Thanks Dave. Dave wrote: > IIRC the domain builder needs to know the VCPUs-max. VCPUs-at-startup > is implemented by a protocol over xenstore where there's a directory: > > cpu = "" > 0 = "" > availability = "online" > 1 = "" > Availability = "online" > > Which tells the PV kernel that it should disable/hotunplug (or not) > certain vCPUs. I'm not sure but I imagine the guest receives the > xenstore watch event, deregisters the vCPU with its scheduler and then > issues a hypercall telling xen to stop scheduling the vCPU too. It's > certainly has to be a co-operative thing, since if xen just stopped > scheduling a vCPU that would probably have some bad effects on the > guest :) It's slightly odd that the protocol allows per-vCPU control, > when I'm not convinced that you can meaningfully tell them apart.
Presumably the in-guest CPU enumeration matches the enumeration in xenstore so it does know which is which. Although xapi currently doesn't allow it I believe independent hot plug/unplug of vCPUs works in Xen and would be useful where topology is exposed to the guest (e.g. if a guest has a unity pinned set of vCPUs == pCPUs and the user wishes to unplug a vCPU on each socket that may mean unplugging vCPU 3 (on socket 0) and vCPU 7 (on socket 1) - I think this comes in xapi NUMA support phase 2 - which reminds me: my original comments were in the context of the current xapi behaviour of not exposing topology to the guest). Cheers, James _______________________________________________ Xen-api mailing list [email protected] http://lists.xen.org/cgi-bin/mailman/listinfo/xen-api
