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Re: [Qemu-devel] Review of monitor commands identifying BDS / BB by name


From: Markus Armbruster
Subject: Re: [Qemu-devel] Review of monitor commands identifying BDS / BB by name
Date: Fri, 05 Dec 2014 13:08:59 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux)

Kevin Wolf <address@hidden> writes:

> Am 05.12.2014 um 10:34 hat Markus Armbruster geschrieben:
>> Eric Blake <address@hidden> writes:
>> 
>> > On 12/04/2014 08:56 AM, Markus Armbruster wrote:
>> >
>> >> 
>> >> @device is a sub-optimal name for this single parameter.  Either we
>> >> accept that and move on, or we deprecate it in favor of a new parameter
>> >> with a better name.  I guess the better name isn't worth that much
>> >> trouble, in particular since the command name is already ugly.
>> >> 
>> >> When @node-name is given, @device must not be given.  So @device is
>> >> mandatory *except* in the @node-name usage we retain for compatibility.
>> >> Deprecate the usage.
>> >> 
>> >> Command documentation could then look like this:
>> >> 
>> >> ##
>> >> # @block-resize
>> >> #
>> >> # Resize a block image while a guest is running.
>> >> #
>> >> # @device: the name of the block backend or node to resize (node names
>> >> # supported since 2.3)
>> >> #
>> >> # @size: new image size in bytes
>> >> #
>> >> # Deprecated usage (since 2.3):
>> >> #
>> >> # @device: #optional the name of the block backend to resize
>> >> #
>> >> # @node-name: #optional name of the node to resize (since 2.0)
>> >> #
>> >> # Either @device or @node-name must be set but not both.
>> >
>> > But this isn't discoverable.  You aren't changing whether any parameters
>> > are optional, only enhancing the semantics of existing parameters.  How
>> > is libvirt supposed to know if qemu is old (node names have to go
>> > through node-name) or new (node names are preferred through device)?
>> 
>> Good point.
>> 
>> > Just blindly try the 'device' argument, and if it fails, fall back to an
>> > attempt with node-name?
>> 
>> Works, but "try interfaces one after the other until you find one that
>> works" is a rather lame discovery technique.
>
> As long as we don't have introspection, it's the only one we have.

We have query-commands.  Lets you check for presence of commands, but no
more.

>> > On the other hand, if 'node-name' becomes the preferred interface, then
>> > libvirt has a solution: if node-name is present (it is easy during
>> > up-front QMP probing to determine whether 'node-name' is a recognized
>> > optional argument or an unknown argument), then always use node-name.
>> > As long as libvirt always names the nodes of each device (which it
>> > should be doing, but that's a patch series for another day and another
>> > list), then a device lookup is never needed.  If 'node-name' is not
>> > present, then only the 'device' form is supported, and libvirt can only
>> > manage the top image of a backend (but can make that point obvious to
>> > the end user that they should upgrade qemu for more functionality).
>> 
>> If we deprecate @device instead of @node-name, we make the appropriate
>> (non-deprecated) use of backend names rather than node names hard to
>> probe.
>> 
>> Command argument introspection could help only if it carried
>> "deprecated" flags.  Might be a good idea anyway, but command
>> introspection is one of those nice-to-haves we can't seem to deliver.
>> 
>> A possible alternative is our common way to cheat at probing: when
>> probing for A is hard, probe for B, and assume support for B implies
>> support for A.
>> 
>> My guess that a "better name [than @device for the single parameter]
>> isn't worth that much trouble" needs to be reevaluated with
>> discoverability in mind.  Yes, it's troublesome, but it's also easily
>> discoverable.
>
> Still requires trying the new argument and then falling back to @device/
> @node-name.

Yes, but you only have to probe for "parameter is accepted", and not for
"parameter is accepted and both backend and node names work".

> But as long as libvirt doesn't support the node-name interface yet
> anyway, I think this discussion is mostly moot.

Tend to agree for this specific instance.



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