linux-kernel - Re: [PATCH v7 1/9] arm64/cpufeature: add FEAT_MTE_TAGGED

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Message-ID: <86y0txcsxf.wl-maz@kernel.org>
Date: Thu, 12 Jun 2025 08:36:44 +0100
From: Marc Zyngier <maz@...nel.org>
To: Yeoreum Yun <yeoreum.yun@....com>
Cc: catalin.marinas@....com,
	pcc@...gle.com,
	will@...nel.org,
	broonie@...nel.org,
	anshuman.khandual@....com,
	joey.gouly@....com,
	yury.khrustalev@....com,
	oliver.upton@...ux.dev,
	frederic@...nel.org,
	akpm@...ux-foundation.org,
	surenb@...gle.com,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org
Subject: Re: [PATCH v7 1/9] arm64/cpufeature: add FEAT_MTE_TAGGED_FAR feature

On Wed, 11 Jun 2025 14:58:10 +0100,
Yeoreum Yun <yeoreum.yun@....com> wrote:
> 
> Add FEAT_MTE_TAGGED_FAR cpucap which makes FAR_ELx report
> all non-address bits on a synchronous MTE tag check fault since Armv8.9
> 
> Signed-off-by: Yeoreum Yun <yeoreum.yun@....com>
> Acked-by: Yury Khrustalev <yury.khrustalev@....com>
> ---
>  arch/arm64/kernel/cpufeature.c | 8 ++++++++
>  arch/arm64/tools/cpucaps       | 1 +
>  2 files changed, 9 insertions(+)
> 
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index b34044e20128..af6a6924a3e8 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -320,6 +320,7 @@ static const struct arm64_ftr_bits ftr_id_aa64pfr1[] = {
>  
>  static const struct arm64_ftr_bits ftr_id_aa64pfr2[] = {
>  	ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR2_EL1_FPMR_SHIFT, 4, 0),
> +	ARM64_FTR_BITS(FTR_VISIBLE, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64PFR2_EL1_MTEFAR_SHIFT, 4, ID_AA64PFR2_EL1_MTEFAR_NI),
>  	ARM64_FTR_END,
>  };

Same thing as the remark I had on FEAT_LSUI: for anything that does
not introduce extra state, please similarly enable the feature in KVM.
There is no point in these discrepancies.

For anything that has extra state, you should at least consider what
needs to be done.

Thanks,

	M.

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