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Subject: Re: [RFC] KVM: optimize the kvm_vcpu_on_spin
To: Marc Zyngier <marc.zyngier@arm.com>,
        "Longpeng(Mike)" <longpeng2@huawei.com>, rkrcmar@redhat.com
Cc: agraf@suse.com, borntraeger@de.ibm.com, cohuck@redhat.com,
        christoffer.dall@linaro.org, james.hogan@imgtec.com,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        weidong.huang@huawei.com, arei.gonglei@huawei.com,
        wangxinxin.wang@huawei.com, longpeng.mike@gmail.com
References: <1501309377-195256-1-git-send-email-longpeng2@huawei.com>
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From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <a7a5e723-700a-db6f-afd4-1be8e7e9999c@redhat.com>
Date: Mon, 31 Jul 2017 16:23:55 +0200
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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