RE: [PATCH v11 1/4] arm64: cpufeature: introduce helper cpu_has_hw_af()

[Justin He (Arm Technology China)]

Hi Catalin

> -----Original Message-----
> From: Catalin Marinas <catalin.marinas@xxxxxxx>
> Sent: Friday, October 11, 2019 12:43 AM
> To: Justin He (Arm Technology China) <Justin.He@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>; Mark Rutland
> <Mark.Rutland@xxxxxxx>; James Morse <James.Morse@xxxxxxx>; Marc
> Zyngier <maz@xxxxxxxxxx>; Matthew Wilcox <willy@xxxxxxxxxxxxx>; Kirill A.
> Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>; linux-arm-
> kernel@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; linux-
> mm@xxxxxxxxx; Suzuki Poulose <Suzuki.Poulose@xxxxxxx>; Borislav
> Petkov <bp@xxxxxxxxx>; H. Peter Anvin <hpa@xxxxxxxxx>; x86@xxxxxxxxxx;
> Thomas Gleixner <tglx@xxxxxxxxxxxxx>; Andrew Morton <akpm@linux-
> foundation.org>; hejianet@xxxxxxxxx; Kaly Xin (Arm Technology China)
> <Kaly.Xin@xxxxxxx>; nd <nd@xxxxxxx>
> Subject: Re: [PATCH v11 1/4] arm64: cpufeature: introduce helper
> cpu_has_hw_af()
> 
> On Wed, Oct 09, 2019 at 04:42:43PM +0800, Jia He wrote:
> > We unconditionally set the HW_AFDBM capability and only enable it on
> > CPUs which really have the feature. But sometimes we need to know
> > whether this cpu has the capability of HW AF. So decouple AF from
> > DBM by a new helper cpu_has_hw_af().
> >
> > Signed-off-by: Jia He <justin.he@xxxxxxx>
> > Suggested-by: Suzuki Poulose <Suzuki.Poulose@xxxxxxx>
> > Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
> 
> I don't think I reviewed this version of the patch.

Sorry about that.
> 
> > diff --git a/arch/arm64/include/asm/cpufeature.h
> b/arch/arm64/include/asm/cpufeature.h
> > index 9cde5d2e768f..1a95396ea5c8 100644
> > --- a/arch/arm64/include/asm/cpufeature.h
> > +++ b/arch/arm64/include/asm/cpufeature.h
> > @@ -659,6 +659,20 @@ static inline u32
> id_aa64mmfr0_parange_to_phys_shift(int parange)
> >  	default: return CONFIG_ARM64_PA_BITS;
> >  	}
> >  }
> > +
> > +/* Check whether hardware update of the Access flag is supported */
> > +static inline bool cpu_has_hw_af(void)
> > +{
> > +	if (IS_ENABLED(CONFIG_ARM64_HW_AFDBM)) {
> 
> Please just return early here to avoid unnecessary indentation:

Okay
> 
> 	if (!IS_ENABLED(CONFIG_ARM64_HW_AFDBM))
> 		return false;
> 
> > +		u64 mmfr1 = read_cpuid(ID_AA64MMFR1_EL1);
> > +
> > +		return !!cpuid_feature_extract_unsigned_field(mmfr1,
> > +
> 	ID_AA64MMFR1_HADBS_SHIFT);
> 
> No need for !!, the return type is a bool already.

But cpuid_feature_extract_unsigned_field has the return type "unsigned int" [1]

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm64/include/asm/cpufeature.h#n444

> 
> Anyway, apart from these nitpicks, the patch is fine you can keep my
> reviewed-by.

Thanks ð
> 
> If later we noticed a potential performance issue on this path, we can
> turn it into a static label as with other CPU features.

Okay

--
Cheers,
Justin (Jia He)