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Subject: Re: [PATCH RESEND v4 2/2] arm/arm64: KVM: Add KVM_GET/SET_VCPU_EVENTS
To:     gengdongjiu <gengdongjiu@huawei.com>
Cc:     rkrcmar@redhat.com, corbet@lwn.net, christoffer.dall@arm.com,
        marc.zyngier@arm.com, linux@armlinux.org.uk,
        catalin.marinas@arm.com, will.deacon@arm.com, kvm@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org, linux-acpi@vger.kernel.org
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 <1528487320-2873-3-git-send-email-gengdongjiu@huawei.com>
 <45e94aae-ed9f-1fb7-f10e-d95c2f969ddd@arm.com>
 <f7f59176-16a1-3b24-5acc-87bf1b4c1490@huawei.com>
 <6887237f-d252-5b9e-02cb-5a44fef27080@arm.com>
 <2455dc33-ae8d-79e6-e915-4181050e1e65@huawei.com>
From:   James Morse <james.morse@arm.com>
Message-ID: <9e5be3fe-e6df-ca56-5eab-9ab7e18d9a99@arm.com>
Date:   Fri, 15 Jun 2018 16:57:48 +0100
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Hi gengdongjiu,

On 12/06/18 16:48, gengdongjiu wrote:
> On 2018/6/12 23:29, James Morse wrote:
>> On 12/06/18 15:50, gengdongjiu wrote:
>>> On 2018/6/11 21:36, James Morse wrote:
>>>> On 08/06/18 20:48, Dongjiu Geng wrote:
>>>>> diff --git a/arch/arm/include/uapi/asm/kvm.h b/arch/arm/include/uapi/asm/kvm.h
>>>>> index caae484..c3e6975 100644
>>>>> --- a/arch/arm/include/uapi/asm/kvm.h
>>>>> +++ b/arch/arm/include/uapi/asm/kvm.h
>>>>> @@ -124,6 +124,18 @@ struct kvm_sync_regs {
>>>>>  struct kvm_arch_memory_slot {
>>>>>  };
>>>>>  
>>>>> +/* for KVM_GET/SET_VCPU_EVENTS */
>>>>> +struct kvm_vcpu_events {
>>>>> +	struct {
>>>>> +		__u8 serror_pending;
>>>>> +		__u8 serror_has_esr;
>>>>> +		/* Align it to 8 bytes */
>>>>> +		__u8 pad[6];
>>>>> +		__u64 serror_esr;
>>>>> +	} exception;
>>>>> +	__u32 reserved[12];
>>>>> +};
>>>>> +
>>>>
>>>> You haven't defined __KVM_HAVE_VCPU_EVENTS for 32bit, so presumably this struct
>>>> will never be used. Why is it here?
>>
>>>   if not add it for 32 bits. the 32 arm platform will build Fail, whether you have good
>>>    idea to avoid this Failure if not add this struct for the 32 bit?
>>
>> How does this 32bit code build without this patch?
>> If do you provide the struct, how will that code build with older headers?
>>
>> As far as I can see, this is what the __KVM_HAVE_VCPU_EVENTS define is for.
>>
>> This should be both, or neither. Having just the struct is useless.
> It because the caller of kvm_arm_vcpu_get/set_events() is in "virt/kvm/arm/arm.c".
> the virt/kvm/arm/arm.c will used by both arm64 and arm.
> so It needs to add kvm_arm_vcpu_get/set_events() for the 32 bits, however, kvm_arm_vcpu_get/set_events() will directly return,

So you are adding a uapi struct that user-space can't actually use, to avoid a
kernel build-error. Fine, it just looks really strange.

32bit user-space shouldn't try to call this as check-extension reports it as not
present. If it does, it gets -EINVAL back, which is also the default for
kvm_arch_vcpu_ioctl().


Thanks,

James