linux-kernel - Re: [PATCH 10/10] arm64: errata: Add workaround for TSB flush failures

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Message-ID: <eb02839d-b7af-0284-e4ef-8c628e0548d9@arm.com>
Date:   Tue, 3 Aug 2021 09:21:48 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Suzuki K Poulose <suzuki.poulose@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        coresight@...ts.linaro.org, will@...nel.org,
        catalin.marinas@....com, james.morse@....com,
        mathieu.poirier@...aro.org, mike.leach@...aro.org,
        leo.yan@...aro.org, mark.rutland@....com
Subject: Re: [PATCH 10/10] arm64: errata: Add workaround for TSB flush
 failures



On 8/2/21 3:05 PM, Marc Zyngier wrote:
> On 2021-08-02 10:12, Anshuman Khandual wrote:
>> On 7/29/21 4:11 PM, Suzuki K Poulose wrote:
>>> On 29/07/2021 10:55, Marc Zyngier wrote:
>>>> On Wed, 28 Jul 2021 14:52:17 +0100,
>>>> Suzuki K Poulose <suzuki.poulose@....com>
> 
> [...]
> 
>>>>> +            __tsb_csync();                        \
>>>>> +            __tsb_csync();                        \
>>>>> +        } else {                            \
>>>>> +            __tsb_csync();                        \
>>>>> +        }                                \
>>>>
>>>> nit: You could keep one unconditional __tsb_csync().
>>>
>>> I thought about that, I was worried if the CPU expects them back to back
>>> without any other instructions in between them. Thinking about it a bit
>>> more, it doesn't look like that is the case. I will confirm this and
>>> change it accordingly.
>> But its a very subtle change which might be difficult to debug and blame
>> later on, if indeed both the instructions need to be back to back. Seems
>> like just better to leave this unchanged.
> 
> Is that an actual requirement? Sounds like you want to find out
> from the errata document.

Sure, will get back on this.

> 
> And if they actually need to be back to back, what ensures that
> this is always called with interrupt disabled?
> 
> You would also need to have them in the same asm block to avoid
> the compiler reordering stuff.

Agreed, both the above constructs will be required to make sure that
the instructions will be executed consecutively (if required).