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

[Max Reitz <mreitz@redhat.com>]

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*

Max