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. Regards, Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :|
