linux-kernel - Re: [PATCH v3 3/4] arm64: mte: Enable async tag check fault

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Date:   Mon, 18 Jan 2021 13:37:35 +0000
From:   Vincenzo Frascino <vincenzo.frascino@....com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kasan-dev@...glegroups.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 3/4] arm64: mte: Enable async tag check fault



On 1/18/21 12:57 PM, Catalin Marinas wrote:
>> +#ifdef CONFIG_KASAN_HW_TAGS
>> +void mte_check_tfsr_el1_no_sync(void)
>> +{
>> +	u64 tfsr_el1;
>> +
>> +	if (!system_supports_mte())
>> +		return;
>> +
>> +	tfsr_el1 = read_sysreg_s(SYS_TFSR_EL1);
>> +
>> +	/*
>> +	 * The kernel should never hit the condition TF0 == 1
>> +	 * at this point because for the futex code we set
>> +	 * PSTATE.TCO.
>> +	 */
>> +	WARN_ON(tfsr_el1 & SYS_TFSR_EL1_TF0);
> I'd change this to a WARN_ON_ONCE() in case we trip over this due to
> model bugs etc. and it floods the log.
> 

I will merge yours and Mark's comment using WARN_ONCE() here. Did not think of
potential bug in the model and you are completely right.

>> +	if (tfsr_el1 & SYS_TFSR_EL1_TF1) {
>> +		write_sysreg_s(0, SYS_TFSR_EL1);
>> +		isb();
> While in general we use ISB after a sysreg update, I haven't convinced
> myself it's needed here. There's no side-effect to updating this reg and
> a subsequent TFSR access should see the new value.

Why there is no side-effect?

> If a speculated load is allowed to update this reg, we'd probably need an
> ISB+DSB (I don't think it does, something to check with the architects).
> 

I will check this with the architects and let you know.

-- 
Regards,
Vincenzo