linux-kernel - Re: [PATCH 2/2] arm64: mm: Implement arch_wants_old_faultaround

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Date:   Wed, 9 Dec 2020 18:46:44 +0000
From:   Will Deacon <will@...nel.org>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org, Jan Kara <jack@...e.cz>,
        Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Vinayak Menon <vinmenon@...eaurora.org>,
        kernel-team@...roid.com
Subject: Re: [PATCH 2/2] arm64: mm: Implement arch_wants_old_faultaround_pte()

On Wed, Dec 09, 2020 at 06:35:09PM +0000, Catalin Marinas wrote:
> On Wed, Dec 09, 2020 at 04:39:50PM +0000, Will Deacon wrote:
> > diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> > index da250e4741bd..3424f5881390 100644
> > --- a/arch/arm64/include/asm/cpufeature.h
> > +++ b/arch/arm64/include/asm/cpufeature.h
> > @@ -764,6 +764,18 @@ static inline bool cpu_has_hw_af(void)
> >  						ID_AA64MMFR1_HADBS_SHIFT);
> >  }
> >  
> > +static inline bool system_has_hw_af(void)
> > +{
> > +	u64 mmfr1;
> > +
> > +	if (!IS_ENABLED(CONFIG_ARM64_HW_AFDBM))
> > +		return false;
> > +
> > +	mmfr1 = read_sanitised_ftr_reg(SYS_ID_AA64MMFR1_EL1);
> > +	return cpuid_feature_extract_unsigned_field(mmfr1,
> > +						ID_AA64MMFR1_HADBS_SHIFT);
> > +}
> 
> Could we not add a new system-wide cpu feature that checks for hardware
> AF? This read_sanitised_ftr_reg() does a binary search on each
> invocation.

I posted a diff [1] which would allow removing the binary search for cases
where we can pass the register coding as a constant (like this), but
honestly, it's not like we have many ID registers so I doubt it really
matters in the grand scheme of things.

That said, I'm spinning a v2 anyway so I can include it for comments
since I haven't posted it as a proper patch before.

Will

[1] https://lore.kernel.org/r/20201202172727.GC29813@willie-the-truck