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

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Message-ID: <08fd066f-e6c5-f82c-544f-536e9f29e6f3@redhat.com>
Date:   Mon, 15 Oct 2018 19:23:18 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "H. Peter Anvin" <hpa@...or.com>, peng.hao2@....com.cn,
        richard.weiyang@...il.com
Cc:     penghao122@...a.com.cn, 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 15/10/2018 19:16, H. Peter Anvin wrote:
> 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.

Even for a value from a 32-bit register?  That would be _BIT, which
doesn't exist.

Paolo