On Mon, Feb 07, 2022 at 10:36:42AM +0100, Peter Krempa wrote: > On Mon, Feb 07, 2022 at 10:18:43 +0100, Igor Mammedov wrote: > > On Mon, 7 Feb 2022 09:14:37 +0100 > > Igor Mammedov <imammedo@xxxxxxxxxx> wrote: > > > > > On Sat, 5 Feb 2022 13:45:24 +0100 > > > Philippe Mathieu-Daudé <f4bug@xxxxxxxxx> wrote: > > > > > > > Previously CPUs were exposed in the QOM tree at a path > > > > > > > > /machine/unattached/device[nn] > > > > > > > > where the 'nn' of the first CPU is usually zero, but can > > > > vary depending on what devices were already created. > > > > > > > > With this change the CPUs are now at > > > > > > > > /machine/cpu[nn] > > > > > > > > where the 'nn' of the first CPU is always zero. > > > > > > Could you add to commit message the reason behind the change? > > > > regardless, it looks like unwarranted movement to me > > prompted by livirt accessing/expecting a QOM patch which is > > not stable ABI. I'd rather get it fixed on libvirt side. > > > > If libvirt needs for some reason access a CPU instance, > > it should use @query-hotpluggable-cpus to get a list of CPUs > > (which includes QOM path of already present CPUs) instead of > > hard-codding some 'well-known' path as there is no any guarantee > > that it will stay stable whatsoever. > > I don't disagree with you about the use of hardcoded path, but the way > of using @query-hotpluggable-cpus is not really aligning well for how > it's being used. > > To shed a bit more light, libvirt uses the following hardcoded path > > #define QOM_CPU_PATH "/machine/unattached/device[0]" > > in code which is used to query CPU flags. That code doesn't care at all > which cpus are present but wants to get any of them. So yes, calling > query-hotpluggable-cpus is possible but a bit pointless. > > In general the code probing cpu flags via qom-get is very cumbersome as > it ends up doing ~400 QMP calls at startup of a VM in cases when we deem > it necessary to probe the cpu fully. Yes, that's one QMP call per CPUID feature bit that QEMU knows about. It is a massive performance bottleneck that we need a much better solution for. We really should have raised this with QEMU right away when we found we had this need for 100's of QMP commands. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
