Re: [Qemu-devel] [PATCH] block: Formats don't need CONSISTENT_READ with NO_IO

[Eric Blake]

On 11/30/2017 10:44 AM, Kevin Wolf wrote:
> Commit 1f4ad7d fixed 'qemu-img info' for raw images that are currently
> in use as a mirror target. It is not enough for image formats, though,
> as these still unconditionally request BLK_PERM_CONSISTENT_READ.
>
> As this permission is meaningless unless you do actual I/O on the image,
> drop the requirement and allow 'qemu-img info' even for image formats
> under conditions where BLK_PERM_CONSISTENT_READ can't be granted.
>
> Signed-off-by: Kevin Wolf <address@hidden>
> ---
>   block.c | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)

> @@ -1936,7 +1938,9 @@ void bdrv_format_default_perms(BlockDriverState *bs, 
> BdrvChild *c,
>   
>           /* bs->file always needs to be consistent because of the metadata. We
>            * can never allow other users to resize or write to it. */
> -        perm |= BLK_PERM_CONSISTENT_READ;
> +        if (!(flags & BDRV_O_NO_IO)) {
> +            perm |= BLK_PERM_CONSISTENT_READ;

I thought BDRV_O_NO_IO only means we aren't doing I/O on guest-visible 
data, but doesn't stop us from reading the metadata.  The comment is 
telling: if we can read metadata, then we depend on CONSISTENT_READ for 
the metadata to be stable (even if we don't care about guest data 
consistency).

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org