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

[Peter Lieven <pl@kamp.de>]

On 20.10.2014 15:22, Max Reitz wrote:
> 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. ;-)

Thank you.

Peter