Re: [Qemu-block] [PATCH v3 1/4] block: Pass bdrv_* methods to bs->file by default

[Eric Blake]

On 07/04/2017 04:44 AM, Kevin Wolf wrote:
> Am 04.07.2017 um 10:40 hat Manos Pitsidianakis geschrieben:
>> The following functions fail if bs->drv does not implement them:
>>

>> @@ -511,6 +513,8 @@ int bdrv_probe_geometry(BlockDriverState *bs, HDGeometry 
>> *geo)
>>  
>>      if (drv && drv->bdrv_probe_geometry) {
>>          return drv->bdrv_probe_geometry(bs, geo);
>> +    } else if (drv && bs->file && bs->file->bs) {
>> +        return bdrv_probe_geometry(bs->file->bs, geo);
>>      }
>>  
>>      return -ENOTSUP;
> 
> The probed geometry would refer to the physical image file, not to the
> actually presented image. So propagating this to bs->file is wrong for
> image formats.
> 
> Possibly checking .is_filter in addition would be enough.

In fact, this made me think: if we define .is_filter as an enum instead
of a bool, we could implement 0 for no filter, 1 for file filter, and 2
for backing filter.  Then instead of having to have 2 separate default
bdrv_get_block_status_for_*() generic functions in patch 4, we could
instead patch bdrv_co_get_block_status() to check .is_filter, and
automatically fall back to bs->file or bs->backing according to the enum
value stored in .is_filter.

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

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