Message-ID: <7ae6b3a6-1fa2-a655-9347-2263a1701c91@arm.com>
Date:   Thu, 23 Sep 2021 11:43:11 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Suzuki K Poulose <suzuki.poulose@....com>,
        linux-arm-kernel@...ts.infradead.org
Cc:     linux-kernel@...r.kernel.org, maz@...nel.org,
        catalin.marinas@....com, mark.rutland@....com, james.morse@....com,
        leo.yan@...aro.org, mike.leach@...aro.org,
        mathieu.poirier@...aro.org, will@...nel.org, lcherian@...vell.com,
        coresight@...ts.linaro.org
Subject: Re: [PATCH v2 09/17] coresight: trbe: Workaround TRBE errata
 overwrite in FILL mode



On 9/21/21 7:11 PM, 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

3 cache lines = 256 bytes on all implementation which might have TRBE ?
OR this skid bytes should be derived from the platform cache line size
instead.

> 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.

Via PERF_AUX_FLAG_TRUNCATED ? Should be specified here to be clear.

> 
> 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@...aro.org>
> Cc: Mathieu Poirier <mathieu.poirier@...aro.org>
> Cc: Anshuman Khandual <anshuman.khandual@....com>
> Cc: Leo Yan <leo.yan@...aro.org>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....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

As mentioned earlier, does it depend on the platform cache line size ?
Otherwise if the skip bytes is something platform independent, should
be mentioned here in a comment.

>  
>  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;

Passing down the perf handle to derive trbe_cpudata seems to be right.

>  
>  	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.
> +	 */

Needs bit formatting/alignment cleanup.

> +	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;
> +

Right, TRBE without the errata should continue to have the write
pointer = base pointer check. Could all TRBE errata checks like
the following be shortened (without the workaround index) for
better readability ? But not something very important.

trbe_has_erratum(cpudata, TRBE_WORKAROUND_OVERWRITE_FILL_MODE)


>  	return TRBE_FAULT_ACT_SPURIOUS;
>  }
>  
> @@ -544,6 +562,8 @@ static unsigned long trbe_get_trace_size(struct perf_output_handle *handle,
>  {
>  	u64 write;
>  	u64 start_off, end_off;
> +	u64 size;
> +	u64 overwrite_skip = TRBE_WORKAROUND_OVERWRITE_FILL_MODE_SKIP_BYTES;
>  
>  	/*
>  	 * If the TRBE has wrapped around the write pointer has
> @@ -559,7 +579,18 @@ static unsigned long trbe_get_trace_size(struct perf_output_handle *handle,
>  
>  	if (WARN_ON_ONCE(end_off < start_off))
>  		return 0;
> -	return (end_off - start_off);
> +
> +	size = end_off - start_off;
> +	/*
> +	 * If the TRBE is affected by the following erratum, we must fill
> +	 * the space we skipped with IGNORE packets. And we are always
> +	 * guaranteed to have at least a PAGE_SIZE space in the buffer.
> +	 */
> +	if (trbe_has_erratum(buf->cpudata, TRBE_WORKAROUND_OVERWRITE_FILL_MODE) &&
> +	    !WARN_ON(size < overwrite_skip))
> +		__trbe_pad_buf(buf, start_off, overwrite_skip);
> +
> +	return size;
>  }
>  
>  static void *arm_trbe_alloc_buffer(struct coresight_device *csdev,
> @@ -678,7 +709,7 @@ static unsigned long arm_trbe_update_buffer(struct coresight_device *csdev,
>  		clr_trbe_irq();
>  		isb();
>  
> -		act = trbe_get_fault_act(status);
> +		act = trbe_get_fault_act(handle, status);
>  		/*
>  		 * If this was not due to a WRAP event, we have some
>  		 * errors and as such buffer is empty.
> @@ -702,21 +733,95 @@ static unsigned long arm_trbe_update_buffer(struct coresight_device *csdev,
>  	return size;
>  }
>  
> +
> +static int trbe_apply_work_around_before_enable(struct trbe_buf *buf)
> +{
> +	/*
> +	 * TRBE_WORKAROUND_OVERWRITE_FILL_MODE causes the TRBE to overwrite a few cache

few cache lines = 3 cache lines ?

> +	 * line size from the "TRBBASER_EL1" in the event of a "FILL".
> +	 * Thus, we could loose some amount of the trace at the base.
> +	 *
> +	 * Before Fix:
> +	 *
> +	 *  normal-BASE     head  normal-PTR              tail normal-LIMIT
> +	 *  |                   \/                       /
> +	 *   -------------------------------------------------------------
> +	 *  |         |          |xyzdefghij..|...  tuvw|                |
> +	 *   -------------------------------------------------------------
> +	 *                      /    |                   \
> +	 * After Fix->  TRBBASER     TRBPTR              TRBLIMITR.LIMIT
> +	 *
> +	 * In the normal course of action, we would set the TRBBASER to the
> +	 * beginning of the ring-buffer (normal-BASE). But with the erratum,
> +	 * the TRBE could overwrite the contents at the "normal-BASE", after
> +	 * hitting the "normal-LIMIT", since it doesn't stop as expected. And
> +	 * this is wrong. So we must always make sure that the TRBBASER is
> +	 * within the region [head, head+size].
> +	 *
> +	 * Also, we would set the TRBPTR to head (after adjusting for
> +	 * alignment) at normal-PTR. This would mean that the last few bytes
> +	 * of the trace (say, "xyz") might overwrite the first few bytes of
> +	 * trace written ("abc"). More importantly they will appear in what\
> +	 * userspace sees as the beginning of the trace, which is wrong. We may
> +	 * not always have space to move the latest trace "xyz" to the correct
> +	 * order as it must appear beyond the LIMIT. (i.e, [head..head+size].
> +	 * Thus it is easier to ignore those bytes than to complicate the
> +	 * driver to move it, assuming that the erratum was triggered and doing
> +	 * additional checks to see if there is indeed allowed space at
> +	 * TRBLIMITR.LIMIT.
> +	 *
> +	 * To summarize, with the work around:
> +	 *
> +	 *  - We always align the offset for the next session to PAGE_SIZE
> +	 *    (This is to ensure we can program the TRBBASER to this offset
> +	 *    within the region [head...head+size]).
> +	 *
> +	 *  - At TRBE enable:
> +	 *     - Set the TRBBASER to the page aligned offset of the current
> +	 *       proposed write offset. (which is guaranteed to be aligned
> +	 *       as above)
> +	 *     - Move the TRBPTR to skip first 256bytes (that might be
> +	 *       overwritten with the erratum). This ensures that the trace
> +	 *       generated in the session is not re-written.
> +	 *
> +	 *  - At trace collection:
> +	 *     - Pad the 256bytes skipped above again with IGNORE packets.
> +	 */
> +	if (trbe_has_erratum(buf->cpudata, TRBE_WORKAROUND_OVERWRITE_FILL_MODE)) {
> +		if (WARN_ON(!IS_ALIGNED(buf->trbe_write, PAGE_SIZE)))
> +			return -EINVAL;
> +		buf->trbe_hw_base = buf->trbe_write;
> +		buf->trbe_write += TRBE_WORKAROUND_OVERWRITE_FILL_MODE_SKIP_BYTES;
> +	}
> +
> +	return 0;
> +}
> +
>  static int __arm_trbe_enable(struct trbe_buf *buf,
>  			     struct perf_output_handle *handle)
>  {
> +	int ret = 0;
> +
>  	perf_aux_output_flag(handle, PERF_AUX_FLAG_CORESIGHT_FORMAT_RAW);
>  	buf->trbe_limit = compute_trbe_buffer_limit(handle);
>  	buf->trbe_write = buf->trbe_base + PERF_IDX2OFF(handle->head, buf);
>  	if (buf->trbe_limit == buf->trbe_base) {
> -		trbe_stop_and_truncate_event(handle);
> -		return -ENOSPC;
> +		ret = -ENOSPC;
> +		goto err;
>  	}
>  	/* Set the base of the TRBE to the buffer base */
>  	buf->trbe_hw_base = buf->trbe_base;
> +
> +	ret = trbe_apply_work_around_before_enable(buf);
> +	if (ret)
> +		goto err;
> +
>  	*this_cpu_ptr(buf->cpudata->drvdata->handle) = handle;
>  	trbe_enable_hw(buf);
>  	return 0;
> +err:
> +	trbe_stop_and_truncate_event(handle);
> +	return ret;
>  }
>  
>  static int arm_trbe_enable(struct coresight_device *csdev, u32 mode, void *data)
> @@ -860,7 +965,7 @@ static irqreturn_t arm_trbe_irq_handler(int irq, void *dev)
>  	if (!is_perf_trbe(handle))
>  		return IRQ_NONE;
>  
> -	act = trbe_get_fault_act(status);
> +	act = trbe_get_fault_act(handle, status);
>  	switch (act) {
>  	case TRBE_FAULT_ACT_WRAP:
>  		truncated = !!trbe_handle_overflow(handle);
> @@ -1000,7 +1105,22 @@ static void arm_trbe_probe_cpu(void *info)
>  	}
>  
>  	trbe_check_errata(cpudata);
> -	cpudata->trbe_align = cpudata->trbe_hw_align;
> +	/*
> +	 * If the TRBE is affected by erratum TRBE_WORKAROUND_OVERWRITE_FILL_MODE,
> +	 * we must always program the TBRPTR_EL1, 256bytes from a page
> +	 * boundary, with TRBBASER_EL1 set to the page, to prevent
> +	 * TRBE over-writing 256bytes at TRBBASER_EL1 on FILL event.
> +	 *
> +	 * Thus make sure we always align our write pointer to a PAGE_SIZE,
> +	 * which also guarantees that we have at least a PAGE_SIZE space in
> +	 * the buffer (TRBLIMITR is PAGE aligned) and thus we can skip
> +	 * the required bytes at the base.
> +	 */
> +	if (trbe_has_erratum(cpudata, TRBE_WORKAROUND_OVERWRITE_FILL_MODE))
> +		cpudata->trbe_align = PAGE_SIZE;
> +	else
> +		cpudata->trbe_align = cpudata->trbe_hw_align;
> +

But like trbe_apply_work_around_before_enable(), trbe_align assignment
should also be wrapped inside a new helper which should contain these
comments and conditional block. Because it makes sense to have errata
work arounds in the leaf level helper functions, rather than TRBE core
operations.

>  	cpudata->trbe_flag = get_trbe_flag_update(trbidr);
>  	cpudata->cpu = cpu;
>  	cpudata->drvdata = drvdata;
> 

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