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block: add flag to indicate that no I/O will be performed

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When opening an image it is useful to know whether the caller
intends to perform I/O on the image or not. In the case of
encrypted images this will allow the block driver to avoid
having to prompt for decryption keys when we merely want to
query header metadata about the image. eg qemu-img info

This flag is enforced at the top level only, since even if
we don't want todo I/O on the 'qcow2' file payload, the
underlying 'file' driver will still need todo I/O to read
the qcow2 header, for example.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Daniel P. Berrange 2016-03-21 14:11:42 +00:00 • committed by Kevin Wolf
parent 5430215699
commit abb06c5ac1
4 changed files with 28 additions and 24 deletions
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2
block/io.c
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@ -844,6 +844,7 @@ static int coroutine_fn bdrv_aligned_preadv(BlockDriverState *bs,
assert((offset & (BDRV_SECTOR_SIZE - 1)) == 0);
assert((bytes & (BDRV_SECTOR_SIZE - 1)) == 0);
assert(!qiov || bytes == qiov->size);
assert((bs->open_flags & BDRV_O_NO_IO) == 0);
/* Handle Copy on Read and associated serialisation */
if (flags & BDRV_REQ_COPY_ON_READ) {
@ -1130,6 +1131,7 @@ static int coroutine_fn bdrv_aligned_pwritev(BlockDriverState *bs,
assert((offset & (BDRV_SECTOR_SIZE - 1)) == 0);
assert((bytes & (BDRV_SECTOR_SIZE - 1)) == 0);
assert(!qiov || bytes == qiov->size);
assert((bs->open_flags & BDRV_O_NO_IO) == 0);
waited = wait_serialising_requests(req);
assert(!waited || !req->serialising);