Re: [PATCH v3 2/4] arm64: mte: Add asynchronous mode support

[Mark Rutland]

On Fri, Jan 15, 2021 at 12:00:41PM +0000, Vincenzo Frascino wrote:
> MTE provides an asynchronous mode for detecting tag exceptions. In
> particular instead of triggering a fault the arm64 core updates a
> register which is checked by the kernel after the asynchronous tag
> check fault has occurred.
> 
> Add support for MTE asynchronous mode.
> 
> The exception handling mechanism will be added with a future patch.
> 
> Note: KASAN HW activates async mode via kasan.mode kernel parameter.
> The default mode is set to synchronous.
> The code that verifies the status of TFSR_EL1 will be added with a
> future patch.
> 
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
> ---
>  arch/arm64/kernel/mte.c | 26 ++++++++++++++++++++++++--
>  1 file changed, 24 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index 53a6d734e29b..df7a1ae26d7c 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -153,8 +153,30 @@ void mte_init_tags(u64 max_tag)
>  
>  void mte_enable_kernel(enum kasan_hw_tags_mode mode)
>  {
> -	/* Enable MTE Sync Mode for EL1. */
> -	sysreg_clear_set(sctlr_el1, SCTLR_ELx_TCF_MASK, SCTLR_ELx_TCF_SYNC);
> +	const char *m;
> +
> +	/* Preset parameter values based on the mode. */
> +	switch (mode) {
> +	case KASAN_HW_TAGS_ASYNC:
> +		/* Enable MTE Async Mode for EL1. */
> +		sysreg_clear_set(sctlr_el1, SCTLR_ELx_TCF_MASK, SCTLR_ELx_TCF_ASYNC);
> +		m = "asynchronous";
> +		break;
> +	case KASAN_HW_TAGS_SYNC:
> +		sysreg_clear_set(sctlr_el1, SCTLR_ELx_TCF_MASK, SCTLR_ELx_TCF_SYNC);
> +		m = "synchronous";
> +		break;
> +	default:
> +		/*
> +		 * kasan mode should be always set hence we should
> +		 * not reach this condition.
> +		 */
> +		WARN_ON_ONCE(1);
> +		return;
> +	}
> +
> +	pr_info_once("MTE: enabled in %s mode at EL1\n", m);
> +
>  	isb();
>  }

For clarity, we should have that ISB before the pr_info_once().

As with my comment on patch 1, I think with separate functions this
would be much clearer and simpler:

static inline void __mte_enable_kernel(const char *mode, unsigned long tcf)
{
	sysreg_clear_set(sctlr_el1, SCTLR_ELx_TCF_MASK, tcf);
	isb();

	pr_info_once("MTE: enabled in %s mode at EL1\n", mode);
}

void mte_enable_kernel_sync(void)
{
	__mte_enable_kernel("synchronous", SCTLR_ELx_TCF_SYNC);
}

void mte_enable_kernel_async(void)
{
	__mte_enable_kernel("asynchronous", SCTLR_ELx_TCF_ASYNC);
}

Thanks,
Mark.