Re: [Qemu-devel] [PATCH] block: change default of .has_zero_init to 0

[Peter Lieven <pl@kamp.de>]

On 28.06.2013 12:09, Kevin Wolf wrote:
> Am 28.06.2013 um 10:25 hat Peter Lieven geschrieben:
>> On 28.06.2013 10:20, Kevin Wolf wrote:
>>> Am 28.06.2013 um 10:16 hat Peter Lieven geschrieben:
>>>> On 28.06.2013 10:06, Kevin Wolf wrote:
>>>>> Am 27.06.2013 um 15:52 hat Peter Lieven geschrieben:
>>>>>> .has_zero_init defaults to 1 for all formats and protocols.
>>>>>>
>>>>>> this is a dangerous default since this means that all
>>>>>> new added drivers need to manually overwrite it to 0 if
>>>>>> they do not ensure that a device is zero initialized
>>>>>> after bdrv_create().
>>>>>>
>>>>>> if a driver needs to explicitly set this value to
>>>>>> 1 its easier to verify the correctness in the review process.
>>>>>>
>>>>>> during review of the existing drivers it turned out
>>>>>> that ssh and gluster had a wrong default of 1.
>>>>>> both protocols support host_devices as backend
>>>>>> which are not by default zero initialized. this
>>>>>> wrong assumption will lead to possible corruption
>>>>>> if qemu-img convert is used to write to such a backend.
>>>>>>
>>>>>> a similar problem with the wrong default existed for
>>>>>> iscsi mose likely because the driver developer did
>>>>>> oversee the default value of 1.
>>>>>>
>>>>>> Signed-off-by: Peter Lieven <pl@kamp.de>
>>>>>> ---
>>>>>>   block.c               |    8 +++++++-
>>>>>>   block/qcow.c          |    1 +
>>>>>>   block/qcow2.c         |    1 +
>>>>>>   block/qed.c           |    1 +
>>>>>>   block/raw-posix.c     |   10 +---------
>>>>>>   block/raw-win32.c     |    7 +------
>>>>>>   block/rbd.c           |    1 +
>>>>>>   block/sheepdog.c      |    1 +
>>>>>>   block/vdi.c           |    1 +
>>>>>>   block/vmdk.c          |    1 +
>>>>>>   include/block/block.h |    1 +
>>>>>>   11 files changed, 17 insertions(+), 16 deletions(-)
>>>>> You forgot cow, which is also a simple case that can be handled in this
>>>>> patch.
>>>> ups.
>>>>> vpc is still easy, but a bit more complicated than a constant return 1,
>>>>> because it depends on the subformat whether a new image will inherit the
>>>>> has_zero_init property from the underlying storage or whether it always
>>>>> produces zeros (for VHD_FIXED type images, it's basically raw + footer).
>>>>> I'll send a separate patch for this.
>>>> shall I leave this to the new 0 default until your patch is ready?
>>> Yes, please leave vpc alone. I guess my patch will be queued before
>>> yours anyway. ;-)
>> once v2 is ready and noone has objections maybe you can put it in your queue? ;-)
> Sure. I'm going to send a pull request today, so it would be good to
> have v2 of this patch soon. (Otherwise it would have to wait for
> Stefan's pull request probably next Friday)
expect it in the next 60 minutes.

Peter