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From: Catalin Marinas <catalin.marinas@arm.com>
To: Peter Collingbourne <pcc@google.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Will Deacon <will@kernel.org>,
	Andrey Konovalov <andreyknvl@gmail.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Evgenii Stepanov <eugenis@google.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2] arm64: mte: avoid clearing PSTATE.TCO on entry unless necessary
Date: Fri, 10 Dec 2021 12:06:10 +0000	[thread overview]
Message-ID: <YbNCsgH0ekceOkmi@arm.com> (raw)
In-Reply-To: <20211110220735.3937127-1-pcc@google.com>

On Wed, Nov 10, 2021 at 02:07:35PM -0800, Peter Collingbourne wrote:
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 2f69ae43941d..a78ec15f5bbc 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -189,6 +189,27 @@ alternative_cb_end
>  #endif
>  	.endm
>  
> +	.macro mte_clear_tco, sctlr
> +	/*
> +	 * Re-enable tag checking (TCO set on exception entry). This is only
> +	 * necessary if MTE is enabled in either the kernel or the userspace
> +	 * task in synchronous mode. With MTE disabled in the kernel and
> +	 * disabled or asynchronous in userspace, tag check faults (including in
> +	 * uaccesses) are not reported, therefore there is no need to re-enable
> +	 * checking. This is beneficial on microarchitectures where re-enabling
> +	 * TCO is expensive.
> +	 */
> +#ifdef CONFIG_ARM64_MTE
> +alternative_cb	kasan_hw_tags_enable
> +	tbz	\sctlr, #SCTLR_EL1_TCF0_SHIFT, 1f
> +alternative_cb_end
> +alternative_if ARM64_MTE
> +	SET_PSTATE_TCO(0)
> +alternative_else_nop_endif
> +1:
> +#endif
> +	.endm

The patch looks fine to me but I recall in a private conversation with
Mark he proposed the idea of moving this to entry-common.c (unless it
was about something completely different). The downside is that we run
with the TCO set for slightly longer. There shouldn't be a major
drawback currently as we don't have stack tagging anyway.

-- 
Catalin

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  parent reply	other threads:[~2021-12-10 12:07 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-11-10 22:07 [PATCH v2] arm64: mte: avoid clearing PSTATE.TCO on entry unless necessary Peter Collingbourne
2021-12-03 18:33 ` Peter Collingbourne
2021-12-10 12:06 ` Catalin Marinas [this message]
2021-12-16  2:44   ` Peter Collingbourne
2021-12-17 18:16     ` Catalin Marinas
2022-01-22  1:03       ` Peter Collingbourne

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