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

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Date:   Mon, 8 Oct 2018 02:25:32 +0000
From:   Wei Yang <richard.weiyang@...il.com>
To:     peng.hao2@....com.cn
Cc:     richard.weiyang@...il.com, penghao122@...a.com.cn,
        pbonzini@...hat.com, 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 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.


-- 
Wei Yang
Help you, Help me