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

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Message-ID: <1c0577c1-bf73-2c00-b137-9f7251afd20e@arm.com>
Date:   Mon, 18 Jan 2021 14:48:52 +0000
From:   Vincenzo Frascino <vincenzo.frascino@....com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Branislav Rankov <Branislav.Rankov@....com>,
        Marco Elver <elver@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Evgenii Stepanov <eugenis@...gle.com>,
        linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com,
        Alexander Potapenko <glider@...gle.com>,
        linux-arm-kernel@...ts.infradead.org,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Will Deacon <will@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH v3 3/4] arm64: mte: Enable async tag check fault

Hi Mark,

On 1/18/21 2:14 PM, Mark Rutland wrote:
> On Mon, Jan 18, 2021 at 01:37:35PM +0000, Vincenzo Frascino wrote:
>> On 1/18/21 12:57 PM, Catalin Marinas wrote:
> 
>>>> +	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?
> 
> Catalin's saying that the value of TFSR_EL1 doesn't affect anything
> other than a read of TFSR_EL1, i.e. there are no indirect reads of
> TFSR_EL1 where the value has an effect, so there are no side-effects.
> 
> Looking at the ARM ARM, no synchronization is requires from a direct
> write to an indirect write (per ARM DDI 0487F.c table D13-1), so I agree
> that we don't need the ISB here so long as there are no indirect reads.
> 
> Are you aware of cases where the TFSR_EL1 value is read other than by an
> MRS? e.g. are there any cases where checks are elided if TF1 is set? If
> so, we may need the ISB to order the direct write against subsequent
> indirect reads.
> 

Thank you for the explanation. I am not aware of any case in which TFSR_EL1 is
read other then by an MRS. Based on the ARM DDI 0487F.c (J1-7626) TF0/TF1 are
always set to '1' without being accessed before. I will check with the
architects for further clarification and if this is correct I will remove the
isb() in the next version.

> Thanks,
> Mark.
> 

-- 
Regards,
Vincenzo