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

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.