linux-kernel - Re: [PATCH 3/4] KVM: add KVM_CREATE

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Message-ID: <d014f264-72a4-a17e-cdb4-38a95b737b3c@redhat.com>
Date:   Tue, 18 Apr 2017 16:30:17 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Radim Krčmář <rkrcmar@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Christoffer Dall <cdall@...aro.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Cornelia Huck <cornelia.huck@...ibm.com>,
        James Hogan <james.hogan@...tec.com>,
        Paul Mackerras <paulus@...abs.org>,
        Alexander Graf <agraf@...e.com>
Subject: Re: [PATCH 3/4] KVM: add KVM_CREATE_VM2 system ioctl



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