Re: [Qemu-block] [PATCH v3 03/25] block: Add BDS.backing_overridden

[Max Reitz]

On 11.01.2017 20:00, Eric Blake wrote:
> On 11/29/2016 07:18 PM, Max Reitz wrote:
>> 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.
>>
>> This commit adds a FIXME which will be remedied by a follow-up commit.
>> Until then, the respective piece of code will not result in any behavior
>> that is worse than what we currently have.
>>
>> Signed-off-by: Max Reitz <address@hidden>
>> ---
> 
>> +++ b/include/block/block_int.h
>> @@ -461,6 +461,7 @@ 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 
>> */
> 
> Should we group all of the bool in this struct so as to reduce its size
> and improve cache locality?  But it's already clunky, so that may be
> appropriate as a separate patch.
> 
> Reviewed-by: Eric Blake <address@hidden>

Yep, makes sense, but we should do that in a separate patch.

(Or maybe we should fix the compiler to automatically do that
reordering? O:-))

Max

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