linux-kernel - Re: [PATCH 0/4] KVM: add KVM_CREATE_VM2 to allow dynamic kvm->vcpus array

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Date:   Tue, 18 Apr 2017 13:11:55 +0200
From:   David Hildenbrand <david@...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>,
        Paolo Bonzini <pbonzini@...hat.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 0/4] KVM: add KVM_CREATE_VM2 to allow dynamic kvm->vcpus
 array

On 13.04.2017 22:19, Radim Krčmář wrote:
> The basic idea is to let userspace provide the desired maximal number of
> VCPUs and allocate only necessary memory for them.
> 
> The goal is to freeze KVM_MAX_VCPUS at its current level and only increase the

KVM_MAX_VCPUS might still increase e.g. if hw support for more VCPUs is
comming.

> new KVM_MAX_CONFIGURABLE_VCPUS, probably directly to INT_MAX/KVM_VCPU_ID, so we
> don't have to worry about it for a while.
> 
> PPC should be interested in this as they set KVM_MAX_VCPUS to NR_CPUS
> and probably waste few pages for every guest this way.

As we just store pointers, this should be a maximum of 4 pages for ppc
(4k pages). Is this really worth yet another VM creation ioctl? Is there
not a nicer way to handle this internally?

An alternative might be to simply realloc the array when it reaches a
certain size (on VCPU creation, maybe protecting the pointer via rcu).
But not sure if something like that could work.

> 
> 
> Radim Krčmář (4):
>   KVM: remove unused __KVM_HAVE_ARCH_VM_ALLOC
>   KVM: allocate kvm->vcpus separately
>   KVM: add KVM_CREATE_VM2 system ioctl
>   KVM: x86: enable configurable MAX_VCPU
> 
>  Documentation/virtual/kvm/api.txt | 28 +++++++++++++++
>  arch/x86/include/asm/kvm_host.h   |  1 +
>  arch/x86/kvm/irq_comm.c           |  4 +--
>  include/linux/kvm_host.h          | 23 +++++-------
>  include/uapi/linux/kvm.h          |  8 +++++
>  virt/kvm/kvm_main.c               | 76 +++++++++++++++++++++++++++++++++------
>  6 files changed, 114 insertions(+), 26 deletions(-)
> 


-- 

Thanks,

David