Re: [PATCH v2 03/33] block: Add BdrvChildRole

[Max Reitz]

On 05.02.20 16:24, Eric Blake wrote:
> On 2/4/20 11:08 AM, Max Reitz wrote:
>> This enum will supplement BdrvChildClass when it comes to what role (or
>> combination of roles) a child takes for its parent.
>>
>> Because empty enums are not allowed, let us just start with it filled.
>>
>> Signed-off-by: Max Reitz <address@hidden>
>> ---
>>   include/block/block.h | 27 +++++++++++++++++++++++++++
>>   1 file changed, 27 insertions(+)
>>
>> diff --git a/include/block/block.h b/include/block/block.h
>> index 38963ef203..0f7e8caa5b 100644
>> --- a/include/block/block.h
>> +++ b/include/block/block.h
>> @@ -279,6 +279,33 @@ enum {
>>       DEFAULT_PERM_UNCHANGED      = BLK_PERM_ALL &
>> ~DEFAULT_PERM_PASSTHROUGH,
>>   };
>>   +typedef enum BdrvChildRole {
>> +    /* Child stores data */
>> +    BDRV_CHILD_DATA         = (1 << 0),
>> +
>> +    /* Child stores metadata */
>> +    BDRV_CHILD_METADATA     = (1 << 1),
>> +
>> +    /* Filtered child */
>> +    BDRV_CHILD_FILTERED     = (1 << 2),
> 
> I'm not sure this comment does justice for what the flag represents, but
> am not sure of what longer comment to put in its place.

You’re right.  I thought I could just rely on our .is_filter
documentation (at least after
https://lists.gnu.org/archive/html/qemu-devel/2019-08/msg01721.html),
but that doesn’t really apply here.

For example, this series makes raw (without further parameters) have a
CHILD_FILTERED child, without raw being a filter itself.

So there should indeed be some definition here.

Maybe:

A child to which the parent forwards all reads and writes.  Therefore,
this child presents exactly the same visible data as the parent.

Would that work?

Max

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