Messages in this thread Patch in this message | | | From | Anshuman Khandual <> | Subject | Re: [PATCH 08/10] coresight: trbe: Workaround TRBE errat overwrite in FILL mode | Date | Tue, 3 Aug 2021 15:55:22 +0530 |
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On 7/28/21 7:22 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 > 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.
If the write pointer is guaranteed to have always started beyond [base + 256] and then wraps around. Why cannot the initial trace at [base .. base + 256] not to be considered for perf ?
> We set the flags already to indicate that some amount of trace > was lost during the FILL event IRQ. So this is fine.
Right, from perf data flag point of view it is not a problem. But I am still wondering why cannot the trace at the base can not be consumed by perf.
> > 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
While enabling TRBE after required reconfiguration, this will cause both TRBBASER_EL1 and TRBPTR_EL1 to change each time around (basically TRBBASER_EL1 follows handle->head) ?
> "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.
Now that base needs to follow handle->head, both needs to be PAGE_SIZE aligned (including the write pointer) because base pointer needs to be PAGE_SIZE aligned ?
> > 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> > --- > drivers/hwtracing/coresight/coresight-trbe.c | 111 +++++++++++++++++-- > 1 file changed, 102 insertions(+), 9 deletions(-) > > diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c > index 9ea28813182b..cd997ed5d918 100644 > --- a/drivers/hwtracing/coresight/coresight-trbe.c > +++ b/drivers/hwtracing/coresight/coresight-trbe.c > @@ -17,6 +17,7 @@ > > #include <asm/barrier.h> > #include <asm/cputype.h> > +#include <asm/cpufeature.h> > > #include "coresight-self-hosted-trace.h" > #include "coresight-trbe.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
Although I am not sure how to achieve it, TRBE_ERRATA_MAX should be derived from errata cpucap start and end markers which identify all TRBE specific erratas. The hard coding above could be avoided.
> + > +/* > + * Safe limit for the number of bytes that may be overwritten > + * when the erratum is triggered.
Specify which errata ?
> + */ > +#define TRBE_WORKAROUND_OVERWRITE_FILL_MODE_SKIP 256
Needs to have _BYTES. TRBE_ERRATA_OVERWRITE_FILL_MODE_SKIP_BYTES ?
> > static unsigned long trbe_errata_cpucaps[TRBE_ERRATA_MAX] = { > + [TRBE_WORKAROUND_OVERWRITE_FILL_MODE] = ARM64_WORKAROUND_TRBE_OVERWRITE_FILL_MODE,
s/TRBE_WORKAROUND_OVERWRITE_FILL_MODE/TRBE_ERRATA_OVERWRITE_FILL_MODE instead ?
ARM64_WORKAROUND_TRBE_XXX -----> TRBE_ERRATA_XXX
> }; > > /* > @@ -531,10 +540,13 @@ 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; > - } > + /* > + * It is not necessary to verify the TRBPTR == TRBBASER to detect > + * a FILL event. Moreover, CPU errata could make this check invalid. > + */
Why should the check be dropped for CPU instances which dont have the errata ?
> + if (is_trbe_wrap(trbsr) && (ec == TRBE_EC_OTHERS) && (bsc == TRBE_BSC_FILLED)) > + return TRBE_FAULT_ACT_WRAP; > + > return TRBE_FAULT_ACT_SPURIOUS; > } > > @@ -544,6 +556,7 @@ static unsigned long trbe_get_trace_size(struct perf_output_handle *handle, > { > u64 write; > u64 start_off, end_off; > + u64 size; > > /* > * If the TRBE has wrapped around the write pointer has > @@ -559,7 +572,18 @@ 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(TRBE_WORKAROUND_OVERWRITE_FILL_MODE, buf->cpudata) && > + !WARN_ON(size < TRBE_WORKAROUND_OVERWRITE_FILL_MODE_SKIP))
Why warn here ? The skid can some times be less than 256 bytes. OR because now write pointer alignment is PAGE_SIZE (from later code), size too needs to be always a PAGE_SIZE multiple ?
> + __trbe_pad_buf(buf, start_off, TRBE_WORKAROUND_OVERWRITE_FILL_MODE_SKIP); > + > + return size; > } > > static void *arm_trbe_alloc_buffer(struct coresight_device *csdev, > @@ -704,20 +728,73 @@ 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. > + * > + * To work around this: > + * - Software must leave 256bytes of space from the base, so that > + * the trace collected now is not overwritten. > + * - Fill the first 256bytes with IGNORE packets for the decoder > + * to ignore at the end of the session, so that the decoder ignores > + * this gap. > + * > + * This also means that, the TRBE driver must set the TRBBASER_EL1 > + * such that, when the erratum is triggered, it doesn't overwrite > + * the "area" outside the area marked by (handle->head, +size). > + * So, we make sure that the handle->head is always PAGE aligned, > + * by tweaking the required alignment for the TRBE (trbe_align). > + * And when we enable the TRBE, > + * > + * - move the TRBPTR_EL1 to 256bytes past the starting point. > + * So that any trace collected in this run is not overwritten. > + * > + * - set the TRBBASER_EL1 to the original trbe_write. This will > + * ensure that, if the TRBE hits the erratum, it would only > + * write within the region allowed for the TRBE. > + * > + * At the trace collection time, we always pad the skipped bytes > + * with IGNORE packets to make sure the decoder doesn't see any > + * overwritten packets. > + */ > + if (trbe_has_erratum(TRBE_WORKAROUND_OVERWRITE_FILL_MODE, buf->cpudata)) { > + 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;
Not sure if I really understood this correctly, but would not the above keep on reducing the usable buffer space for TRBE each time around ? BTW buffer adjustment complexity here requires a proper ASCII diagram based illustration like before.
> + } > + > + return 0; > +} > + > static int __arm_trbe_enable(struct trbe_buf *buf, > struct perf_output_handle *handle) > { > + int ret = 0; > + > 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) > @@ -1003,7 +1080,23 @@ static void arm_trbe_probe_cpu(void *info) > pr_err("Unsupported alignment on cpu %d\n", cpu); > goto cpu_clear; > } > - cpudata->trbe_align = cpudata->trbe_hw_align; > + > + /* > + * If the TRBE is affected by erratum TRBE_WORKAROUND_OVERWRITE_FILL_MODE,
Better to address it with the original errata name i.e ARM64_WORKAROUND_TRBE_OVERWRITE_FILL_MODE instead. But not a big issue though.
> + * 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. > + */
Should not TRBPTR_EL1 be aligned to 256 bytes instead, as TRBBASER_EL1 is always at PAGE_SIZE boundary. Hence TRBPTR_EL1 could never be in between base and [base + 256 bytes]. Why alignment needs to go all the way upto PAGE_SIZE instead ? OR am I missing something here ?
> + if (trbe_has_erratum(TRBE_WORKAROUND_OVERWRITE_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; >
I will visit this patch again. Not sure if I really understand all this changes correctly enough.
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