On Fri, Apr 11, 2025 at 12:40:46PM +0200, Markus Armbruster wrote:
> Daniel P. Berrangé <berra...@redhat.com> writes:
>
> > On Wed, Apr 09, 2025 at 09:58:13AM +0200, Peter Krempa via Devel wrote:
> >> On Wed, Apr 09, 2025 at 09:39:02 +0200, Markus Armbruster via Devel wrote:
> >> > Hi Steve, I apologize for the slow response.
> >> >
> >> > Steve Sistare <steven.sist...@oracle.com> writes:
> >> >
> >> > > Using qom-list and qom-get to get all the nodes and property values in
> >> > > a
> >> > > QOM tree can take multiple seconds because it requires 1000's of
> >> > > individual
> >> > > QOM requests. Some managers fetch the entire tree or a large subset
> >> > > of it when starting a new VM, and this cost is a substantial fraction
> >> > > of
> >> > > start up time.
> >> >
> >> > "Some managers"... could you name one?
> >>
> >> libvirt is at ~500 qom-get calls during an average startup ...
> >>
> >> > > To reduce this cost, consider QAPI calls that fetch more information in
> >> > > each call:
> >> > > * qom-list-get: given a path, return a list of properties and values.
> >> > > * qom-list-getv: given a list of paths, return a list of properties
> >> > > and
> >> > > values for each path.
> >> > > * qom-tree-get: given a path, return all descendant nodes rooted at
> >> > > that
> >> > > path, with properties and values for each.
> >> >
> >> > Libvirt developers, would you be interested in any of these?
> >>
> >> YES!!!
> >
> > Not neccessarily, see below... !!!!
> >
> >>
> >> The getter with value could SO MUCH optimize the startup sequence of a
> >> VM where libvirt needs to probe CPU flags:
> >>
> >> (note the 'id' field in libvirt's monitor is sequential)
> >>
> >> buf={"execute":"qom-get","arguments":{"path":"/machine/unattached/device[0]","property":"realized"},"id":"libvirt-8"}
> >> buf={"execute":"qom-get","arguments":{"path":"/machine/unattached/device[0]","property":"hotplugged"},"id":"libvirt-9"}
> >> buf={"execute":"qom-get","arguments":{"path":"/machine/unattached/device[0]","property":"hotpluggable"},"id":"libvirt-10"}
> >>
> >> [...]
> >>
> >> buf={"execute":"qom-get","arguments":{"path":"/machine/unattached/device[0]","property":"hv-apicv"},"id":"libvirt-470"}
> >> buf={"execute":"qom-get","arguments":{"path":"/machine/unattached/device[0]","property":"xd"},"id":"libvirt-471"}
> >> buf={"execute":"qom-get","arguments":{"path":"/machine/unattached/device[0]","property":"sse4_1"},"id":"libvirt-472"}
> >> buf={"execute":"qom-get","arguments":{"path":"/machine/unattached/device[0]","property":"unavailable-features"},"id":"libvirt-473"}
> >>
> >> First and last line's timestamps:
> >>
> >> 2025-04-08 14:44:28.882+0000: 1481190: info : qemuMonitorIOWrite:340 :
> >> QEMU_MONITOR_IO_WRITE: mon=0x7f4678048360
> >> buf={"execute":"qom-get","arguments":{"path":"/machine/unattached/device[0]","property":"realized"},"id":"libvirt-8"}
> >>
> >> 2025-04-08 14:44:29.149+0000: 1481190: info : qemuMonitorIOWrite:340 :
> >> QEMU_MONITOR_IO_WRITE: mon=0x7f4678048360
> >> buf={"execute":"qom-get","arguments":{"path":"/machine/unattached/device[0]","property":"unavailable-features"},"id":"libvirt-473"}
> >>
> >> Libvirt spent ~170 ms probing cpu flags.
> >
> > One thing I would point out is that qom-get can be considered an
> > "escape hatch" to get information when no better QMP command exists.
> > In this case, libvirt has made the assumption that every CPU feature
> > is a QOM property.
> >
> > Adding qom-list-get doesn't appreciably change that, just makes the
> > usage more efficient.
> >
> > Considering the bigger picture QMP design, when libvirt is trying to
> > understand QEMU's CPU feature flag expansion, I would ask why we don't
> > have something like a "query-cpu" command to tell us the current CPU
> > expansion, avoiding the need for poking at QOM properties directly.
>
> How do the existing query-cpu-FOO fall short of what management
> applications such as libvirt needs?
It has been along while since I looked at them, but IIRC they were
returning static info about CPU models, whereas libvirt wanted info
on the currently requested '-cpu ARGS'
With regards,
Daniel
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