Re: [Libvir] CPU pinning of domains at creation time

There are a few things I gathered on this issue. This affects NUMA setups, where basically if a domain must be placed on a given cell it is not good to let the hypervisor place it first with its own heuristics and then later migrate it to a different set of CPU, but better to instruct the hypervisor to start said domain on the given set. - For Xen it is possible to instruct the hypervisor by passing (cpus '2,3') in the SExpr where the argument is a list of the physical processors allowed

- For KVM I think the standard way would be to select the cpuset using sched_setaffinity() between the fork of the current process and the exec of the qemu process

- there is no need (from a NUMA perspective) to do fine grained allocation at that point, as long as the domain can be restricted to a given cell at startup, then if needed virDomainPinVcpu() can be used later to do more precise pinning in order to try to optimize placement

- to be able to instruct the hypervisor at creation time adding the information in the domain XML description looks the more natural way (another option would be to force to use virDomainDefineXML, add a call using the resulting virDomainPtr to define the set, and then virDomainCreate would be used to do the actual start) + the good point of having this embedded in the XML is that we still have all informations about the domain settings in the XML, if we want to restart it later + the bad point is that we need to fetch and carry this extra information when doing XML dumps to not loose it for example when manipulating the domain to add or remove devices - extracting a cpuset can still be an heavy operation, for example if using xend on need one RPC per vcpu in the domain, the cpuset being constructed by OR'ing logically all cpumaps used by the vcpus of the domain (though in most case this will be the full map after the first CPU and can be stopped immediately)

- for the mapping at the XML level I suggest to use a simple extension to the <vcpu>n</vcpu> and extend it to <vcpu cpuset='2,3'>n</vcpu> with a limited syntax which is just the comma separated list of allowed CPU numbers (if the code actually detects such a cpuset is in effect i.e. in general this won't be added).

Internally implementing this should not be too hard, I would probably refactor some of the existing parsing code, provide functions to get the cpuset and the number of physical processors. Does this sounds okay ?