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Subject: Re: [PATCH 2/3] KVM: uapi: Remove KVM_DEV_TYPE_ARM_PV_TIME in
 kvm_device_type
To:     Marc Zyngier <maz@kernel.org>
References: <20200817033729.10848-1-zhukeqian1@huawei.com>
 <20200817033729.10848-3-zhukeqian1@huawei.com>
 <f97633b4a39c301f916bb76030dcabf0@kernel.org>
CC:     <linux-kernel@vger.kernel.org>,
        <linux-arm-kernel@lists.infradead.org>,
        <kvmarm@lists.cs.columbia.edu>, <kvm@vger.kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        James Morse <james.morse@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        "Steven Price" <steven.price@arm.com>, <wanghaibin.wang@huawei.com>
From:   zhukeqian <zhukeqian1@huawei.com>
Message-ID: <4cd543a2-4d5b-882c-38d6-f5055512f0dc@huawei.com>
Date:   Mon, 17 Aug 2020 16:43:14 +0800
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Hi Marc,

On 2020/8/17 15:39, Marc Zyngier wrote:
> On 2020-08-17 04:37, Keqian Zhu wrote:
>> ARM64 PV-time ST is configured by userspace through vCPU attribute,
>> and KVM_DEV_TYPE_ARM_PV_TIME is unused.
>>
>> Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
>> ---
>>  include/uapi/linux/kvm.h       | 2 --
>>  tools/include/uapi/linux/kvm.h | 2 --
>>  2 files changed, 4 deletions(-)
>>
>> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
>> index 4fdf303..9a6b97e 100644
>> --- a/include/uapi/linux/kvm.h
>> +++ b/include/uapi/linux/kvm.h
>> @@ -1258,8 +1258,6 @@ enum kvm_device_type {
>>  #define KVM_DEV_TYPE_ARM_VGIC_ITS    KVM_DEV_TYPE_ARM_VGIC_ITS
>>      KVM_DEV_TYPE_XIVE,
>>  #define KVM_DEV_TYPE_XIVE        KVM_DEV_TYPE_XIVE
>> -    KVM_DEV_TYPE_ARM_PV_TIME,
>> -#define KVM_DEV_TYPE_ARM_PV_TIME    KVM_DEV_TYPE_ARM_PV_TIME
>>      KVM_DEV_TYPE_MAX,
>>  };
>>
>> diff --git a/tools/include/uapi/linux/kvm.h b/tools/include/uapi/linux/kvm.h
>> index 4fdf303..9a6b97e 100644
>> --- a/tools/include/uapi/linux/kvm.h
>> +++ b/tools/include/uapi/linux/kvm.h
>> @@ -1258,8 +1258,6 @@ enum kvm_device_type {
>>  #define KVM_DEV_TYPE_ARM_VGIC_ITS    KVM_DEV_TYPE_ARM_VGIC_ITS
>>      KVM_DEV_TYPE_XIVE,
>>  #define KVM_DEV_TYPE_XIVE        KVM_DEV_TYPE_XIVE
>> -    KVM_DEV_TYPE_ARM_PV_TIME,
>> -#define KVM_DEV_TYPE_ARM_PV_TIME    KVM_DEV_TYPE_ARM_PV_TIME
>>      KVM_DEV_TYPE_MAX,
>>  };
> 
> No. You can't drop anything from UAPI, used or not. Doing so will
> break the compilation of any userspace that, for any reason, references
> this value. We cannot reuse this value in the future either, as it would
> create a we wouldn't know which device to create.
> 
> It is pretty unfortunate that PV time has turned into such a train wreck,
> but that's what we have now, and it has to stay.
Well, I see. It is a sad thing indeed.

Thanks,
Keqian