Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array

[Cornelia Huck <cohuck@redhat.com>]

On Thu, 17 Aug 2017 09:04:14 +0200
Alexander Graf <agraf@suse.de> 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",
> > http://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1377285.html
> > 
> > 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.