Re: [libvirt] [Qemu-devel] [RFC 12/42] pc: initialize legacy hotplug only for 2.6 and older machine types

On Thu, May 12, 2016 at 15:20:51 +0200, Igor Mammedov wrote: > On Thu, 12 May 2016 13:04:23 +0200 > Peter Krempa <pkrempa(a)redhat.com&gt; wrote: > > > On Thu, May 12, 2016 at 07:52:23 -0300, Eduardo Habkost wrote: > > > I don't think we should do that, unless users already had time to > > > update their scripts and libvirt had time to implement code > > > supporting the new method. > > > > > > I believe libvirt (and people's scripts) use maxcpus only when > > > they want CPU hotplug, so making max_cpus > smp_cpus enable CPU > > > hotplug implicitly would probably solve the compatibility issue. > > > > Libvirt uses maxcpus only if the configuration explicitly has less > > active cpus than the maximum number. This option would be the best IMO. > > > > > If we want to deprecate the use of maxcpus to enable CPU hotplug, > > > then we can make it print a warning for a few releases, so people > > > have time to update their code. > > > > At that point libvirt also needs a way to detect that the new argument > > is supported by qemu, so we can start passing it on the command line > > basically every time we now pass 'maxcpus'. > > > > The warning will get most probably ignored by people using libvirt as > > the stdout/err of qemu is not visible to them. > Ok, to make things less complicated I'll drop machine.cpu-hotplug > and leave it always enabled as it used to be and as Michael suggested. > > I'll drop following patches: 12, 13, 14, 20, 23 and respin series I actually don't mind disabling it. But I'd be glad if it was based on the max_cpus value as Eduardo suggested.

On Wed, 11 May 2016 15:50:39 +0200 Igor Mammedov <imammedo(a)redhat.com&gt; wrote: > On Tue, 10 May 2016 17:24:14 -0300 > Eduardo Habkost <ehabkost(a)redhat.com&gt; wrote: [...] > > Should max_cpus > smp_cpus automatically set > > cpu-hotplug=on? > I'd prefer dumb explicit feature enablement, > as it doesn't put any assumptions on other options and > QEMU + mgmt don't have to maintain logic for implicit > rules that might enable it. > > and if I didn't manage to push 'device_add x86cpu' in 2.7 time frame, > guess work gets a bit confusing with current cpu-add semantic, > consider current: > > SRC-QEMU -smp 1,maxcpus=2 > cpu-add 1 > DST-QEMU -smp 2,maxcpus=2 > > vs would be device_add: > > SRC-QEMU -smp 1,maxcpus=2 > device_add cpu > DST-QEMU -smp 1,maxcpus=2 -device cpu > > so instead of qemu/users guessing, I suggest to make it explictly > enabled to get feature (which is mostly optional) or > cleanly fail qemu start if confusing options are specified > with a clear error message.