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

[Markus Armbruster <armbru@redhat.com>]

Eric Blake <eblake@redhat.com> 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.

> 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.

>> Let's get the easy case out of the way first: a query that reports only
>> backend properties and not node properties can return an array where
>> each element carries a backend name, like query-block does now.
>> 
>> For queries that report node properties, we have a couple of options:
>> 
>> * Flat array with node names
>> 
>>   Like current query-named-block-nodes.
>> 
>>   Can't report on nodes without names.  Jeff's project to give all nodes
>>   names would moot this issue.
>
> I could live with this.
>
>> 
>> * Array of trees mirroring the actual node forest,
>> 
>>   Similar to current query-blockstats.
>> 
>>   Tree roots correspond to backends, and backends have names.
>> 
>>   Unfortunately, the nodes don't actually form a forest: node trees may
>>   be shared.  To turn it into make a forest, we'd have to duplicate the
>>   shared trees.
>> 
>> * Hybrid: array of trees, but named sub-trees are represented by name
>> 
>>   Like the previous one, except the recursion stops at named nodes.
>>   Instead of including such a sub-tree, we reference it by name, then
>>   add it to the array if it's not already there.
>
> This one seems like it might be easier for avoiding the reconstruction
> of relationships; but if management doesn't already know relationships,
> I'm not sure it is worth the complexity.
>
>> 
>> "Flat array" seems simplest.
>
> Simplest to implement.  Management can't easily reconstruct the tree,
> but for looking up information about a known node, iterating over the
> simpler structure will be faster than trying to find a known node in a
> complex structure.

Tree reconstruction is possible only if all nodes have names, and the
array elements refer to their children by name.