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

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Message-ID: <2fde5816-35a7-1e21-e42c-f6e413f30aec@arm.com>
Date:   Sat, 16 Jan 2021 13:49:14 +0000
From:   Vincenzo Frascino <vincenzo.frascino@....com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kasan-dev@...glegroups.com,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Evgenii Stepanov <eugenis@...gle.com>,
        Branislav Rankov <Branislav.Rankov@....com>,
        Andrey Konovalov <andreyknvl@...gle.com>
Subject: Re: [PATCH v3 2/4] arm64: mte: Add asynchronous mode support

On 1/15/21 3:13 PM, Mark Rutland wrote:
> 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@....com>
>> Cc: Will Deacon <will@...nel.org>
>> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@....com>
>> ---
>>  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().
>

Good point, I will fix it in v4.

> 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);
> }
> 

Ok, seems cleaner like this, will adapt my code accordingly.

> Thanks,
> Mark.
> 

-- 
Regards,
Vincenzo