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

[linyujun (C)]

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@xxxxxxx> 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@xxxxxxxxxx>
> > > ---
> > >   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
> .