RE: [PATCH v4 1/3] arm64: cpufeature: introduce helper cpu_has_hw_af()

[Justin He (Arm Technology China)]

Hi Suzuki

> -----Original Message-----
> From: Catalin Marinas <catalin.marinas@xxxxxxx>
> Sent: 2019å9æ19æ 0:46
> To: Suzuki Poulose <Suzuki.Poulose@xxxxxxx>
> Cc: Justin He (Arm Technology China) <Justin.He@xxxxxxx>; 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; Punit Agrawal
> <punitagrawal@xxxxxxxxx>; Anshuman Khandual
> <Anshuman.Khandual@xxxxxxx>; Jun Yao <yaojun8558363@xxxxxxxxx>;
> Alex Van Brunt <avanbrunt@xxxxxxxxxx>; Robin Murphy
> <Robin.Murphy@xxxxxxx>; Thomas Gleixner <tglx@xxxxxxxxxxxxx>;
> Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>; JÃrÃme Glisse
> <jglisse@xxxxxxxxxx>; Ralph Campbell <rcampbell@xxxxxxxxxx>;
> hejianet@xxxxxxxxx; Kaly Xin (Arm Technology China) <Kaly.Xin@xxxxxxx>
> Subject: Re: [PATCH v4 1/3] arm64: cpufeature: introduce helper
> cpu_has_hw_af()
>
> On Wed, Sep 18, 2019 at 03:20:41PM +0100, Suzuki K Poulose wrote:
> > On 18/09/2019 14:19, Jia He wrote:
> > > diff --git a/arch/arm64/include/asm/cpufeature.h
> b/arch/arm64/include/asm/cpufeature.h
> > > index c96ffa4722d3..206b6e3954cf 100644
> > > --- a/arch/arm64/include/asm/cpufeature.h
> > > +++ b/arch/arm64/include/asm/cpufeature.h
> > > @@ -390,6 +390,7 @@ extern DECLARE_BITMAP(boot_capabilities,
> ARM64_NPATCHABLE);
> > >           for_each_set_bit(cap, cpu_hwcaps, ARM64_NCAPS)
> > >   bool this_cpu_has_cap(unsigned int cap);
> > > +bool cpu_has_hw_af(void);
> > >   void cpu_set_feature(unsigned int num);
> > >   bool cpu_have_feature(unsigned int num);
> > >   unsigned long cpu_get_elf_hwcap(void);
> > > diff --git a/arch/arm64/kernel/cpufeature.c
> b/arch/arm64/kernel/cpufeature.c
> > > index b1fdc486aed8..c5097f58649d 100644
> > > --- a/arch/arm64/kernel/cpufeature.c
> > > +++ b/arch/arm64/kernel/cpufeature.c
> > > @@ -1141,6 +1141,12 @@ static bool has_hw_dbm(const struct
> arm64_cpu_capabilities *cap,
> > >           return true;
> > >   }
> > > +/* Decouple AF from AFDBM. */
> > > +bool cpu_has_hw_af(void)
> > > +{
> > Sorry for not having asked this earlier. Are we interested in,
> >
> > "whether *this* CPU has AF support ?" or "whether *at least one*
> > CPU has the AF support" ? The following code does the former.
> >
> > > + return (read_cpuid(ID_AA64MMFR1_EL1) & 0xf);
>
> In a non-preemptible context, the former is ok (per-CPU).

Yes, just as what Catalin explained, we need the former because the
pagefault occurred in every cpus

--
Cheers,
Justin (Jia He)


>
> --
> Catalin
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