Re: [Qemu-devel] [PATCH] block: add a knob to disable multiwrite_merge

[Peter Lieven <pl@kamp.de>]

On 20.10.2014 15:55, Kevin Wolf wrote:
> Am 20.10.2014 um 15:47 hat Peter Lieven geschrieben:
>> On 20.10.2014 15:31, Kevin Wolf wrote:
>>> Am 20.10.2014 um 15:22 hat Max Reitz geschrieben:
>>>> On 20.10.2014 at 15:19, Peter Lieven wrote:
>>>>> On 20.10.2014 15:15, Max Reitz wrote:
>>>>>> On 20.10.2014 at 14:48, Peter Lieven wrote:
>>>>>>> On 20.10.2014 14:19, Max Reitz wrote:
>>>>>>>> On 2014-10-20 at 14:16, Peter Lieven wrote:
>>>>>>>>> On 20.10.2014 13:51, Max Reitz wrote:
>>>>>>>>>> On 2014-10-20 at 12:03, Peter Lieven wrote:
>>>>>>>>>> [...]
>>>>>>>>> Can you further help here. I think my problem was that I
>>>>>>>>> don't have access to the commandline options in
>>>>>>>>> bdrv_open?!
>>>>>>>> You do. It's the "options" QDict. :-)
>>>>>>> Maybe I just don't get it.
>>>>>>>
>>>>>>> If I specify
>>>>>>>
>>>>>>> qemu -drive if=virtio,file=image.qcow2,write-merging=off
>>>>>>>
>>>>>>> and check with
>>>>>>>
>>>>>>> qdict_get_try_bool(options, "write-merging", true);
>>>>>>>
>>>>>>> in bdrv_open() directly before bdrv_swap I always get true.
>>>>>> Hm, judging from fprintf(stderr, "%s\n",
>>>>>> qstring_get_str(qobject_to_json_pretty(QOBJECT(options))));,
>>>>>> it's there for me (directly after qdict_del(options,
>>>>>> "node-name"). The output is:
>>>>>>
>>>>>> Qemu wrote:
>>>>>>> {
>>>>>>>     "filename": "image.qcow2"
>>>>>>> }
>>>>>>> {
>>>>>>>     "write-merging": "off"
>>>>>>> }
>>>>>>> qemu-system-x86_64: -drive
>>>>>>> if=virtio,file=image.qcow2,write-merging=off: could not open
>>>>>>> disk image image.qcow2: Block format 'qcow2' used by device
>>>>>>> 'virtio0' doesn't support the option 'write-merging'
>>>>>> But as you can see, it's a string and not a bool. So the problem
>>>>>> is that there are (at least) two parameter "types" in qemu: One
>>>>>> is just giving a QDict, and the other are QemuOpts. QDicts are
>>>>>> just the raw user input and the user can only input strings, so
>>>>>> everything is just a string. As far as I know, typing everything
>>>>>> correctly is done by converting the QDict to a QemuOpts object
>>>>>> (as you can see in generally every block driver which supports
>>>>>> some options (e.g. qcow2) and also in blockdev_init(), it's
>>>>>> qemu_opts_absorb_qdict()).
>>>>>>
>>>>>> Sooo, right, I forgot that. Currently, there are no non-string
>>>>>> non-block-driver-specific options for mid-tree BDS (in contrast
>>>>>> to the root BDS, which are parsed in blockdev_init()), so you
>>>>>> now have the honorable task of introducing such a QemuOptsList
>>>>>> along with qemu_opts_absorb_qdict() and everything to
>>>>>> bdrv_open_common(). *cough*
>>>>> I would appreciate if someone with better knowledge of this whole
>>>>> stuff would start this. Or we postpone this know until all the
>>>>> ongoing conversions are done.
>>>> I can try and create some barebone which your patches can then be
>>>> based on. I probably don't have the knowledge either, but I'm daring
>>>> enough to do it anyway. ;-)
>>> Actually I have some patches somewhere [1] that introduce a QemuOpts for
>>> bdrv_open_common(). I intended to use that for cache modes, but as I
>>> explained in our KVM Forum presentation, it's not quite as easy as I
>>> thought it would be and so the patch series isn't ready yet.
>>>
>>> Anyway, having the QemuOpts there for driver-independent options is
>>> probably the way to go. Feel free to remove the caching from my
>>> patch and keep only the node-name part. Then it can be a preparatory
>>> patch for your series where you simply add a new option to the list.
>>>
>>> Kevin
>>>
>>> [1] http://repo.or.cz/w/qemu/kevin.git/commitdiff/9c22aee04cf0bdf6a3858340bc6ff27d6805254f
>> Thank you.
>>
>> Would it be legit to recycle qemu_common_drive_opts from blockdev.c for this?
> No, I don't think so. That one should in theory be only for BlockBackend
> options. For the short term, it still mixes BB and BDS options, but BDS
> options should be moved out step by step. In any case, it is only used
> for the top level.
>
> Any option that is parsed with qemu_opts_absorb_qdict() in
> bdrv_open_common() must also be handled there. If you don't ensure that
> and extract all the options that blockdev_init() knows without actually
> handling them, it can happen that invalid options are silently ignored
> (e.g. backing.werror should error out, but would be accepted).
>
> And please coordinate with Max, if both of you write a patch, that's
> wasted time.

Max, if you don't have started I would use Kevins patch as basis?

Peter