Re: [libvirt] [PATCH] qemu: Remove limit enforcing when setting processor count

On 09/10/12 11:11, Daniel P. Berrange wrote: >On Fri, Sep 07, 2012 at 11:07:43AM -0600, Eric Blake wrote: >>On 09/07/2012 09:13 AM, Daniel P. Berrange wrote: >>>On Fri, Sep 07, 2012 at 02:51:00PM +0200, Peter Krempa wrote: >>>>When setting processor count for a domain using the API libvirt enforced >>>>a maximum processor count that was determined using an IOCTL on >>>>/dev/kvm. Unfortunately this value isn't representative enough and qemu >>>>happily accepts and starts with values greater than the reported value. >>> >>>Really ? If KVM is accepting a greater vCPU count than it reports >>>via /dev/kvm, then we should fix the latter >> >>Why? We already allow oversubscription (you can run 3 guests with 1 >>vcpu each on a 2-cpu machine); we also allow pinning a 2-vcpu guest to 1 >>host cpu; so why can't we allow oversubscription from within a single >>VM? Sure, performance will be lousier, but that doesn't mean the >>request is invalid. > >This isn't anything todo with oversubscription. This check is validating >that the user did not request more vCPUs than the hypervisor is able to >support, regardless of physical CPU count. Yes. But the problem is that even if /dev/kvm reports that it supports 160 CPUs you can start guests with 230 processors with any problem. (Maybe apart from that it will consume a lot of CPU time).