Re: [PATCH 3/4] KVM: add KVM_CREATE_VM2 system ioctl

[Paolo Bonzini <pbonzini@redhat.com>]

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. :)

Paolo