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

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Date:   Mon, 15 Oct 2018 10:16:50 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     peng.hao2@....com.cn, richard.weiyang@...il.com
Cc:     penghao122@...a.com.cn, pbonzini@...hat.com, rkrcmar@...hat.com,
        tglx@...utronix.de, mingo@...hat.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 10/7/18 6:04 PM, peng.hao2@....com.cn wrote:
\>>>>>
>>>>> #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.

The right way to do this would be to use the _BITUL() (or _BITULL()) macro.

	-hpa