linux-kernel - Re: [PATCH v7 28/28] coresight: Add support for v8.4 SelfHosted tracing

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Date:   Fri, 12 Feb 2021 15:36:33 +0000
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     Mike Leach <mike.leach@...aro.org>
Cc:     linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Coresight ML <coresight@...ts.linaro.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Leo Yan <leo.yan@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jonathan Zhou <jonathan.zhouwen@...wei.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH v7 28/28] coresight: Add support for v8.4 SelfHosted
 tracing

Hi Mike

On 2/12/21 10:34 AM, Mike Leach wrote:
> Hi Mathieu, Suzuki,
> 
> Sorry for the really late response on this patch, but I noticed a
> problem while doing a review of the ETE / TRBE set. (TRBE specs
> mention TRFCR_ELx, so I was confirming a couple of things).
> 
> On Sun, 10 Jan 2021 at 22:49, Suzuki K Poulose <suzuki.poulose@....com> wrote:
>>
>> From: Jonathan Zhou <jonathan.zhouwen@...wei.com>
>>
>> v8.4 tracing extensions added support for trace filtering controlled
>> by TRFCR_ELx. This must be programmed to allow tracing at EL1/EL2 and
>> EL0. The timestamp used is the virtual time. Also enable CONTEXIDR_EL2
>> tracing if we are running the kernel at EL2.
>>
>> Cc: Catalin Marinas <catalin.marinas@....com>
>> Cc: Mike Leach <mike.leach@...aro.org>
>> Cc: Will Deacon <will@...nel.org>
>> Reviewed-by: Mathieu Poirier <mathieu.poirier@...aro.org>
>> Signed-off-by: Jonathan Zhou <jonathan.zhouwen@...wei.com>
>> [ Move the trace filtering setup etm_init_arch_data() and
>>   clean ups]
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
>> ---
>>   .../coresight/coresight-etm4x-core.c          | 25 +++++++++++++++++++
>>   1 file changed, 25 insertions(+)
>>
>> diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c
>> index 3d3165dd09d4..18c1a80abab8 100644
>> --- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
>> +++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
>> @@ -859,6 +859,30 @@ static bool etm4_init_csdev_access(struct etmv4_drvdata *drvdata,
>>          return false;
>>   }
>>
>> +static void cpu_enable_tracing(void)
>> +{
>> +       u64 dfr0 = read_sysreg(id_aa64dfr0_el1);
>> +       u64 trfcr;
>> +
>> +       if (!cpuid_feature_extract_unsigned_field(dfr0, ID_AA64DFR0_TRACE_FILT_SHIFT))
>> +               return;
>> +
>> +       /*
>> +        * If the CPU supports v8.4 SelfHosted Tracing, enable
>> +        * tracing at the kernel EL and EL0, forcing to use the
>> +        * virtual time as the timestamp.
>> +        */
>> +       trfcr = (TRFCR_ELx_TS_VIRTUAL |
>> +                TRFCR_ELx_ExTRE |
>> +                TRFCR_ELx_E0TRE);
>> +
>> +       /* If we are running at EL2, allow tracing the CONTEXTIDR_EL2. */
>> +       if (is_kernel_in_hyp_mode())
>> +               trfcr |= TRFCR_EL2_CX;
>> +
> 
> This is wrong - CX bit is present on TRFCR_EL2, not TRFCR_EL1.

Why is this wrong ? We do this only when we are in EL2.

> Moreover, TRFCR_EL2 has a separate enables for tracing at EL0 and EL2.
> 

True, that is for EL0&2 translation regimes. i.e, tracing EL0 with
the kernel running at EL2. But bits TRFCR_EL2.E2TRE == TRFCR_EL1.E1TRE
If notice, we name the bit TRFCR_ELx_ExTRE. And E0TRE == E0HTRE.

So we do the following :

   1) When kernel running at EL2:
     Enable tracing at EL2 and EL0 and context tracking
   2) When kernel running at EL1:
     Enable tracing at EL1 and EL0.


> Secondly - is this correct in principal?  Should the driver not be
> reading the access it is permitted by the kernel, rather than giving
> itself unfettered access to trace where it wants to.

I dont follow the "access permitted by the kernel" here. What are we referrring to ?

> Surely TRFCR_ELx  levels should be chosen in KConfig  and then should
> be set up in kernel initialisation?

I disagree with yet another Kconfig. This basic requirement for
enabling the trace collection. It is not something that we can optionally
use from the architecture. So we should transparently do the right
thing for making sure that we set up the system for something that
didn't require any other steps. Or in other words, if we add a Kconfig
option for TRFCR programming, if someone forgets to select it
when they upgraded the kernel they are in for a surprisingly long
debugging to find why the trace doesnt work.

As for the TRFCR programming, we have two choices. etm4x driver
or generic boot up for the CPU. I preferred to do this in the
driver as we can enable it only if trace drivers are available.

Cheers
Suzuki

> 
> Regards
> 
> Mike
> 
> 
> 
>> +       write_sysreg_s(trfcr, SYS_TRFCR_EL1);
>> +}
>> +
>>   static void etm4_init_arch_data(void *info)
>>   {
>>          u32 etmidr0;
>> @@ -1044,6 +1068,7 @@ static void etm4_init_arch_data(void *info)
>>          /* NUMCNTR, bits[30:28] number of counters available for tracing */
>>          drvdata->nr_cntr = BMVAL(etmidr5, 28, 30);
>>          etm4_cs_lock(drvdata, csa);
>> +       cpu_enable_tracing();
>>   }
>>
>>   static inline u32 etm4_get_victlr_access_type(struct etmv4_config *config)
>> --
>> 2.24.1
>>
> 
> 
> --
> Mike Leach
> Principal Engineer, ARM Ltd.
> Manchester Design Centre. UK
>