[libvirt] virsh capabilities vs. domcapabilities

[Eric Blake]

We have some inconsistencies in the node capabilities (which shows guest
capabilities for some default binaries) and domcapabilities (which shows
guest capabilities for a specified binary).  It might be nicer for
client applications if the two XML components share a common schema and
output layout, so that the same client code can be used to parse either
(sub-tree of) XML, modulo the name of the top-most element containing
the tree.

Furthermore, I'm trying to figure out how to advertise whether a given
domain will support active commit (and similarly, Peter's patches for
relative backing name preservation).  Advertising the feature just
through 'virsh capabilities' is insufficient, because that only covers
the default binary; so it seems like the sort of thing that should also
be in 'virsh domcapabilities'.

Since virConnectGetDomainCapabilities is brand new to 1.2.7, we still
have time to change its XML.  But I want consensus on whether we need
things to match, or whether we intentionally want people to rely only on
the newer XML format of the new API call (that is, don't bloat 'virsh
capabilities'/virConnectGetCapabilities any further, and learning
whether active commit is supported will have to be done via 'virsh
domcapabilities'/virConnectGetDomainCapabilities).  That is, I'm
wondering if <domainCapabilities> should use <capabilities>/<guest> as
its starting point, rather than completely inventing new XML.

I'm also finding 'virsh domcapabilities' a bit hard to use - even though
it allows all its arguments to be optional at the RPC level, the qemu
implementation isn't so happy:

# tools/virsh domcapabilities
error: failed to get emulator capabilities
error: virttype_str in qemuConnectGetDomainCapabilities must not be NULL

but how am I supposed to know what --virttype strings are valid?

# tools/virsh domcapabilities --virttype kvm
error: failed to get emulator capabilities
error: invalid argument: at least one of emulatorbin or architecture
fields must be present

Would it be nicer to behave the same as 'virsh capabilities' and give
the details of the default binary in this case?



Now, for a comparison between the two XML per binary:

'virsh capabilities' gives me this segment:

  <guest>
    <os_type>hvm</os_type>
    <arch name='alpha'>
      <wordsize>64</wordsize>
      <emulator>/usr/bin/qemu-system-alpha</emulator>
      <machine maxCpus='4'>clipper</machine>
      <domain type='qemu'>
      </domain>
    </arch>
    <features>
      <deviceboot/>
      <disksnapshot default='on' toggle='no'/>
    </features>
  </guest>

while 'virsh domcapabilities --emulatorbin /usr/bin/qemu-system-alpha
--virttype kvm' gives this:

<domainCapabilities>
  <path>/usr/bin/qemu-system-alpha</path>
  <domain>kvm</domain>
  <machine>clipper</machine>
  <arch>alpha</arch>
  <vcpu max='4'/>
  <devices>
    <disk supported='yes'>
      <enum name='diskDevice'>
        <value>disk</value>
        <value>cdrom</value>
        <value>floppy</value>
        <value>lun</value>
      </enum>
      <enum name='bus'>
        <value>ide</value>
        <value>fdc</value>
        <value>scsi</value>
        <value>virtio</value>
        <value>usb</value>
      </enum>
    </disk>
    <hostdev supported='yes'>
      <enum name='mode'>
        <value>subsystem</value>
      </enum>
      <enum name='startupPolicy'>
        <value>default</value>
        <value>mandatory</value>
        <value>requisite</value>
        <value>optional</value>
      </enum>
      <enum name='subsysType'>
        <value>usb</value>
        <value>pci</value>
        <value>scsi</value>
      </enum>
      <enum name='capsType'/>
      <enum name='pciBackend'/>
    </hostdev>
  </devices>
</domainCapabilities>

 I'm okay that the domcapabilites output is longer, and don't think we
need to backport any of the new stuff to the older API.  But any
information present in the old API should be easily retrieved using the
new API, so that the information isn't lost, and so that a client can
learn the same amount of detail about a non-default binary as they can
about the defaults.

Look at the difference in XPath to get to some of the same information:

/guest/os_type vs. ? - where is os_type in domcapabilities?

/guest/arch at name vs. /domainCapabilities/arch - why is one an attribute
and the other an element?

/guest/arch/wordsize vs. nothing - where is wordsize in domcapabilities?

/guest/arch/emulator vs. /domainCapabilities/path - why 3 levels vs. 2,
and different naming?

/guest/arch/machine at maxCpus vs. /domainCapabilities/vcpu at max - why 3
levels vs. 2, with different naming?

/guest/arch/machine vs. /domainCapabilities/machine - why 3 levels vs. 2?

/guest/arch/domain at type vs. /domainCapabilities/domain - why attribute
of 3 levels vs. element at 2 levels?  And did I expose an error when I
passed --virrtype kvm, even though qemu-system-alpha is only a qemu
emulator on my machine (since kvm emulators is only for hardware-assit
emulation, which is not possible when I'm doing cross architecture)?

/guest/features vs. ? - where are the features in domcapabilities?



-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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