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Subject: Re: [PATCH v2 09/17] coresight: trbe: Workaround TRBE errata
 overwrite in FILL mode
To:     Mathieu Poirier <mathieu.poirier@linaro.org>
Cc:     linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        maz@kernel.org, catalin.marinas@arm.com, mark.rutland@arm.com,
        james.morse@arm.com, anshuman.khandual@arm.com, leo.yan@linaro.org,
        mike.leach@linaro.org, will@kernel.org, lcherian@marvell.com,
        coresight@lists.linaro.org
References: <20210921134121.2423546-1-suzuki.poulose@arm.com>
 <20210921134121.2423546-10-suzuki.poulose@arm.com>
 <20211001171522.GB3148492@p14s>
From:   Suzuki K Poulose <suzuki.poulose@arm.com>
Message-ID: <085e398c-767c-1b9b-0780-bed830d936fb@arm.com>
Date:   Mon, 4 Oct 2021 09:46:07 +0100
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Hi Mathieu

On 01/10/2021 18:15, Mathieu Poirier wrote:
> On Tue, Sep 21, 2021 at 02:41:13PM +0100, Suzuki K Poulose wrote:
>> ARM Neoverse-N2 (#2139208) and Cortex-A710(##2119858) suffers from
>> an erratum, which when triggered, might cause the TRBE to overwrite
>> the trace data already collected in FILL mode, in the event of a WRAP.
>> i.e, the TRBE doesn't stop writing the data, instead wraps to the base
>> and could write upto 3 cache line size worth trace. Thus, this could
>> corrupt the trace at the "BASE" pointer.
>>
>> The workaround is to program the write pointer 256bytes from the
>> base, such that if the erratum is triggered, it doesn't overwrite
>> the trace data that was captured. This skipped region could be
>> padded with ignore packets at the end of the session, so that
>> the decoder sees a continuous buffer with some padding at the
>> beginning. The trace data written at the base is considered
>> lost as the limit could have been in the middle of the perf
>> ring buffer, and jumping to the "base" is not acceptable.
>> We set the flags already to indicate that some amount of trace
>> was lost during the FILL event IRQ. So this is fine.
>>
>> One important change with the work around is, we program the
>> TRBBASER_EL1 to current page where we are allowed to write.
>> Otherwise, it could overwrite a region that may be consumed
>> by the perf. Towards this, we always make sure that the
>> "handle->head" and thus the trbe_write is PAGE_SIZE aligned,
>> so that we can set the BASE to the PAGE base and move the
>> TRBPTR to the 256bytes offset.
>>
>> Cc: Mike Leach <mike.leach@linaro.org>
>> Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
>> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
>> Cc: Leo Yan <leo.yan@linaro.org>
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
>> ---
>> Change since v1:
>>   - Updated comment with ASCII art
>>   - Add _BYTES suffix for the space to skip for the work around.
>> ---
>>   drivers/hwtracing/coresight/coresight-trbe.c | 144 +++++++++++++++++--
>>   1 file changed, 132 insertions(+), 12 deletions(-)
>>
>> diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c
>> index f569010c672b..983dd5039e52 100644
>> --- a/drivers/hwtracing/coresight/coresight-trbe.c
>> +++ b/drivers/hwtracing/coresight/coresight-trbe.c
>> @@ -16,6 +16,7 @@
>>   #define pr_fmt(fmt) DRVNAME ": " fmt
>>   
>>   #include <asm/barrier.h>
>> +#include <asm/cpufeature.h>
>>   #include <asm/cputype.h>
>>   
>>   #include "coresight-self-hosted-trace.h"
>> @@ -84,9 +85,17 @@ struct trbe_buf {
>>    * per TRBE instance, we keep track of the list of errata that
>>    * affects the given instance of the TRBE.
>>    */
>> -#define TRBE_ERRATA_MAX			0
>> +#define TRBE_WORKAROUND_OVERWRITE_FILL_MODE	0
>> +#define TRBE_ERRATA_MAX				1
>> +
>> +/*
>> + * Safe limit for the number of bytes that may be overwritten
>> + * when the erratum is triggered.
>> + */
>> +#define TRBE_WORKAROUND_OVERWRITE_FILL_MODE_SKIP_BYTES	256
>>   
>>   static unsigned long trbe_errata_cpucaps[TRBE_ERRATA_MAX] = {
>> +	[TRBE_WORKAROUND_OVERWRITE_FILL_MODE] = ARM64_WORKAROUND_TRBE_OVERWRITE_FILL_MODE,
>>   };
>>   
>>   /*
>> @@ -519,10 +528,13 @@ static void trbe_enable_hw(struct trbe_buf *buf)
>>   	set_trbe_limit_pointer_enabled(buf->trbe_limit);
>>   }
>>   
>> -static enum trbe_fault_action trbe_get_fault_act(u64 trbsr)
>> +static enum trbe_fault_action trbe_get_fault_act(struct perf_output_handle *handle,
>> +						 u64 trbsr)
>>   {
>>   	int ec = get_trbe_ec(trbsr);
>>   	int bsc = get_trbe_bsc(trbsr);
>> +	struct trbe_buf *buf = etm_perf_sink_config(handle);
>> +	struct trbe_cpudata *cpudata = buf->cpudata;
>>   
>>   	WARN_ON(is_trbe_running(trbsr));
>>   	if (is_trbe_trg(trbsr) || is_trbe_abort(trbsr))
>> @@ -531,10 +543,16 @@ static enum trbe_fault_action trbe_get_fault_act(u64 trbsr)
>>   	if ((ec == TRBE_EC_STAGE1_ABORT) || (ec == TRBE_EC_STAGE2_ABORT))
>>   		return TRBE_FAULT_ACT_FATAL;
>>   
>> -	if (is_trbe_wrap(trbsr) && (ec == TRBE_EC_OTHERS) && (bsc == TRBE_BSC_FILLED)) {
>> -		if (get_trbe_write_pointer() == get_trbe_base_pointer())
>> -			return TRBE_FAULT_ACT_WRAP;
>> -	}
>> +	/*
>> +	 * If the trbe is affected by TRBE_WORKAROUND_OVERWRITE_FILL_MODE,
>> +	 * it might write data after a WRAP event in the fill mode.
>> +	 * Thus the check TRBPTR == TRBBASER will not be honored.
>> +	 */
>> +	if ((is_trbe_wrap(trbsr) && (ec == TRBE_EC_OTHERS) && (bsc == TRBE_BSC_FILLED)) &&
>> +	    (trbe_has_erratum(cpudata, TRBE_WORKAROUND_OVERWRITE_FILL_MODE) ||
>> +	     get_trbe_write_pointer() == get_trbe_base_pointer()))
>> +		return TRBE_FAULT_ACT_WRAP;
>> +
> 
> I'm very perplexed by the trbe_has_erratum() infrastructure... Since this is a
> TRBE the code will always run on the CPU it is associated with, and if
> I'm correct here we could call this_cpu_has_cap() directly with the same
> outcome.  I doubt that all divers using the cpucaps subsystem carry a shadow
> structure to keep the same information.

Very valid question. Of course, we can use the this_cpu_has_cap()
helper. Unlike the cpus_have_*_cap() - which gives you the system
wide status of the erratum - the cpucap doesn't keep a cache of which
CPUs are affected by a given erratum. Thus this_cpu_has_cap() would
involve running the detection on the current CPU everytime we call it.
i.e, scanning the MIDR of the CPU through the list of affected MIDRs
for the given erratum. This is a bit of overhead.

Given that we already have CPU specific information in trbe_cpudata, I
chose to cache the affected errata locally. This gives us quick access
to the erratum for individual TRBE instances. Of course this list is
initialised at TRBE probe and thus avoids us having to do the costly
check, each time we need it. I could make this clear in the patch
which introduces the framework.


Thanks for the review

Suzuki

> Thanks,
> Mathieu