linux-kernel - Re: [RESEND PATCH v6 1/3] arm64: Add BBM Level 2 cpu feature

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Message-ID: <20250506083607.GA45762@e133081.arm.com>
Date: Tue, 6 May 2025 09:36:07 +0100
From: Mikołaj Lenczewski <miko.lenczewski@....com>
To: ALOK TIWARI <alok.a.tiwari@...cle.com>
Cc: ryan.roberts@....com, suzuki.poulose@....com,
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Subject: Re: [RESEND PATCH v6 1/3] arm64: Add BBM Level 2 cpu feature

Hi Alok,

Apologies for the delay.

On Mon, Apr 28, 2025 at 11:25:07PM +0530, ALOK TIWARI wrote:
> 
> 
> On 28-04-2025 21:05, Mikołaj Lenczewski wrote:
> > +config ARM64_BBML2_NOABORT
> > +	bool "Enable support for Break-Before-Make Level 2 detection and usage"
> > +	default y
> > +	help
> > +	  FEAT_BBM provides detection of support levels for break-before-make
> > +	  sequences. If BBM level 2 is supported, some TLB maintenance requirements
> > +	  can be relaxed to improve performance. We additonally require the
> 
> typo additonally  -> additionally
Yes, had missed this. Will fix.

> > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > index 9c4d6d552b25..7a85a1bdc6e9 100644
> > --- a/arch/arm64/kernel/cpufeature.c
> > +++ b/arch/arm64/kernel/cpufeature.c
> > @@ -2200,6 +2200,70 @@ static bool hvhe_possible(const struct arm64_cpu_capabilities *entry,
> >   	return arm64_test_sw_feature_override(ARM64_SW_FEATURE_OVERRIDE_HVHE);
> >   }
> > +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.
> > +	 */
> 
> use bbml2 -> BBML2 to maintain consistency
> 
OK, will go through and use BBML2 throughout for consistency.

-- 
Kind regards,
Mikołaj Lenczewski