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

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Date:   Sat, 6 Oct 2018 07:08:49 +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 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?

>>--
>>Wei Yang
>>Help you, Help me


-- 
Wei Yang
Help you, Help me