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


From: Peter Lieven
Subject: Re: [Qemu-devel] [PATCH] block: add a knob to disable multiwrite_merge
Date: Mon, 20 Oct 2014 15:19:41 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.2.0

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.

Peter



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