRITE_UNCHANGED | BLK_PERM_WRITE)); /* Cover entire cluster so no additional backing file I/O is required when * allocating cluster in the image file. */ bdrv_round_to_clusters(bs, offset, bytes, &cluster_offset, &cluster_bytes); trace_bdrv_co_do_copy_on_readv(bs, offset, bytes, cluster_offset, cluster_bytes); iov.iov_len = cluster_bytes; 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 = bdrv_driver_preadv(bs, cluster_offset, cluster_bytes, &bounce_qiov, 0); if (ret < 0) { goto err; } bdrv_debug_event(bs, BLKDBG_COR_WRITE); if (drv->bdrv_co_pwrite_zeroes && buffer_is_zero(bounce_buffer, iov.iov_len)) { /* FIXME: Should we (perhaps conditionally) be setting * BDRV_REQ_MAY_UNMAP, if it will allow for a sparser copy * that still correctly reads as zero? */ ret = bdrv_co_do_pwrite_zeroes(bs, cluster_offset, cluster_bytes, 0); } else { /* This does not change the data on the disk, it is not necessary * to flush even in cache=writethrough mode. */ ret = bdrv_driver_pwritev(bs, cluster_offset, cluster_bytes, &bounce_qiov, 0); } 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 = offset - cluster_offset; qemu_iovec_from_buf(qiov, 0, bounce_buffer + skip_bytes, bytes); err: qemu_vfree(bounce_buffer); return ret; }