linux-kernel - Re: [PATCH v1 1/3] arm64: Add BBM Level 2 cpu feature

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Message-ID: <Z7ZqbLdlbmeVX5a0@linux.dev>
Date: Wed, 19 Feb 2025 15:34:04 -0800
From: Oliver Upton <oliver.upton@...ux.dev>
To: Mikołaj Lenczewski <miko.lenczewski@....com>
Cc: ryan.roberts@....com, yang@...amperecomputing.com,
	catalin.marinas@....com, will@...nel.org, joey.gouly@....com,
	broonie@...nel.org, mark.rutland@....com, james.morse@....com,
	yangyicong@...ilicon.com, robin.murphy@....com,
	anshuman.khandual@....com, maz@...nel.org, liaochang1@...wei.com,
	akpm@...ux-foundation.org, david@...hat.com, baohua@...nel.org,
	ioworker0@...il.com, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 1/3] arm64: Add BBM Level 2 cpu feature

Hi Miko,

On Wed, Feb 19, 2025 at 02:38:38PM +0000, Mikołaj Lenczewski wrote:
> +config ARM64_ENABLE_BBML2

nit: consider calling this ARM64_BBML2_NOABORT or similar, since this
assumes behavior that exceeds the BBML2 baseline.

> +	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. Selecting N causes the kernel to
> +	  fallback to BBM level 0 behaviour even if the system supports BBM level 2.
> +

[...]

> +static bool has_bbml2_noconflict(const struct arm64_cpu_capabilities *entry,
> +				 int scope)
> +{
> +	if (!IS_ENABLED(CONFIG_ARM64_ENABLE_BBML2))
> +		return false;
> +
> +	/* 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, fulfil the

typo: fullfill

> +	 * 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).
> +	 */

We should be *very* specific of what qualifies a 'known-good'
implementation here. Implementations shouldn't be added to this list
based on the observed behavior, only if *the implementer* states their
design will not generate conflict aborts for BBML2 mapping granularity
changes.

> +	static const struct midr_range supports_bbml2_without_abort_list[] = {
> +		MIDR_REV_RANGE(MIDR_CORTEX_X4, 0, 3, 0xf),
> +		MIDR_REV_RANGE(MIDR_NEOVERSE_V3, 0, 2, 0xf),
> +		{}
> +	};
> +
> +	if (!is_midr_in_range_list(read_cpuid_id(), supports_bbml2_without_abort_list))
> +		return false;
> +
> +	return true;
> +}
> +
>  #ifdef CONFIG_ARM64_PAN
>  static void cpu_enable_pan(const struct arm64_cpu_capabilities *__unused)
>  {
> @@ -2926,6 +2951,13 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>  		.matches = has_cpuid_feature,
>  		ARM64_CPUID_FIELDS(ID_AA64MMFR2_EL1, EVT, IMP)
>  	},
> +	{
> +		.desc = "BBM Level 2 without conflict abort",
> +		.capability = ARM64_HAS_BBML2_NOCONFLICT,
> +		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
> +		.matches = has_bbml2_noconflict,
> +		ARM64_CPUID_FIELDS(ID_AA64MMFR2_EL1, BBM, 2)
> +	},
>  	{
>  		.desc = "52-bit Virtual Addressing for KVM (LPA2)",
>  		.capability = ARM64_HAS_LPA2,
> diff --git a/arch/arm64/tools/cpucaps b/arch/arm64/tools/cpucaps
> index 1e65f2fb45bd..8d67bb4448c5 100644
> --- a/arch/arm64/tools/cpucaps
> +++ b/arch/arm64/tools/cpucaps
> @@ -26,6 +26,7 @@ HAS_ECV
>  HAS_ECV_CNTPOFF
>  HAS_EPAN
>  HAS_EVT
> +HAS_BBML2_NOCONFLICT

Please add this cap to cpucap_is_possible() test for the config option.

Thanks,
Oliver