linux-kernel - Re: [RFC] KVM: optimize the kvm_vcpu_on

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Date:   Mon, 31 Jul 2017 16:23:55 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Marc Zyngier <marc.zyngier@....com>,
        "Longpeng(Mike)" <longpeng2@...wei.com>, rkrcmar@...hat.com
Cc:     agraf@...e.com, borntraeger@...ibm.com, cohuck@...hat.com,
        christoffer.dall@...aro.org, james.hogan@...tec.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        weidong.huang@...wei.com, arei.gonglei@...wei.com,
        wangxinxin.wang@...wei.com, longpeng.mike@...il.com
Subject: Re: [RFC] KVM: optimize the kvm_vcpu_on_spin

On 31/07/2017 15:42, Marc Zyngier wrote:
>> If the vcpu(me) exit due to request a usermode spinlock, then
>> the spinlock-holder may be preempted in usermode or kernmode.
>> But if the vcpu(me) is in kernmode, then the holder must be
>> preempted in kernmode, so we should choose a vcpu in kernmode
>> as the most eligible candidate.
>
> That seems to preclude any form of locking between userspace and kernel
> (which probably wouldn't be Linux). Are you sure that this form of
> construct is not used anywhere? I have the feeling this patch could
> break this scenario...

It's just a heuristic; it would only be broken if you overcommit, and it
would be just as broken as if KVM didn't implement directed yield at all.

Paolo