On Mon, Feb 25, 2019 at 09:50:26AM +0100, Markus Armbruster wrote: > Stefan Hajnoczi <stefa...@redhat.com> writes: > > > QMP clients can usually detect the presence of features via schema > > introspection. There are rare features that do not involve schema > > changes and are therefore impossible to detect with schema > > introspection. > > > > This patch adds the query-qemu-capabilities command. It returns a list > > of capabilities that this QEMU supports. > > The name "capabilities" could be confusing, because we already have QMP > capabilities, complete with command qmp_capabilities. Would "features" > work?
Sure, will fix.
> > The decision to make this a command rather than something statically
> > defined in the schema is intentional. It allows QEMU to decide which
> > capabilities are available at runtime, if necessary.
> >
> > This new interface is necessary so that QMP clients can discover that
> > migrating disk image files is safe with cache.direct=off on Linux.
> > There is no other way to detect whether or not QEMU supports this.
>
> I think what's unsaid here is that we don't want to make a completely
> arbitrary schema change just to carry this bit of information. We
> could, but we don't want to. Correct?
Yes, exactly.
> > +{ 'struct': 'QemuCapabilities',
> > + 'data': { 'capabilities': ['QemuCapability'] } }
> > +
> > +##
> > +# @query-qemu-capabilities:
> > +#
> > +# Return the capabilities supported by this QEMU. Most features can be
> > +# detected via schema introspection but some are not observable from the
> > +# schema. This command offers a way to check for the presence of such
> > +# features.
> > +#
> > +# Since: 4.0
> > +##
> > +{ 'command': 'query-qemu-capabilities',
> > + 'returns': 'QemuCapabilities' }
>
> Capabilities are flags: either present or absent, thus effectively
> boolean. I don't have a specific reason to doubt limiting capabilities
> to boolean. However, doing QemuCapabilities like
>
> { 'struct': 'QemuCapabilities',
> 'data': { 'migrate-with-cache-direct-off-on-linux': 'bool' } }
>
> could grow to support other values more easily. What do you think?
Oops, I didn't think of the obvious solution! Thank you, will fix.
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