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

From: "Radim Krčmář" <rkrcmar@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
	Christoffer Dall <cdall@linaro.org>,
	Marc Zyngier <marc.zyngier@arm.com>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Cornelia Huck <cornelia.huck@de.ibm.com>,
	James Hogan <james.hogan@imgtec.com>,
	Paul Mackerras <paulus@ozlabs.org>,
	Alexander Graf <agraf@suse.com>
Subject: Re: [PATCH 3/4] KVM: add KVM_CREATE_VM2 system ioctl
Date: Mon, 24 Apr 2017 18:22:55 +0200	[thread overview]
Message-ID: <20170424162255.GA5713@potion> (raw)
In-Reply-To: <d014f264-72a4-a17e-cdb4-38a95b737b3c@redhat.com>

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.