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?
