On 17/08/2017 09:36, Cornelia Huck wrote:

quoted

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.

Paolo