Re: [PATCH v5 10/15] coresight: trbe: Workaround TRBE errata overwrite in FILL mode

Anshuman Khandual
2021-10-19T05:42:47Z

On 10/15/21 4:01 AM, 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>
> Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>

Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>

> ---
> Changes since v2:
> - Updated the ASCII art to include better description of
> all the steps in the work around
> 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 | 169 +++++++++++++++++--
> 1 file changed, 158 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c
> index 314e5e7374c7..b56b166b2dec 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 "coresight-self-hosted-trace.h"
> #include "coresight-trbe.h"
> @@ -88,14 +89,22 @@ struct trbe_buf {
> * - Not duplicating the detection logic
> * - Streamlined detection of erratum across the system
> */
> +#define TRBE_WORKAROUND_OVERWRITE_FILL_MODE 0
>
> static int trbe_errata_cpucaps[] = {
> + [TRBE_WORKAROUND_OVERWRITE_FILL_MODE] = ARM64_WORKAROUND_TRBE_OVERWRITE_FILL_MODE,
> -1, /* Sentinel, must be the last entry */
> };
>
> /* The total number of listed errata in trbe_errata_cpucaps */
> #define TRBE_ERRATA_MAX (ARRAY_SIZE(trbe_errata_cpucaps) - 1)
>
> +/*
> + * Safe limit for the number of bytes that may be overwritten
> + * when ARM64_WORKAROUND_TRBE_OVERWRITE_FILL_MODE is triggered.
> + */
> +#define TRBE_WORKAROUND_OVERWRITE_FILL_MODE_SKIP_BYTES 256
> +
> /*
> * struct trbe_cpudata: TRBE instance specific data
> * @trbe_flag - TRBE dirty/access flag support
> @@ -146,6 +155,11 @@ static inline bool trbe_has_erratum(struct trbe_cpudata *cpudata, int i)
> return (i < TRBE_ERRATA_MAX) && test_bit(i, cpudata->errata);
> }
>
> +static inline bool trbe_may_overwrite_in_fill_mode(struct trbe_cpudata *cpudata)
> +{
> + return trbe_has_erratum(cpudata, TRBE_WORKAROUND_OVERWRITE_FILL_MODE);
> +}
> +
> static int trbe_alloc_node(struct perf_event *event)
> {
> if (event->cpu == -1)
> @@ -549,10 +563,13 @@ 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))
> @@ -561,10 +578,16 @@ 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_may_overwrite_in_fill_mode(cpudata) ||
> + get_trbe_write_pointer() == get_trbe_base_pointer()))
> + return TRBE_FAULT_ACT_WRAP;
> +
> return TRBE_FAULT_ACT_SPURIOUS;
> }
>
> @@ -573,6 +596,8 @@ 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
> @@ -593,7 +618,18 @@ 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,
> @@ -712,7 +748,7 @@ 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.
> @@ -736,21 +772,117 @@ 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
> + * 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-TRBPTR) tail (normal-LIMIT)
> + * | \/ /
> + * -------------------------------------------------------------
> + * | Pg0 | Pg1 | | | PgN |
> + * -------------------------------------------------------------
> + *
> + * 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. This could result in overwriting trace collected in
> + * one of the previous runs, being consumed by the user. So we must
> + * always make sure that the TRBBASER is within the region
> + * [head, head+size]. Note that TRBBASER must be PAGE aligned,
> + *
> + * After moving the BASE:
> + *
> + * normal-BASE head (normal-TRBPTR) tail (normal-LIMIT)
> + * | \/ /
> + * -------------------------------------------------------------
> + * | | |xyzdef. |.. tuvw| |
> + * -------------------------------------------------------------
> + * /
> + * New-BASER
> + *
> + * 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.
> + *
> + * Thus the full workaround will move the BASE and the PTR and would
> + * look like (after padding at the skipped bytes at the end of
> + * session) :
> + *
> + * normal-BASE head (normal-TRBPTR) tail (normal-LIMIT)
> + * | \/ /
> + * -------------------------------------------------------------
> + * | | |///abc.. |.. rst| |
> + * -------------------------------------------------------------
> + * / |
> + * New-BASER New-TRBPTR
> + *
> + * 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)
> @@ -890,7 +1022,7 @@ 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);
> @@ -1038,7 +1170,22 @@ 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_may_overwrite_in_fill_mode(cpudata))
> + cpudata->trbe_align = PAGE_SIZE;
> + else
> + cpudata->trbe_align = cpudata->trbe_hw_align;
> +
> cpudata->trbe_flag = get_trbe_flag_update(trbidr);
> cpudata->cpu = cpu;
> cpudata->drvdata = drvdata;
>