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Subject: Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array
To: Cornelia Huck <cohuck@redhat.com>
Cc: Alexander Graf <agraf@suse.de>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        linux-mips@linux-mips.org, linux-arm-kernel@lists.infradead.org,
        kvm@vger.kernel.org, kvm-ppc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org,
        Marc Zyngier <marc.zyngier@arm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        James Hogan <james.hogan@imgtec.com>,
        Christoffer Dall <cdall@linaro.org>,
        Paul Mackerras <paulus@ozlabs.org>,
        David Hildenbrand <david@redhat.com>
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 <20170817093612.024cc4bc.cohuck@redhat.com>
 <b69bc1e0-9d7b-5412-ba56-a5261d539a5b@redhat.com>
 <20170817112829.7795820a.cohuck@redhat.com>
From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <69d47131-2405-b2be-60d7-e40fbd17592a@redhat.com>
Date: Thu, 17 Aug 2017 11:44:47 +0200
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On 17/08/2017 11:28, Cornelia Huck wrote:
> On Thu, 17 Aug 2017 11:16:59 +0200
> Paolo Bonzini <pbonzini@redhat.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 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