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Subject: Re: [PATCH 0/4] KVM: add KVM_CREATE_VM2 to allow dynamic kvm->vcpus
 array
To: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org
References: <20170413201951.11939-1-rkrcmar@redhat.com>
Cc: Christoffer Dall <cdall@linaro.org>,
        Marc Zyngier <marc.zyngier@arm.com>,
        Paolo Bonzini <pbonzini@redhat.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>
From: David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <bb0cda85-6222-5cb7-718f-bbc70cd6297c@redhat.com>
Date: Tue, 18 Apr 2017 13:11:55 +0200
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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