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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <b240630b-e991-5c1e-a2c7-8def744f275e@redhat.com>
Date:   Thu, 17 Aug 2017 12:18:03 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...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>
Subject: Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array

On 17/08/2017 11:55, David Hildenbrand wrote:
> On 17.08.2017 11:44, Paolo Bonzini wrote:
>> On 17/08/2017 11:28, Cornelia Huck wrote:
>>> 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 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).
> 
> Unpopular opinion: Let's keep it simple first (straight rcu) and
> optimize later on.

RCU vs. SRCU is about correctness, not optimization...

Paolo