Re: [Qemu-devel] [PATCH v3 3/5] qcow2: Introduce an option for sufficient L2 cache for the entire image

[Leonid Bloch]

On 07/27/2018 12:28 AM, Kevin Wolf wrote>
> That makes sense. You probably need to clone the QDict before passing
> it to qcow2_update_options(), with qdict_clone_shallow().

Sounds like a solution! Great, thanks!

> > > > Can QAPI be the solution? I've seen some examples, and it looks like
> > > the
> > > > qcow2 driver already uses QAPI to some extent - I mean all the qdict
> > > stuff
> > > > is from QAPI, no? Can you please point me to an example of how QAPI
> > > can
> > > > solve the issue of an option that can accept both a size and a
> > > string?
> > >
> > > By using QAPI I mean using types like BlockdevOptionsQcow2. These types
> > > are directly generated from the JSON schema, where it's possible to
> > > specify an alternate of a string and an integer.
> > >
> > > One example for an alternate in the JSON schema is BlockdevRef, where
> > > you can give a string (to reference a node-name) or an inline
> > > declaration of a block device. In JSON, it looks like this:
> > >
> > >     { 'alternate': 'BlockdevRef',
> > >       'data': { 'definition': 'BlockdevOptions',
> > >                 'reference': 'str' } }
> > >
> > > And the generated C type for it is:
> > >
> > >     struct BlockdevRef {
> > >         QType type;
> > >         union { /* union tag is @type */
> > >             BlockdevOptions definition;
> > >             char *reference;
> > >         } u;
> > >     };
> > >
> > > In our case, we would get a union of int64_t and char* instead, and
> > > still the QType type as a discriminator.
> >
> > Thanks for the explanation!
> > But qcow2 already has the options in the JSON file, they're just not
> > used for some reason?
>
> The block layer configuration is a bit of a mess because of backwards
> compatibility. The JSON schema is in fact used for -blockdev and for the
> blockdev-add QMP command, but not for the old option -drive and
> drive_add.
>
> In the case of -blockdev/blockdev-add, after validating the input
> against the schema, the QAPI objects are converted to QDicts so they can
> use the same code path as the old code. Eventually, we want to use QAPI
> objects all the way, but that requires either expressing all -drive
> options in terms of QAPI object or getting rid of -driver altogether.
> This is still going to take some time.

Thanks for the details! It's clear now!

Leonid.

> Kevin