[PATCH v2 09/17] coresight: trbe: Workaround TRBE errata overwrite in FILL mode
Anshuman Khandual
anshuman.khandual at arm.com
Thu Sep 30 21:21:27 PDT 2021
On 9/28/21 4:10 PM, Suzuki K Poulose wrote:
> On 23/09/2021 07:13, Anshuman Khandual wrote:
>>
>>
>> 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.
>
> 256bytes is the aligned (to the power of 2) value for the safe guard.
> Not 3 cache lines. Ideally, if there is another CPU that has larger
> cache line size, affected by the erratum, yes, we must do that.
> But for now this is sufficient.
Okay.
>
>>
>>> 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.
>
> Please note that setting the flag is not a side effect of the
> work around. And as such I don't think this needs to be mentioned
> here. e.g, we changed this to COLLISION recently for WRAP events.
> It makes sense to keep the details to the driver.
Okay.
>
>>
>>>
>
>>> 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 at linaro.org>
>>> Cc: Mathieu Poirier <mathieu.poirier at linaro.org>
>>> Cc: Anshuman Khandual <anshuman.khandual at arm.com>
>>> Cc: Leo Yan <leo.yan at linaro.org>
>>> Signed-off-by: Suzuki K Poulose <suzuki.poulose at 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
>>
>> 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.
>
> I could add 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)
>
> Do you mean something like :
>
> trbe_has_erratum(cpudata, TRBE_WORKAROUND_OVERWRITE_FILL_MODE) ->
> trbe_may_overwrite_in_fill_mode(cpudata) ?
Right, something similar which absorbs the work around index in
its name itself.
>
>
>>
>>
>>> 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 ?
>
> Yes, upto 3.
>
>>
>>> + * 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.
>
> That would imply we re-initialize the trbe_align in the new helper after
> setting the value here for all other unaffected TRBEs. I would rather
> leave it as it is, until we have more work arounds that touch this area.
> This is one of code called per TRBE instance.
Okay.
More information about the linux-arm-kernel
mailing list