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Re: [Qemu-block] [Qemu-devel] [PATCH v14 12/21] option: allow qemu_opts_

From: Markus Armbruster
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH v14 12/21] option: allow qemu_opts_to_qdict to merge repeated options
Date: Wed, 12 Oct 2016 19:46:00 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux)

"Daniel P. Berrange" <address@hidden> writes:

> If given an option string such as
>   size=1024,nodes=10,nodes=4-5,nodes=1-2,policy=bind
> the qemu_opts_to_qdict() method will currently overwrite
> the values for repeated option keys, so only the last
> value is in the returned dict:
>     size=QString("1024")
>     nodes=QString("1-2")
>     policy=QString("bind")
> With this change the caller can optionally ask for all
> the repeated values to be stored in a QList. In the
> above example that would result in 'nodes' being a
> QList, so the returned dict would contain
>     size=QString("1024")
>     nodes=QList([QString("10"),
>                  QString("4-5"),
>                  QString("1-2")])
>     policy=QString("bind")
> Note that the conversion has no way of knowing whether
> any given key is expected to be a list upfront - it can
> only figure that out when seeing the first duplicated
> key. Thus the caller has to be prepared to deal with the
> fact that if a key 'foo' is a list, then the returned
> qdict may contain either a QString or a QList for the
> key 'foo'.
> In a third mode, it is possible to ask for repeated
> options to be reported as an error, rather than silently
> dropping all but the last one.

To serve as a replacement for the options visitor, this needs to be able
to behave exactly the same together with a suitably hacked up QObject
input visitor.  Before I dive into the actual patch, let me summarize
QemuOpts and options visitor behavior.

Warning, this is going to get ugly.

QemuOpts faithfully represents a key=value,... string as a list of
QemuOpt.  Each QemuOpt represents one key=value.  They are in the same
order.  If key occurs multiple times in the string, it occurs just the
same in the list.

*Except* key "id" is special: it's stored outside the list, and all but
the first one are silently ignored.

Most users only ever get the last value of a key.  Any non-last
key=value are silently ignored.

We actually exploit this behavior to do defaults, by *prepending* them
to the list.  See the use of qemu_opts_set_defaults() in main().

A few users get all values of keys (other than key "id"):

* -device, in qdev_device_add() with callback set_property().

  We first get "driver" and "bus" normally (silently ignoring non-last
  values, as usual).  All other keys are device properties.  To set
  them, we get all (key, value), ignore keys "driver" and "bus", and set
  the rest.  If a key occurs multiple times, it gets set multiple times.
  This effectively ignores all but the last one, silently.

* -semihosting-config, in main() with callback add_semihosting_arg().

  We first get a bunch of keys normally.  Key "arg" is special: it may
  be repeated to build a list.  To implement that, we get all (key,
  value), ignore keys other than "arg", and accumulate the values.

* -machine & friends, in main() with callback machine_set_property()

  Similar to -device, only for machines, with "type" instead of "driver"
  and "bus".

* -spice, in qemu_spice_init() with callback add_channel()

  Keys "tls-channel" and "plaintext-channel" may be used repeated to
  specify multiple channels.  To implement that, we get all (key,
  value), ignore keys other than "tls-channel" and "plaintext-channel",
  and set up a channel for each of the others.

* -writeconfig, in config_write_opts() with callback config_write_opt()

  We write out all keys in order.

* The options visitor, in opts_start_struct()

  We convert the list of (key, value) to a hash table of (key, list of
  values).  Most of the time, the list of values has exactly one

  When the visitor's user asks for a scalar, we return the last element
  of the list of values, in lookup_scalar().

  When the user asks for list elements, we return the elements of the
  list of values in order, in opts_next_list(), or if there are none,
  the empty list in opts_start_list().

Unlike the options visitor, this patch (judging from your description)
makes a list only when keys are repeated.  The QObject visitor will have
to cope with finding both scalars and lists.  When it finds a scalar,
but needs a list, it'll have to wrap it in a list (PATCH 09, I think).
When it finds a list, but needs a scalar, it'll have to fish it out of
the list (where is that?).

> All existing callers are all converted to explicitly
> request the historical behaviour of only reporting the
> last key. Later patches will make use of the new modes.
> Signed-off-by: Daniel P. Berrange <address@hidden>

Out of steam for today.

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