linux-kernel - Re: [PATCH v8 1/4] KVM: X86: Add vCPU running/preempted state

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Date:   Wed, 13 Dec 2017 12:45:21 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH v8 1/4] KVM: X86: Add vCPU running/preempted state

On 13.12.2017 12:38, Wanpeng Li wrote:
> 2017-12-13 18:20 GMT+08:00 David Hildenbrand <david@...hat.com>:
>> On 13.12.2017 02:33, Wanpeng Li wrote:
>>> From: Wanpeng Li <wanpeng.li@...mail.com>
>>>
>>> This patch reuses the preempted field in kvm_steal_time, and will export
>>> the vcpu running/pre-empted information to the guest from host. This will
>>> enable guest to intelligently send ipi to running vcpus and set flag for
>>> pre-empted vcpus. This will prevent waiting for vcpus that are not running.
>>>
>>> Cc: Paolo Bonzini <pbonzini@...hat.com>
>>> Cc: Radim Krčmář <rkrcmar@...hat.com>
>>> Cc: Peter Zijlstra <peterz@...radead.org>
>>> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
>>> ---
>>>  arch/x86/include/uapi/asm/kvm_para.h | 3 +++
>>>  arch/x86/kernel/kvm.c                | 2 +-
>>>  arch/x86/kvm/x86.c                   | 4 ++--
>>>  3 files changed, 6 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
>>> index 09cc064..763b692 100644
>>> --- a/arch/x86/include/uapi/asm/kvm_para.h
>>> +++ b/arch/x86/include/uapi/asm/kvm_para.h
>>> @@ -51,6 +51,9 @@ struct kvm_steal_time {
>>>       __u32 pad[11];
>>>  };
>>>
>>> +#define KVM_VCPU_NOT_PREEMPTED      (0 << 0)
>>> +#define KVM_VCPU_PREEMPTED          (1 << 0)
>>
>> Is it really helpful to have two flags?
>>
>> Just use KVM_VCPU_PREEMPTED  and clear that one in record_steal_time()
> 
> I think it is fine since there is a third flag introduced in patch
> 2/4, it is more clear currently.
> 
> Regards,
> Wanpeng Li
> 

Having two flags representing the same thing is not clear to me.

-- 

Thanks,

David / dhildenb