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

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Date:   Wed, 13 Dec 2017 13:31:16 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Hildenbrand <david@...hat.com>,
        Wanpeng Li <kernellwp@...il.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>,
        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 12:45, David Hildenbrand wrote:
> 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.

I agree that KVM_VCPU_NOT_PREEMPTED is not particularly necessary, but
it is not correct to clear KVM_VCPU_PREEMPTED; instead, the entire field
must be cleared to zero.

Also, this patch is not justified very well by the commit message.  A
better wording would be:

The next patch will add another bit to the preempted field in
kvm_steal_time.  Define a constant for bit 0 (the only one that is
currently used).