2017-04-18 16:30+0200, Paolo Bonzini: > On 18/04/2017 16:16, Paolo Bonzini wrote: >>> This patch allows userspace to tell how many VCPUs it is going to use, >>> which can save memory when allocating the kvm->vcpus array. This will >>> be done with a new KVM_CREATE_VM2 IOCTL. >>> >>> An alternative would be to redo kvm->vcpus as a list or protect the >>> array with RCU. RCU is slower and a list is not even practical as >>> kvm->vcpus are being used for index-based accesses. >>> >>> We could have an IOCTL that is called in between KVM_CREATE_VM and first >>> KVM_CREATE_VCPU and sets the size of the vcpus array, but we'd be making >>> one useless allocation. Knowing the desired number of VCPUs from the >>> beginning is seems best for now. >>> >>> This patch also prepares generic code for architectures that will set >>> KVM_CONFIGURABLE_MAX_VCPUS to a non-zero value. >> Why is KVM_MAX_VCPU_ID or KVM_MAX_VCPUS not enough? > > Ok, for KVM_MAX_VCPUS I should have read the cover letter more carefully. :) KVM_MAX_VCPU_ID makes sense as the upper bound, I just didn't want to mingle the concepts, because the kvm->vcpus array is not indexed by VCPU_ID ... In hindsight, it would be best to change that and get rid of the search. I'll see how that looks in v2.
