On 17/08/2017 11:28, Cornelia Huck wrote: > On Thu, 17 Aug 2017 11:16:59 +0200 > Paolo Bonzini <pbonzini@xxxxxxxxxx> 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). Paolo
