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 22:22:37 +0200	[thread overview]
Message-ID: <20170424202237.GA7776@potion> (raw)
In-Reply-To: <20170424162255.GA5713@potion>

2017-04-24 18:22+0200, Radim Krčmář:
> 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.

I realized why not:
 * the major user of kvm->vcpu is kvm_for_each_vcpu and it works best
   with a packed array
 * at least arm KVM_IRQ_LINE uses the order in which cpus were created
   to communicate with userspace

Putting this work into a drawer with the "do not share data structure
between kvm_for_each_vcpu and kvm_get_vcpu" idea. :)