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From: Ian Campbell <ian.campbell@citrix.com> xend prior to 4.0 understands vcpus as maxvcpus and vcpu_avail as a bit map of which cpus are online (default is all). xend from 4.0 onwards understands maxvcpus as maxvcpus and vcpus as the number which are online (from 0..N-1). The upstream commit (68a94cf528e6 "xm: Add maxvcpus support") claims that if maxvcpus is omitted then the old behaviour (i.e. obeying vcpu_avail) is retained, but AFAICT it was not, in this case vcpu==maxcpus==online cpus. This is good for us because handling anything else would be fiddly. This patch changes parsing of the virDomainDef maxvcpus and vcpus entries to use the corresponding 'maxvcpus' and 'vcpus' settings from xm and xl config. It also drops use of the old Xen 3.x 'vcpu_avail' setting. The change also removes the maxvcpus limit of MAX_VIRT_VCPUS (since maxvcpus is simply a count, not a bit mask), which is particularly crucial on ARM where MAX_VIRT_CPUS == 1 (since all guests are expected to support vcpu placement, and therefore only the boot vcpu's info lives in the shared info page). Existing tests adjusted accordingly, and new tests added for the 'maxvcpus' setting.
LibVirt : simple API for virtualization
Libvirt is a C toolkit to interact with the virtualization capabilities
of recent versions of Linux (and other OSes). It is free software
available under the GNU Lesser General Public License. Virtualization of
the Linux Operating System means the ability to run multiple instances of
Operating Systems concurrently on a single hardware system where the basic
resources are driven by a Linux instance. The library aim at providing
long term stable C API initially for the Xen paravirtualization but
should be able to integrate other virtualization mechanisms if needed.
Daniel Veillard <veillard@redhat.com>