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

[Peter Lieven]

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