Re: [Qemu-devel] [PATCH 02/19] block: Add BDS.backing_overridden

[Max Reitz]

On 02.05.2016 17:35, Kevin Wolf wrote:
> Am 26.04.2016 um 23:32 hat Max Reitz geschrieben:
>> If the backing file is overridden, this most probably does change the
>> guest-visible data of a BDS. Therefore, we will need to consider this in
>> bdrv_refresh_filename().
>>
>> Adding a new field to the BDS is not nice, but it is very simple and
>> exactly keeps track of whether the backing file has been overridden.
>>
>> Signed-off-by: Max Reitz <address@hidden>
> 
> What about live snapshots or block jobs that manipulate bs->backing?
> They don't use bdrv_open_backing_file(), but bdrv_append() or
> bdrv_replace_in_backing_chain() in order to assign a backing file, which
> may or may not be the same as in the image header.

I'll take a look into each case, thanks.

> Also, do we want to consider a backing file overridden when the user
> specified it explicitly, but it matches the image header?

For simplicity's sake, I would. I think we've realized before that
trying to identify whether to file names point to the same image is
rather difficult.

Max

> 
> Kevin
> 
>>  block.c                   | 2 ++
>>  include/block/block_int.h | 1 +
>>  2 files changed, 3 insertions(+)
>>
>> diff --git a/block.c b/block.c
>> index e349e83..e178488 100644
>> --- a/block.c
>> +++ b/block.c
>> @@ -1299,6 +1299,7 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict 
>> *parent_options,
>>  
>>      reference = qdict_get_try_str(parent_options, bdref_key);
>>      if (reference || qdict_haskey(options, "file.filename")) {
>> +        bs->backing_overridden = true;
>>          backing_filename[0] = '\0';
>>      } else if (bs->backing_file[0] == '\0' && qdict_size(options) == 0) {
>>          QDECREF(options);
>> @@ -1589,6 +1590,7 @@ static int bdrv_open_inherit(BlockDriverState **pbs, 
>> const char *filename,
>>      backing = qdict_get_try_str(options, "backing");
>>      if (backing && *backing == '\0') {
>>          flags |= BDRV_O_NO_BACKING;
>> +        bs->backing_overridden = true;
>>          qdict_del(options, "backing");
>>      }
>>  
>> diff --git a/include/block/block_int.h b/include/block/block_int.h
>> index 10d8759..d73e9ce 100644
>> --- a/include/block/block_int.h
>> +++ b/include/block/block_int.h
>> @@ -409,6 +409,7 @@ struct BlockDriverState {
>>      char backing_file[PATH_MAX]; /* if non zero, the image is a diff of
>>                                      this file image */
>>      char backing_format[16]; /* if non-zero and backing_file exists */
>> +    bool backing_overridden; /* backing file has been specified by the user 
>> */
>>  
>>      QDict *full_open_options;
>>      char exact_filename[PATH_MAX];
>> -- 
>> 2.8.0
>>

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