Re: [Qemu-devel] [PATCH v4 7/8] qmp: Support abstract classes on device-list-properties

[Eduardo Habkost]

On Mon, Nov 07, 2016 at 03:40:57PM +0100, Markus Armbruster wrote:
> Eduardo Habkost <address@hidden> writes:
> 
> > On Mon, Nov 07, 2016 at 09:09:58AM +0100, Markus Armbruster wrote:
> >> Eduardo Habkost <address@hidden> writes:
> >> 
> >> > On Fri, Nov 04, 2016 at 04:45:17PM +0100, Markus Armbruster wrote:
> >> >> Eduardo Habkost <address@hidden> writes:
> >> >> 
> >> >> > (CCing libvirt people, as I forgot to CC them)
> >> >> >
> >> >> > On Mon, Oct 31, 2016 at 03:07:23PM +0100, Igor Mammedov wrote:
> >> >> >> On Fri, 28 Oct 2016 23:48:06 -0200
> >> >> >> Eduardo Habkost <address@hidden> wrote:
> >> >> >> 
> >> >> >> > When an abstract class is used on device-list-properties, we can
> >> >> >> > simply return the class properties registered for the class.
> >> >> >> > 
> >> >> >> > This will be useful if management software needs to query for
> >> >> >> > supported options that apply to all devices of a given type (e.g.
> >> >> >> > options supported by all CPU models, options supported by all PCI
> >> >> >> > devices).
> >> >> >> Patch looks fine to me but I'm not qmp interface guru
> >> >> >> so I'd leave review up to maintainers.
> >> >> >> 
> >> >> >> One question though,
> >> >> >> How would management software discover typename of abstract class?
> >> >> >
> >> >> > It depends on the use case. On some cases, management may already
> >> >> > have bus-specific logic that will know what's the base type it
> >> >> > needs to query (e.g. it may query "pci-device" to find out if all
> >> >> > PCI devices support a given option). On other cases, it may be
> >> >> > discovered using other commands.
> >> >> 
> >> >> The stated purpose of this feature is to let management software "query
> >> >> for supported options that apply to all devices of a given type".  I
> >> >> suspect that when management software has a notion of "a given type", it
> >> >> knows its name.
> >> >> 
> >> >> Will management software go fishing for subtype relationships beyond the
> >> >> types it knows?  I doubt it.  Of course, management software developers
> >> >> are welcome to educate me :)
> >> >> 
> >> >> > For the CPU case, I will propose adding the base QOM CPU typename
> >> >> > in the query-target command.
> >> >> 
> >> >> Does this type name vary?  If yes, can you give examples?
> >> >
> >> > It does. x86-specific CPU properties are on the x86_64-cpu and
> >> > i386-cpu classes. arm-specific CPU properties are on the arm-cpu
> >> > class.
> >> 
> >> I see we have concrete CPUs (such as "Westmere-x86_64-cpu"), which are
> >> subtypes of an abstract CPU (such as "x86_64-cpu"), which is a subtype
> >> of "cpu", which is a subtype of "device", which is a subtype of
> >> "object".
> >> 
> >> The chain "cpu" - "device" - "object" is fixed and well-known.
> >> 
> >> The link from there to the concrete CPU varies.  Whether it could be
> >> considered well-known or not is debatable.
> >> 
> >> My true question is: should we have a special purpose interface to get
> >> the abstract supertype of concrete CPU types, or should be have general
> >> purpose means to introspect the subtype hierarchy?
> >> 
> >> Note that we have the latter already, although in a rather cumbersome
> >> form:
> >> 
> >>     { "execute": "qom-list-types",
> >>       "arguments": { "implements": T, "abstract": true } }
> >> 
> >> lists all subtypes of T.  You can filter out the concrete subtypes by
> >> subtracting the same query with "abstract": false.  Start with the
> >> type you're interested in, find all its abstract supertypes.  If you
> >> need to know more, repeat for the types you found.
> >
> > Looks cumbersome, because I don't see a way to find all
> > supertypes of a given type without walking the whole tree
> > starting from "object" (is there one?). But it could be improved
> > a bit if we added a "implements" field to ObjectTypeInfo.
> 
> My point is: we can skip discussing whether we should expose the subtype
> relation, because we already do.

Correct. My only problem is that it seems to add extra
assumptions to the code (e.g. that there's only one abstract CPU
type). But if libvirt is careful, it doesn't need to make any
assumptions: it can explore the type hierarchy and confirm that
the assumptions are correct.

> 
> > But, maybe we should take a step back: my original goal was to
> > let libvirt know which properties are supported by any CPU model
> > when using "-cpu".
> 
> Why is that useful?

libvirt wants to know if the QEMU binary supports a given -cpu
option (normally CPU features that can be enabled/disabled using
"+foo"/"-foo").

> 
> >                    Maybe we should make QEMU do the QOM->option
> > translation and implement "-cpu" support in
> > query-command-line-options? We already do something similar with
> > "-machine".
> 
> query-command-line-options is flawed, and the less it's used, the
> happier I am.

No problem, let's just forget my suggestion.  :)

> 
> Real command line introspection would be nice, but getting it will
> involve cleaning up the unholy mess our command line has become.  Don't
> hold your breath.
> 
> My preferred workaround is to find something equivalent to introspect
> via QMP.

Sounds good to me.

> 
> That said, if you come up with a patch that solves a real problem now, I
> won't object to it just because it involves query-command-line-options.

I think I will keep the current approach. Changing
query-command-line-option would probably be more complex and
solve only the -cpu problem. Making QOM introspection better
looks simpler and will probably be useful for other use cases.

I am still tempted to try to make QOM->query-command-line-option
mapping easier and simpler, but it doesn't seem to be the answer
to the current problem.

-- 
Eduardo