linux-kernel - Re: [PATCH] coresight: trbe: Move check for kernelspace unmapped at EL0 to probe

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Message-ID: <0cfe7af6-d401-b244-d508-9fcd35ebe07c@arm.com>
Date:   Thu, 3 Feb 2022 12:21:12 +0000
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     Sudeep Holla <sudeep.holla@....com>,
        Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Mike Leach <mike.leach@...aro.org>,
        Leo Yan <leo.yan@...aro.org>, coresight@...ts.linaro.org
Subject: Re: [PATCH] coresight: trbe: Move check for kernelspace unmapped at
 EL0 to probe

On 03/02/2022 12:04, Sudeep Holla wrote:
> On Thu, Feb 03, 2022 at 11:55:58AM +0530, Anshuman Khandual wrote:
>>
>>
>> On 2/1/22 5:52 PM, Sudeep Holla wrote:
>>> Currently with the check present in the module initialisation, it shouts
>>> on all the systems irrespective of presence of coresight trace buffer
>>> extensions.
>>
>> IIUC a system with CONFIG_CORESIGHT_TRBE enabled but without a TRBE DT
>> i.e "arm,trace-buffer-extension" complains about kernel space unmapping
>> at EL0 (even though it does not even really have TRBE HW to initialize).
> 
> 
> Correct. Basically, this error will be seen on all systems(DT and ACPI) when
> the module is compiled. It really doesn't matter if the system supports TRBE.
> 
>>>
>>> Similar to Arm SPE perf driver, move the check for kernelspace unmapping
>>> when running at EL0 to the device probe instead of module initialisation.
>>
>> Makes sense.
>>
> 
> Thanks.
> 
>>>
>>> Cc: Mathieu Poirier <mathieu.poirier@...aro.org>
>>> Cc: Suzuki K Poulose <suzuki.poulose@....com>
>>> Cc: Mike Leach <mike.leach@...aro.org>
>>> Cc: Leo Yan <leo.yan@...aro.org>
>>> Cc: Anshuman Khandual <anshuman.khandual@....com>
>>> Cc: coresight@...ts.linaro.org
>>> Signed-off-by: Sudeep Holla <sudeep.holla@....com>
>>> ---
>>>   drivers/hwtracing/coresight/coresight-trbe.c | 10 +++++-----
>>>   1 file changed, 5 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c
>>> index 276862c07e32..3fe2ce1ba5bf 100644
>>> --- a/drivers/hwtracing/coresight/coresight-trbe.c
>>> +++ b/drivers/hwtracing/coresight/coresight-trbe.c
>>> @@ -1423,6 +1423,11 @@ static int arm_trbe_device_probe(struct platform_device *pdev)
>>>   	struct device *dev = &pdev->dev;
>>>   	int ret;
>>>   
>>
>> Could you please add a similar comment like SPE driver regarding how
>> the TRBE buffer will be inaccessible, if kernel gets unmapped at EL0
>> and trace capture will terminate.
>>
> 
> Sure I can add that. But if the device probe fails, will you be able to even
> start the trace capture, sorry I didn't get what you mean by "trace capture
> will terminate". I assume it must be "trace capture is not possible or not
> allowed" IIUC.
> 

"Trace capture is not possible with kernel page table isolation"

  is good enough.

Thanks for making these changes

Cheers
Suzuki