linux-kernel - Re: [PATCH v3 1/3] arm64: Add BBM Level 2 cpu feature

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Message-ID: <20250313180833.GA40525@mazurka.cambridge.arm.com>
Date: Thu, 13 Mar 2025 18:08:33 +0000
From: Mikołaj Lenczewski <miko.lenczewski@....com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: suzuki.poulose@....com, yang@...amperecomputing.com, corbet@....net,
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Subject: Re: [PATCH v3 1/3] arm64: Add BBM Level 2 cpu feature

On Thu, Mar 13, 2025 at 04:13:22PM +0000, Ryan Roberts wrote:
> On 13/03/2025 10:41, Mikołaj Lenczewski wrote: 
> > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > index d561cf3b8ac7..b936e0805161 100644
> > --- a/arch/arm64/kernel/cpufeature.c
> > +++ b/arch/arm64/kernel/cpufeature.c
> > @@ -2176,6 +2176,76 @@ static bool hvhe_possible(const struct arm64_cpu_capabilities *entry,
> >  	return arm64_test_sw_feature_override(ARM64_SW_FEATURE_OVERRIDE_HVHE);
> >  }
> >  
> > +static inline bool bbml2_possible(void)
> > +{
> > +	return !arm64_test_sw_feature_override(ARM64_SW_FEATURE_OVERRIDE_NOBBML2);
> 
> If you're going to keep this helper, I think it really needs to be:
> 
> return IS_ENABLED(CONFIG_ARM64_BBML2_NOABORT) &&
>        !arm64_test_sw_feature_override(ARM64_SW_FEATURE_OVERRIDE_NOBBML2);
> 
> Then you would simplify the caller to remove it's own
> IS_ENABLED(CONFIG_ARM64_BBML2_NOABORT) check.
> 
> But personally I would remove the helper and just fold the test into
> has_bbml2_noabort().
> 
> Thanks,
> Ryan

I was debating folding it into has_bbml2_noabort(), but went ahead and
implemented it separately to match hvhe_possible(), which was another sw
feature helper.

But I agree, folding it will be simpler and read just as easily (if not
easier). Will do so.

> > +}
> > +
> > +static bool cpu_has_bbml2_noabort(unsigned int cpu_midr)
> > +{
> > +	/* We want to allow usage of bbml2 in as wide a range of kernel contexts
> > +	 * as possible. This list is therefore an allow-list of known-good
> > +	 * implementations that both support bbml2 and additionally, fulfill the
> > +	 * extra constraint of never generating TLB conflict aborts when using
> > +	 * the relaxed bbml2 semantics (such aborts make use of bbml2 in certain
> > +	 * kernel contexts difficult to prove safe against recursive aborts).
> > +	 *
> > +	 * Note that implementations can only be considered "known-good" if their
> > +	 * implementors attest to the fact that the implementation never raises
> > +	 * TLBI conflict aborts for bbml2 mapping granularity changes.
> > +	 */
> > +	static const struct midr_range supports_bbml2_noabort_list[] = {
> > +		MIDR_REV_RANGE(MIDR_CORTEX_X4, 0, 3, 0xf),
> > +		MIDR_REV_RANGE(MIDR_NEOVERSE_V3, 0, 2, 0xf),
> > +		{}
> > +	};
> > +
> > +	return is_midr_in_range_list(cpu_midr, supports_bbml2_noabort_list);
> > +}
> > +
> > +static inline unsigned int __cpu_read_midr(int cpu)
> 
> nit: why the double underscrore prefix?

Again copying other helpers I saw that seemed to do similar things.
Didn't know if this was the expected style, so did as other helpers did.
Will remove.

Thank you for the review.

-- 
Kind regards,
Mikołaj Lenczewski