----- Original Message -----
On 08/23/2013 07:24 AM, Andrew Jones wrote:
The comment in kvm_max_vcpus() states that it's using the recommended procedure from the kernel API documentation to get the max number of vcpus that kvm supports. It is, but by always returning the maximum number supported. The maximum number should only be used for development purposes. qemu should check KVM_CAP_NR_VCPUS for the recommended number of vcpus. This patch adds a warning if a user specifies a number of cpus between the recommended and max.
v2: Incorporate tests for max_cpus, which specifies the maximum number of hotpluggable cpus. An additional note is that the message for the fail case was slightly changed, 'exceeds max cpus' to 'exceeds the maximum cpus'. If this is unacceptable change for users like libvirt, then I'll need to spin a v3.
A quick grep of libvirt does not show any dependence on the particular wording "exceeds max cpus", so you are probably fine changing that.
What I'm more worried about is what number is libvirt supposed to show to the end user, and should libvirt enforce the lower recommended max, or the larger kernel absolute max? Which of the two values does the QMP 'MachineInfo' type return in its 'cpu-max' field during the 'query-machines' command? Should we be modifying QMP to return both values, so that libvirt can also expose the logic to the end user of allowing a recommended vs. larger development max?
Machine definitions maintain yet another 'max_cpus'. And it appears that qmp would return that value. It would probably be best if it returned max(qemu_machine.max_cpus, kvm_max_cpus) though. I'm starting to think that we should just keep things simple for most of the virt stack by sticking to enforcing the larger developer max. And then on a production kernel we should just compile KVM_MAX_VCPUS = KVM_SOFT_MAX_VCPUS and be done with it. With that thought, this patch could be dropped too. The alternative seems to be supporting a run-time selectable experimental mode throughout the whole virt stack. drew