Re: Management applications and CPU feature flags (was: [PATCH V1 0/6] fast qom tree get)

Daniel P. Berrangé <berrange(a)redhat.com&gt; 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.sistare(a)oracle.com&gt; 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?