From: MORITA Kazutaka <morita.kazut...@lab.ntt.co.jp>
While Asias is debugging an issue creating qcow2 images on top of
non-file protocols. It boils down to this example using NBD:
$ qemu-io -c 'open -g nbd+unix:///?socket=/tmp/nbd.sock' -c 'read -v 0 512'
Notice the open -g option to set bs->growable. This means you can
read/write beyond end of file. Reading beyond end of file is supposed
to produce zeroes.
We rely on this behavior in qcow2_create2() during qcow2 image
creation. We create a new file and then write the qcow2 header
structure using bdrv_pwrite(). Since QCowHeader is not a multiple of
sector size, block.c first uses bdrv_read() on the empty file to fetch
the first sector (should be all zeroes).
Here is the output from the qemu-io NBD example above:
$ qemu-io -c 'open -g nbd+unix:///?socket=/tmp/nbd.sock' -c 'read -v 0 512'
00000000: ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ................
00000010: ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ................
00000020: ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ................
...
We are not zeroing the buffer! As a result qcow2 image creation on top
of protocols is not guaranteed to work even when file creation is
supported by the protocol.
Signed-off-by: MORITA Kazutaka <morita.kazut...@lab.ntt.co.jp>
Signed-off-by: Asias He <as...@redhat.com>
---
block.c | 30 +++++++++++++++++++++++++++++-
1 file changed, 29 insertions(+), 1 deletion(-)
diff --git a/block.c b/block.c
index 01b66d8..deaf0a0 100644
--- a/block.c
+++ b/block.c
@@ -2544,7 +2544,35 @@ static int coroutine_fn
bdrv_co_do_readv(BlockDriverState *bs,
}
}
- ret = drv->bdrv_co_readv(bs, sector_num, nb_sectors, qiov);
+ if (!bs->drv->protocol_name) {
+ ret = drv->bdrv_co_readv(bs, sector_num, nb_sectors, qiov);
+ } else {
+ /* NBD doesn't support reading beyond end of file. */
+ int64_t len, total_sectors, max_nb_sectors;
+
+ len = bdrv_getlength(bs);
+ if (len < 0) {
+ ret = len;
+ goto out;
+ }
+
+ total_sectors = len >> BDRV_SECTOR_BITS;
+ max_nb_sectors = MAX(0, total_sectors - sector_num);
+ if (max_nb_sectors > 0) {
+ ret = drv->bdrv_co_readv(bs, sector_num,
+ MIN(nb_sectors, max_nb_sectors), qiov);
+ } else {
+ ret = 0;
+ }
+
+ /* Reading beyond end of file is supposed to produce zeroes */
+ if (ret == 0 && total_sectors < sector_num + nb_sectors) {
+ size_t offset = MAX(0, total_sectors - sector_num);
+ size_t bytes = (sector_num + nb_sectors - offset) *
+ BDRV_SECTOR_SIZE;
+ qemu_iovec_memset(qiov, offset * BDRV_SECTOR_SIZE, 0, bytes);
+ }
+ }
out:
tracked_request_end(&req);
--
1.8.3.1
