Daniel P. Berrangé <berra...@redhat.com> writes:
> 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'
Libvirt developers, please work with us on design of new commands or
improvements to existing ones to better meet libvirt's needs in this
area.
