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Re: [libvirt] [PATCH 3/5] domaincaps: Report graphics spiceGL



On Tue, May 10, 2016 at 07:15:01PM -0400, Cole Robinson wrote:
> On 05/10/2016 11:50 AM, Daniel P. Berrange wrote:
> > On Tue, May 10, 2016 at 11:26:29AM -0400, Cole Robinson wrote:
> >> On 05/10/2016 05:10 AM, Daniel P. Berrange wrote:
> >>> On Mon, May 09, 2016 at 06:53:01PM -0400, Cole Robinson wrote:
> >>>> On 05/09/2016 09:48 AM, Daniel P. Berrange wrote:
> >>>
> >>> IMHO it is a total failure if we require the application to extend its
> >>> parser every time we add a new enum to the domain capabilities. We have
> >>> the ability to design something that is data driven - we should not build
> >>> something it is forced to be code driven with code changes for every
> >>> libvirt addition.
> >>
> >> This is ignoring the point I made previously that the schema is not already
> >> fully introspectable. Most of the current enums cannot be programmatically
> >> consumed anyways, because there's no way to map an enum name to its domain XML
> >> property. So if that's our goal to be data driven, we should address that
> >> issue first.
> > 
> > Knowing the mapping of the capabilities enums to the domain schema is
> > a pre-requisite for consuming the capabilities data, no matter which
> > approach discussed in this thread is chosen. Assuming the app knows
> > that mapping, then the enum conditionals can be programmatically
> > handled with the approach I describe.
> > 
> > Providing a way for the app to automatically determine the mapping
> > from capabilities enums to the domain schema would be a nice
> > addition, but it isn't a pre-requisite blocker for what's discussed
> > here. It is something that can be deal with in parallel.
> > 
> >> I can't really think of a good way to represent that without nesting
> >> deeply or using specially formatted strings. Do you have a suggestion
> >> for that?
> > 
> > Probably the best thing is to use a very simplified xpath style notation.
> > If we add a 'mapping' attribute to the <enum> that expresses the attribute
> > or element it is associated with, relative to the parent container element.
> > 
> > eg, consider the tests/domaincapsschemadata/domaincaps-qemu_1.6.50-1.xml
> > data file with the mapping info:
> > 
> > <domainCapabilities>
> >   <path>/usr/bin/qemu-system-x86_64</path>
> >   <domain>kvm</domain>
> >   <machine>pc-1.2</machine>
> >   <arch>x86_64</arch>
> >   <os supported='yes'>
> >     <loader supported='yes'>
> >       <value>/usr/share/AAVMF/AAVMF_CODE.fd</value>
> >       <value>/usr/share/OVMF/OVMF_CODE.fd</value>
> >       <enum name='type' mapping="@type">
> >         <value>rom</value>
> >         <value>pflash</value>
> >       </enum>
> >       <enum name='readonly' mapping="@readonly">
> >         <value>yes</value>
> >         <value>no</value>
> >       </enum>
> >     </loader>
> >   </os>
> >   <devices>
> >     <disk supported='yes'>
> >       <enum name='diskDevice' mapping="@device">
> >         <value>disk</value>
> >         <value>cdrom</value>
> >         <value>floppy</value>
> >         <value>lun</value>
> >       </enum>
> >       <enum name='bus' mapping="target/@bus">
> >         <value>ide</value>
> >         <value>fdc</value>
> >         <value>scsi</value>
> >         <value>virtio</value>
> >         <value>usb</value>
> >       </enum>
> >     </disk>
> >     <hostdev supported='yes'>
> >       <enum name='mode' mapping="@mode">
> >         <value>subsystem</value>
> >       </enum>
> >       <enum name='startupPolicy' mapping="source/@startupPolicy">
> >         <value>default</value>
> >         <value>mandatory</value>
> >         <value>requisite</value>
> >         <value>optional</value>
> >       </enum>
> >       <enum name='subsysType' mapping="@type">
> >         <value>usb</value>
> >         <value>pci</value>
> >         <value>scsi</value>
> >       </enum>
> >       <enum name='capsType' mapping="@type"/>
> >       <enum name='pciBackend' mapping="driver/@name">
> >         <value>default</value>
> >         <value>kvm</value>
> >         <value>vfio</value>
> >       </enum>
> >     </hostdev>
> >   </devices>
> >   <features>
> >     <gic supported='no'/>
> >   </features>
> > </domainCapabilities>
> > 
> > 
> > NB, with my proposed conditionals, the hostdev enums above would actually
> > want to be different. eg it would want to look like this:
> > 
> >       <enum name='mode' mapping="@mode">
> >         <value>subsystem</value>
> >       </enum>
> >       <enum name='startupPolicy' mapping="source/@startupPolicy">
> >         <value>default</value>
> >         <value>mandatory</value>
> >         <value>requisite</value>
> >         <value>optional</value>
> >       </enum>
> >       <enum name='type' mapping="@type">
> >         <condition>
> > 	  <enum name='mode' value='subsystem'/>
> > 	</condition>
> >         <value>usb</value>
> >         <value>pci</value>
> >         <value>scsi</value>
> >       </enum>
> >       <enum name='driver' mapping="driver/@name">
> >         <condition>
> > 	  <enum name='mode' value='subsystem'/>
> > 	  <enum name='type' value='pci'/>
> > 	</condition>
> >         <value>default</value>
> >         <value>kvm</value>
> >         <value>vfio</value>
> >       </enum>
> > 
> 
> Yeah that mapping= bit makes sense to me.
> 
> I don't have time to see pick this up now though, so I've stuffed it in a bug:

Thinking about this some more last night, I realize that once we have
the 'mapping' attribute, then the choice of values in the 'name' attribute
becomes totally irrelevant.

IOW, we could use your suggestion for giving each enum a unique name,
eg 'spiceGL', 'vncGL', etc, etc. So apps which want to consume this
data have a choice between just matching on explicit enum names and
ignoring the <condition> rules, or they can match on the 'mapping'
attribute and use the <condition> rules.. So we get the best of both
ideas.

Regards,
Daniel
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