Re: [PATCH -next] arm64: Optimize the comparison of unsigned expressions to avoid compiling error

["linyujun (C)" <linyujun809@huawei.com>]

Thanks for your advice


在 2023/2/21 23:10, Ard Biesheuvel 写道:
> On Tue, 21 Feb 2023 at 11:37, Mark Rutland <mark.rutland@arm.com> wrote:
>> On Tue, Feb 21, 2023 at 09:27:40AM +0800, Lin Yujun wrote:
>>> while compile arch/arm64/include/asm/cpufeature.h with
>>> -Werror=type-limits enabled, errors shown as below:
>>>
>>> ./arch/arm64/include/asm/cpufeature.h: In function 'system_supports_4kb_granule':
>>> ./arch/arm64/include/asm/cpufeature.h:653:14: error: comparison of unsigned expression >= 0 is always true [-Werror=type-limits]
>>>    return (val >= ID_AA64MMFR0_TGRAN4_SUPPORTED_MIN) &&
>>>                ^~
>>> ./arch/arm64/include/asm/cpufeature.h: In function 'system_supports_64kb_granule':
>>> ./arch/arm64/include/asm/cpufeature.h:666:14: error: comparison of unsigned expression >= 0 is always true [-Werror=type-limits]
>>>    return (val >= ID_AA64MMFR0_TGRAN64_SUPPORTED_MIN) &&
>>>                ^~
> The usefulness of this diagnostic is debatable even when the right
> hand is a literal '0' but warning about symbolic constants like this
> is actively evil.
>
> In general, preprocessor #defined values can depend on Kconfig
> settings or other build configuration switches, and this warning
> encourages us to remove the expression altogether, which could mean
> introducing a bug if the macro may assume values >0 in other
> configurations.
>
> Ergo, we must ignore -Wtype-limits until it is fixed, and can at least
> distinguish literal 0 constants from ones instantiated by a CPP macro.
>
>
>> When is the `-Werror=type-limits` flag enabled by the build system?
>>
>> We have patterns like this all over the kernel, and I don't think this is
>> indicative of a real problem, and I don't think that we need to change code to
>> make this warning disappear.
>>
>>> Modify the return judgment statement, use
>>> "((val - min) < (val - max - 1))" to confirm that returns
>>> true in “min <= val <= max” cases, false in other cases.
>> That expression is far less clear than the existing code, so I do not think
>> that is a good idea.
>>
>>> Fixes: 79d82cbcbb3d ("arm64/kexec: Test page size support with new TGRAN range values")
>> What functional error does this fix?
>>
>> What configuration is broken?
>>
>> Thanks,
>> Mark.
>>
>>> Signed-off-by: Lin Yujun <linyujun809@huawei.com>
>>> ---
>>>   arch/arm64/include/asm/cpufeature.h | 18 ++++++++++++------
>>>   1 file changed, 12 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
>>> index 03d1c9d7af82..0a6bda025141 100644
>>> --- a/arch/arm64/include/asm/cpufeature.h
>>> +++ b/arch/arm64/include/asm/cpufeature.h
>>> @@ -54,6 +54,9 @@ enum ftr_type {
>>>   #define FTR_VISIBLE_IF_IS_ENABLED(config)            \
>>>        (IS_ENABLED(config) ? FTR_VISIBLE : FTR_HIDDEN)
>>>
>>> +#define IN_RANGE_INCLUSIVE(val, min, max)            \
>>> +     (((val) - (min)) < ((val) - (max) - 1))
>>> +
>>>   struct arm64_ftr_bits {
>>>        bool            sign;   /* Value is signed ? */
>>>        bool            visible;
>>> @@ -693,8 +696,9 @@ static inline bool system_supports_4kb_granule(void)
>>>        val = cpuid_feature_extract_unsigned_field(mmfr0,
>>>                                                ID_AA64MMFR0_EL1_TGRAN4_SHIFT);
>>>
>>> -     return (val >= ID_AA64MMFR0_EL1_TGRAN4_SUPPORTED_MIN) &&
>>> -            (val <= ID_AA64MMFR0_EL1_TGRAN4_SUPPORTED_MAX);
>>> +     return IN_RANGE_INCLUSIVE(val,
>>> +             ID_AA64MMFR0_EL1_TGRAN4_SUPPORTED_MIN,
>>> +             ID_AA64MMFR0_EL1_TGRAN4_SUPPORTED_MAX);
>>>   }
>>>
>>>   static inline bool system_supports_64kb_granule(void)
>>> @@ -706,8 +710,9 @@ static inline bool system_supports_64kb_granule(void)
>>>        val = cpuid_feature_extract_unsigned_field(mmfr0,
>>>                                                ID_AA64MMFR0_EL1_TGRAN64_SHIFT);
>>>
>>> -     return (val >= ID_AA64MMFR0_EL1_TGRAN64_SUPPORTED_MIN) &&
>>> -            (val <= ID_AA64MMFR0_EL1_TGRAN64_SUPPORTED_MAX);
>>> +     return IN_RANGE_INCLUSIVE(val,
>>> +             ID_AA64MMFR0_EL1_TGRAN64_SUPPORTED_MIN,
>>> +             ID_AA64MMFR0_EL1_TGRAN64_SUPPORTED_MAX);
>>>   }
>>>
>>>   static inline bool system_supports_16kb_granule(void)
>>> @@ -719,8 +724,9 @@ static inline bool system_supports_16kb_granule(void)
>>>        val = cpuid_feature_extract_unsigned_field(mmfr0,
>>>                                                ID_AA64MMFR0_EL1_TGRAN16_SHIFT);
>>>
>>> -     return (val >= ID_AA64MMFR0_EL1_TGRAN16_SUPPORTED_MIN) &&
>>> -            (val <= ID_AA64MMFR0_EL1_TGRAN16_SUPPORTED_MAX);
>>> +     return IN_RANGE_INCLUSIVE(val,
>>> +             ID_AA64MMFR0_EL1_TGRAN16_SUPPORTED_MIN,
>>> +             ID_AA64MMFR0_EL1_TGRAN16_SUPPORTED_MAX);
>>>   }
>>>
>>>   static inline bool system_supports_mixed_endian_el0(void)
>>> --
>>> 2.34.1
>>>
> .