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SubjectRe: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array
On 17/08/2017 11:28, Cornelia Huck wrote:
> On Thu, 17 Aug 2017 11:16:59 +0200
> Paolo Bonzini <> wrote:
>> On 17/08/2017 09:36, Cornelia Huck wrote:
>>>> 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.
>> What Alex is proposing _is_ RCU, except with a homegrown
>> synchronize_rcu. Using kvm->srcu seems to be the best of both worlds.
> I'm worried a bit about the 'homegrown' part, though.

I agree, that's why I'm suggesting SRCU instead. But it's a trick that
has its uses. For example, if you were only doing reads from a work
queue, flush_work_queue could be used as the "homegrown
synchronize_rcu". In KVM you might use kvm_make_all_cpus_request, I guess.

> I also may be misunderstanding what Alex means with "vcpu move"...

My interpretation was "resizing the array" (so it moves in memory).


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