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SubjectRe: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array
On Thu, 17 Aug 2017 09:04:14 +0200
Alexander Graf <> wrote:

> On 16.08.17 21:40, Radim Krčmář wrote:
> > The goal is to increase KVM_MAX_VCPUS without worrying about memory
> > impact of many small guests.
> >
> > This is a second out of three major "dynamic" options:
> > 1) size vcpu array at VM creation time
> > 2) resize vcpu array when new VCPUs are created
> > 3) use a lockless list/tree for VCPUs
> >
> > The disadvantage of (1) is its requirement on userspace changes and
> > limited flexibility because userspace must provide the maximal count on
> > start. The main advantage is that kvm->vcpus will work like it does
> > now. It has been posted as "[PATCH 0/4] KVM: add KVM_CREATE_VM2 to
> > allow dynamic kvm->vcpus array",
> >
> >
> > The main problem of (2), this series, is that we cannot extend the array
> > in place and therefore require some kind of protection when moving it.
> > RCU seems best, but it makes the code slower and harder to deal with.
> > The main advantage is that we do not need userspace changes.
> Creating/Destroying vcpus is not something I consider a fast path, so
> why should we optimize for it? The case that needs to be fast is execution.
> What if we just sent a "vcpu move" request to all vcpus with the new
> pointer after it moved? That way the vcpu thread itself would be
> responsible for the migration to the new memory region. Only if all
> vcpus successfully moved, keep rolling (and allow foreign get_vcpu again).
> That way we should be basically lock-less and scale well. For additional
> icing, feel free to increase the vcpu array x2 every time it grows to
> not run into the slow path too often.

I'd prefer the rcu approach: This is a mechanism already understood
well, no need to come up with a new one that will likely have its own
share of problems.

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