Re: [Qemu-devel] [PATCH v2 10/11] block: add option 'backing' to -drive options

[Paolo Bonzini <pbonzini@redhat.com>]

Il 17/07/2013 11:42, Fam Zheng ha scritto:
> This option allows overriding backing hd of drive. If the target drive
> exists, it's referenced as the backing file and refcount incremented.
> 
> Example:
>     qemu-system-x86_64 -drive \
>         file.filename=foo.qcow2,if=none,id=foo \
>         -drive file=bar.qcow2,backing=foo

I guess this is where we need the soft reference.

This has a _lot_ of potential for misuse, I think Kevin bashed me and
Federico very heavily when we tried to do something similar.

block/backup.c is the right place where we can override the backing hd
of the drive.  Perhaps we can add a way to open a file with
BDRV_O_NO_BACKING from the command line.

Paolo

> Signed-off-by: Fam Zheng <famz@redhat.com>
> ---
>  block.c | 30 ++++++++++++++++++++++++------
>  1 file changed, 24 insertions(+), 6 deletions(-)
> 
> diff --git a/block.c b/block.c
> index 147a448..31fab07 100644
> --- a/block.c
> +++ b/block.c
> @@ -1083,12 +1083,30 @@ int bdrv_open(BlockDriverState *bs, const char *filename, QDict *options,
>  
>      /* If there is a backing file, use it */
>      if ((flags & BDRV_O_NO_BACKING) == 0) {
> -        QDict *backing_options;
> -
> -        extract_subqdict(options, &backing_options, "backing.");
> -        ret = bdrv_open_backing_file(bs, backing_options);
> -        if (ret < 0) {
> -            goto close_and_fail;
> +        const char *backing_drive;
> +        backing_drive = qdict_get_try_str(options, "backing");
> +        if (backing_drive) {
> +            bs->backing_hd = bdrv_find(backing_drive);
> +            if (bs->backing_hd) {
> +                bdrv_ref(bs->backing_hd, false);
> +                pstrcpy(bs->backing_file, sizeof(bs->backing_file),
> +                        bs->backing_hd->filename);
> +                pstrcpy(bs->backing_format, sizeof(bs->backing_format),
> +                        bs->backing_hd->drv->format_name);
> +            } else {
> +                qerror_report(ERROR_CLASS_DEVICE_NOT_FOUND,
> +                        "Can't find drive with name %s\n", backing_drive);
> +                ret = -EINVAL;
> +                goto close_and_fail;
> +            }
> +            qdict_del(options, "backing");
> +        } else {
> +            QDict *backing_options;
> +            extract_subqdict(options, &backing_options, "backing.");
> +            ret = bdrv_open_backing_file(bs, backing_options);
> +            if (ret < 0) {
> +                goto close_and_fail;
> +            }
>          }
>      }
>  
>