linux-kernel - Re: [PATCH] kvm/x86 : avoid shifting signed 32-bit value by 31 bits

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Message-ID: <b5d38cfe-b617-835c-8444-9419f95bea1c@redhat.com>
Date:   Mon, 15 Oct 2018 19:12:30 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wei Yang <richard.weiyang@...il.com>, peng.hao2@....com.cn
Cc:     penghao122@...a.com.cn, rkrcmar@...hat.com, tglx@...utronix.de,
        mingo@...hat.com, hpa@...or.com, joro@...tes.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH] kvm/x86 : avoid shifting signed 32-bit value by 31 bits

On 08/10/2018 04:25, Wei Yang wrote:
> On Mon, Oct 08, 2018 at 09:04:34AM +0800, peng.hao2@....com.cn wrote:
>>> On Sat, Oct 06, 2018 at 11:31:04AM +0800, peng.hao2@....com.cn wrote:
>>>>> On Thu, Oct 04, 2018 at 01:47:18PM -0400, Peng Hao wrote:
>>>>>>
>>>>>> From: Peng Hao <peng.hao2@....com.cn>
>>>>>>
>>>>>>  modify AVIC_LOGICAL_ID_ENTRY_VALID_MASK to unsigned
>>>>>>
>>>>>> Signed-off-by: Peng Hao <peng.hao2@....com.cn>
>>>>>> ---
>>>>>> arch/x86/kvm/svm.c | 2 +-
>>>>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
>>>>>> index d96092b..bf1ded4 100644
>>>>>> --- a/arch/x86/kvm/svm.c
>>>>>> +++ b/arch/x86/kvm/svm.c
>>>>>> @@ -262,7 +262,7 @@ struct amd_svm_iommu_ir {
>>>>>> };
>>>>>>
>>>>>> #define AVIC_LOGICAL_ID_ENTRY_GUEST_PHYSICAL_ID_MASK    (0xFF)
>>>>>> -#define AVIC_LOGICAL_ID_ENTRY_VALID_MASK        (1 << 31)
>>>>>> +#define AVIC_LOGICAL_ID_ENTRY_VALID_MASK        (1UL << 31)
>>>>
>>>>> It is reasonable to change to unsigned, while not necessary to unsigned
>>>>> long?
>>>> AVIC_LOGICAL_ID_ENTRY_VALID_MASK is used in function avic_ldr_write.
>>>> here I think it doesn't matter if you use unsigned or unsigned long. Do you have any suggestions?
>>
>>> In current case, AVIC_LOGICAL_ID_ENTRY_VALID_MASK is used to calculate
>>> the value of new_entry with type of u32. So the definition here is not
>>> harmful.
>>
>>> Also, I did a quick grep and found similar definition (1 << 31) is popular
>>> in the whole kernel tree.
>>
>>> The reason to make this change is not that strong to me. Would you
>>> minding sharing more reason behind this change?
>> oh, I'm just thinking logically, not more reason.
> 
> This definition may introduce problem when this value is used to
> calculate a 64bit data.
> 
> Since current entry is 32bit, we may leave it as it is for now.

I agree.

Paolo