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

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Message-ID: <86ikocomvd.wl-maz@kernel.org>
Date: Thu, 13 Mar 2025 17:34:46 +0000
From: Marc Zyngier <maz@...nel.org>
To: Mikołaj Lenczewski <miko.lenczewski@....com>
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Subject: Re: [PATCH v3 1/3] arm64: Add BBM Level 2 cpu feature

On Thu, 13 Mar 2025 10:41:10 +0000,
Mikołaj Lenczewski <miko.lenczewski@....com> wrote:
> 
> diff --git a/arch/arm64/kernel/pi/idreg-override.c b/arch/arm64/kernel/pi/idreg-override.c
> index c6b185b885f7..9728faa10390 100644
> --- a/arch/arm64/kernel/pi/idreg-override.c
> +++ b/arch/arm64/kernel/pi/idreg-override.c
> @@ -209,6 +209,7 @@ static const struct ftr_set_desc sw_features __prel64_initconst = {
>  		FIELD("nokaslr", ARM64_SW_FEATURE_OVERRIDE_NOKASLR, NULL),
>  		FIELD("hvhe", ARM64_SW_FEATURE_OVERRIDE_HVHE, hvhe_filter),
>  		FIELD("rodataoff", ARM64_SW_FEATURE_OVERRIDE_RODATA_OFF, NULL),
> +		FIELD("nobbml2", ARM64_SW_FEATURE_OVERRIDE_NOBBML2, NULL),
>  		{}
>  	},
>  };
> @@ -246,6 +247,7 @@ static const struct {
>  	{ "rodata=off",			"arm64_sw.rodataoff=1" },
>  	{ "arm64.nolva",		"id_aa64mmfr2.varange=0" },
>  	{ "arm64.no32bit_el0",		"id_aa64pfr0.el0=1" },
> +	{ "arm64.nobbml2",		"arm64_sw.nobbml2=1" },

Why is that a SW feature? This looks very much like a HW feature to
me, and you should instead mask out ID_AA64MMFR2_EL1.BBM, and be done
with it. Something like:

diff --git a/arch/arm64/kernel/pi/idreg-override.c b/arch/arm64/kernel/pi/idreg-override.c
index c6b185b885f70..803a0c99f7b46 100644
--- a/arch/arm64/kernel/pi/idreg-override.c
+++ b/arch/arm64/kernel/pi/idreg-override.c
@@ -102,6 +102,7 @@ static const struct ftr_set_desc mmfr2 __prel64_initconst = {
 	.override	= &id_aa64mmfr2_override,
 	.fields		= {
 		FIELD("varange", ID_AA64MMFR2_EL1_VARange_SHIFT, mmfr2_varange_filter),
+		FIELD("bbm", ID_AA64MMFR2_EL1_BBM_SHIFT, NULL),
 		{}
 	},
 };
@@ -246,6 +247,7 @@ static const struct {
 	{ "rodata=off",			"arm64_sw.rodataoff=1" },
 	{ "arm64.nolva",		"id_aa64mmfr2.varange=0" },
 	{ "arm64.no32bit_el0",		"id_aa64pfr0.el0=1" },
+	{ "arm64.nobbml2",		"id_aa64mmfr2.bbm=0" },
 };
 
 static int __init parse_hexdigit(const char *p, u64 *v)


Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.