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

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