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

H. Peter Anvin
2018-10-15T17:16:50Z

On 10/7/18 6:04 PM, peng.hao2@zte.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