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

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Date:   Thu, 17 Aug 2017 11:28:29 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Alexander Graf <agraf@...e.de>,
        Radim Krčmář <rkrcmar@...hat.com>,
        linux-mips@...ux-mips.org, linux-arm-kernel@...ts.infradead.org,
        kvm@...r.kernel.org, kvm-ppc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        Marc Zyngier <marc.zyngier@....com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        James Hogan <james.hogan@...tec.com>,
        Christoffer Dall <cdall@...aro.org>,
        Paul Mackerras <paulus@...abs.org>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array

On Thu, 17 Aug 2017 11:16:59 +0200
Paolo Bonzini <pbonzini@...hat.com> 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 also may be misunderstanding what Alex means with "vcpu move"...