On Mon, 7 Feb 2022 10:36:42 +0100 Peter Krempa <pkrempa@xxxxxxxxxx> 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. Even though query-hotpluggable-cpus is cumbersome it still lets you avoid hardcodding QOM path and let you get away with keeping "_400 QMP calls" probing while something better comes along. > 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. > > It would be much better (And would sidestep the issue altoghether) if we > had a more sane interface to probe all cpu flags in one go, and ideally > the argument specifying the cpu being optional. > > Libvirt can do the adjustment, but for now IMO the path to the first cpu > (/machine/unattached/device[0]) became de-facto ABI by the virtue that > it was used by libvirt and if I remember correctly it was suggested by > the folks dealing with the CPU when the code was added originally. I would've argued against that back then as well, there weren't any guarantee and I wouldn't like precedent of QOM abuse becoming de-facto ABI. Note: this patch breaks this so called ABI as well and introduces yet another harcodded path without any stability guarantee whatsoever. > > Even if we change it in libvirt right away, changing qemu will break > forward compatibility. While we don't guarantee it, it still creates > user grief. >
