Re: [Qemu-devel] [PATCH v3 2/4] block: Add 'ignore-backing' field to BlockdevOptionsGenericCOWFormat

[Eric Blake]

On 09/10/2015 07:39 AM, Alberto Garcia wrote:
> If set to true, the image will be opened with the BDRV_O_NO_BACKING
> flag. This is useful for creating snapshots using images opened with
> blockdev-add, since they are not supposed to have a backing image
> before the operation.
> 
> Signed-off-by: Alberto Garcia <address@hidden>
> ---
>  block.c              | 5 +++++
>  qapi/block-core.json | 6 +++++-
>  2 files changed, 10 insertions(+), 1 deletion(-)

> 
> diff --git a/block.c b/block.c
> index 22d3b0e..4be32fb 100644
> --- a/block.c
> +++ b/block.c
> @@ -1469,6 +1469,11 @@ static int bdrv_open_inherit(BlockDriverState **pbs, 
> const char *filename,
>  
>      assert(drvname || !(flags & BDRV_O_PROTOCOL));
>  
> +    if (qdict_get_try_bool(options, "ignore-backing", false)) {
> +        flags |= BDRV_O_NO_BACKING;
> +    }
> +    qdict_del(options, "ignore-backing");

What happens if the user specified "ignore-backing":true, "backing":...?
 Should that be a hard error?

>  { 'struct': 'BlockdevOptionsGenericCOWFormat',
>    'base': 'BlockdevOptionsGenericFormat',
> -  'data': { '*backing': 'BlockdevRef' } }
> +  'data': { '*backing': 'BlockdevRef',
> +            '*ignore-backing': 'bool' } }

Depending on whether the answer to my question is that we already behave
sanely and don't leave a BlockdevRef dangling if the caller mixes the
two approaches, then:
Reviewed-by: Eric Blake <address@hidden>

But design-wise, would it make sense to support:

"backing":null

as an explicit request to not open a backing file?  Right now, qapi does
not have a way to express 'null' as part of an alternate type; but if it
did, BlockdevRef would merely add 'null' as one of its allowed
alternates.  Then we wouldn't need ignore-backing from the QMP
perspective. But I'm still not sure how it would map to the command line
perspective.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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