Re: [Qemu-devel] [PATCH] vl: add -libvirt-caps option for libvirt to stop parsing help output

[Anthony Liguori <aliguori@us.ibm.com>]

"Daniel P. Berrange" <berrange@redhat.com> writes:

> On Thu, Jul 26, 2012 at 01:47:23PM +0100, Daniel P. Berrange wrote:
>> On Wed, Jul 25, 2012 at 02:47:57PM -0500, Anthony Liguori wrote:
>> > The help output is going to change dramatically for 0.13.  We've spent too long
>> > waiting for a perfect solution to capabilities handling and the end loser is
>> > the direct consumer of QEMU because the help output is awful.
>> > 
>> > I will not apply any patches that change help output until 0.13 development
>> > opens up.  This should give libvirt adequate time to implement support for
>> > dealing with this new option.
>> 
>> I completely agree with this. We have spent far too long making do with
>> "help output" and its about time we finish with this once & for all.
>> I'm assuming you mean the 1.3 release here. If so I'll agree that it
>> is an acceptable plan to change help output at the start of the 1.3
>> release.
>> 
>> This gives us enough time to agree on what todo to support apps needing
>> to query QEMU.
>> 
>> > This capabilities set comes directly from libvirt's source code so it's entirely
>> > adequate for libvirt's purposes.  We can still explore more sophisticated
>> > approaches that are more general purpose but the help output will be changing.
>> 
>> While I appreciate what you're trying todo here, I think this would be a
>> serious mistake on many counts, and even be incorrect in some ways.
>> 
>>  - It ignores the needs of other apps using QEMU. In particular Richard
>>    Jones has frequently requested a way to detect QEMU capabilities
>>    to satisfy libguestfs. I think it is unreasonable to expect libguestfs
>>    to use the libvirt capabilities described here. Likewise other apps
>> 
>>  - This is just a point in time snapshot of what libvirt currently uses
>>    from QEMU. If, for example, someone provided a patch to libvirt to
>>    support the bluetooth devices we'd be stuck, because although QEMU
>>    already supports bluetooth, this is not expressed in any of the
>>    caps libvirt already has.
>> 
>>  - Not all of this information actually comes from the help output.
>>    A bunch of it is stuff we detect from '-device ?' and
>>    '-device name,?'. Although, (AFAIR) no one has objections to use
>>    parsing the -device output because it is much better defined &
>>    stable than -help, I think we could use some improvement to make
>>    the parsing 100% long term maintainable by QEMU/apps. Similarly
>>    we do  '-cpu ?' and '-machine ?'.  Some of the caps are set based
>>    on the machine architecture, or QEMU version.
>> 
>>  - Some of the caps are set based on what libvirt is actually
>>    able to handle from a command line generation POV, so having
>>    QEMU report these unconditionally is misleading/wrong. It is
>>    not about what QEMU supports, it is about what libvirt is able
>>    to cope with.
>> 
>>  - In the future some of the caps may be describing supported
>>    monitor commands, detected via 'query-commands' QMP cmd.
>> 
>>  - Users of libvirt are asking us to expose information about
>>    what QEMU supports, in particular wrt to devices, but also
>>    other aspects of configuration.
>> 
>> 
>> 
>> A long time back I proposed a '-capabilities' command line argument
>> for querying QEMU.
>> 
>>   https://lists.gnu.org/archive/html/qemu-devel/2010-06/msg00921.html
>> 
>> There was a long discussion about this & many aspects of it were
>> disliked. In retrospect I agree with many of the comments, and
>> am glad we didn't adopt this. I think, however, there is a merit
>> in trying something vaguely related again, but with some key
>> differences. Basically I'd sum up my new idea as "just use QMP".
>> 
>> 
>>  * No new command line arguments like -capabilities
>> 
>>  * libvirt invokes something like
>> 
>>      $QEMUBINARY -qmp CHARDEV -nodefault -nodefconfig -nographics
>> 
>>  * libvirt then runs a number of  QMP commands to find out
>>    what it needs to know. I'd expect the following existing
>>    commands would be used
>> 
>>      - query-version             - already supported
>>      - query-commands            - already supported
>>      - query-events              - already supported
>>      - query-kvm                 - already supported
>>      - qom-{list,list-types,get} - already supported
>>      - query-spice/vnc           - already supported
>> 
>>    And add the following new commands
>> 
>>      - query-devices             - new, -device ?, and/or -device NAME,? data in QMP
>>      - query-machines            - new, -M ? in QMP
>>      - query-cpu-types           - new, -cpu ? in QMP
>> 
>> The above would take care of probably 50% of the current libvirt
>> capabilities probing, including a portion of the -help stuff. Then
>> there is all the rest of the crap we detect from the -help. We could
>> just take the view, that "as of 1.2", we assume everything we previously
>> detected is just available by default, and thus don't need to probe
>> it.  For stuff that is QOM based, I expect we'll be able to detect new
>> features in the future using the qom-XXX monitor commands. For stuff
>> that is non-qdev, and non-qom, libvirt can just do a plain version
>> number check, unless we decide there is specific info worth exposing
>> via other new 'query-XXX' monitor commands.
>> 
>> So in summary, as of 1.2 QEMU, libvirt would
>> 
>>   - Remove all use of -help, and other -XXXX command line args
>>     for detecting capabilities
>> 
>>   - Use -qmp and issue commands above to detect whatever it
>>     can
>> 
>>   - Any other existing capabilities are just "enable by default"
>>     for QEMU >= 1.2
>> 
>>   - Detect future stuff via existing monitor commands, otherwise
>>     just do it by QEMU version number.
>> 
>> 
>> So in terms of QEMU work, all I'm asking is to be allowed to
>> implement the query-devices, query-machines & query-cpu-types
>> QMP monitor commands. I'll happily do this work myself, if it
>> brings an end to the -help madness.
>
> Oh, if it is possible to get this data via QOM commands already,
> or it can be easily made to work with QOM commands, then obviously
> I wouldn't ask for these new query-XXX commands.

Yup.

You could do:

qom-list-types implements=TYPE_DEVICE

And that will give you the various types.  We'll need to add a:

device-list-properties typename=FOO

I've got a patch locally for that that I'm testing right now.  Paolo and
I never came to an agreement on how to do this generically for Objects
but I'm happy with a device-specific interface for the short term.

Regards,

Anthony Liguori

>
> Regards,
> Daniel
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