Re: [PATCH 3/4] perf arm_spe: Decode Arm N1 IMPDEF events

[Ian Rogers]

On Tue, Apr 7, 2026 at 5:35 AM James Clark <james.clark@xxxxxxxxxx> wrote:
>
>
>
> On 02/04/2026 4:26 pm, Ian Rogers wrote:
> > On Wed, Apr 1, 2026 at 7:26 AM James Clark <james.clark@xxxxxxxxxx> wrote:
> >>
> >>  From the TRM [1], N1 has one IMPDEF event which isn't covered by the
> >> common list. Add a framework so that more cores can be added in the
> >> future and that the N1 IMPDEF event can be decoded. Also increase the
> >> size of the buffer because we're adding more strings and if it gets
> >> truncated it falls back to a hex dump only.
> >>
> >> [1]: https://developer.arm.com/documentation/100616/0401/Statistical-Profiling-Extension/implementation-defined-features-of-SPE
> >> Suggested-by: Al Grant <al.grant@xxxxxxx>
> >> Signed-off-by: James Clark <james.clark@xxxxxxxxxx>
> >> ---
> >>   tools/perf/util/arm-spe-decoder/Build              |  2 +
> >>   .../util/arm-spe-decoder/arm-spe-pkt-decoder.c     | 45 ++++++++++++++++++++--
> >>   .../util/arm-spe-decoder/arm-spe-pkt-decoder.h     |  5 ++-
> >>   tools/perf/util/arm-spe.c                          | 13 ++++---
> >>   4 files changed, 54 insertions(+), 11 deletions(-)
> >>
> >> diff --git a/tools/perf/util/arm-spe-decoder/Build b/tools/perf/util/arm-spe-decoder/Build
> >> index ab500e0efe24..97a298d1e279 100644
> >> --- a/tools/perf/util/arm-spe-decoder/Build
> >> +++ b/tools/perf/util/arm-spe-decoder/Build
> >> @@ -1 +1,3 @@
> >>   perf-util-y += arm-spe-pkt-decoder.o arm-spe-decoder.o
> >> +
> >> +CFLAGS_arm-spe-pkt-decoder.o += -I$(srctree)/tools/arch/arm64/include/ -I$(OUTPUT)arch/arm64/include/generated/
> >> diff --git a/tools/perf/util/arm-spe-decoder/arm-spe-pkt-decoder.c b/tools/perf/util/arm-spe-decoder/arm-spe-pkt-decoder.c
> >> index c880b0dec3a1..42a7501d4dfe 100644
> >> --- a/tools/perf/util/arm-spe-decoder/arm-spe-pkt-decoder.c
> >> +++ b/tools/perf/util/arm-spe-decoder/arm-spe-pkt-decoder.c
> >> @@ -15,6 +15,8 @@
> >>
> >>   #include "arm-spe-pkt-decoder.h"
> >>
> >> +#include "../../arm64/include/asm/cputype.h"
> >
> > Sashiko spotted:
> > https://sashiko.dev/#/patchset/20260401-james-spe-impdef-decode-v1-0-ad0d372c220c%40linaro.org
> > """
> > This isn't a bug, but does this include directive rely on accidental
> > path normalization?
> >
> > The relative path ../../arm64/include/asm/cputype.h does not exist relative
> > to arm-spe-pkt-decoder.c. It only compiles because the Build file adds
> > -I$(srctree)/tools/arch/arm64/include/ to CFLAGS.
> >
> > Would it be cleaner to use #include <asm/cputype.h> to explicitly rely on
> > the include path?
> > [ ... ]
> > """
> > I wouldn't use <asm/cputype.h> due to cross-compilation and the like,
> > instead just add the extra "../" into the include path.
> >
>
> Do you mean change the #include to this?
>
>    #include "../../../arm64/include/asm/cputype.h"
>
> I still need to add:
>
>    CFLAGS_arm-spe-pkt-decoder.o += -I$(srctree)/tools/arch/arm64/include/
>
> To make the this include in cputype.h work:
>
>    #include <asm/sysreg.h>
>
> Which probably only works because there isn't a sysreg.h on other
> architectures. But I'm not sure what the significance of ../../ vs
> ../../../ is if either compile? arm-spe.c already does it with ../../
> which is what I copied.

Hmm.. maybe the path should be
"../../../arch/arm64/include/asm/cputype.h". The include preference is
for a path relative to the source file and
../../arm64/include/asm/cputype.h doesn't exist. It is kind of horrid
to add an include path and then use a relative path to escape into a
higher-level directory. arm-spe.c is a little different as it is one
directory higher in the directory layout.

Thanks,
Ian

> >> +
> >>   static const char * const arm_spe_packet_name[] = {
> >>          [ARM_SPE_PAD]           = "PAD",
> >>          [ARM_SPE_END]           = "END",
> >> @@ -307,6 +309,11 @@ static const struct ev_string common_ev_strings[] = {
> >>          { .event = 0, .desc = NULL },
> >>   };
> >>
> >> +static const struct ev_string n1_event_strings[] = {
> >> +       { .event = 12, .desc = "LATE-PREFETCH" },
> >> +       { .event = 0, .desc = NULL },
> >> +};
> >> +
> >>   static u64 print_event_list(int *err, char **buf, size_t *buf_len,
> >>                              const struct ev_string *ev_strings, u64 payload)
> >>   {
> >> @@ -318,14 +325,44 @@ static u64 print_event_list(int *err, char **buf, size_t *buf_len,
> >>          return payload;
> >>   }
> >>
> >> +struct event_print_handle {
> >> +       const struct midr_range *midr_ranges;
> >> +       const struct ev_string *ev_strings;
> >> +};
> >> +
> >> +#define EV_PRINT(range, strings)                       \
> >> +       {                                       \
> >> +               .midr_ranges = range,           \
> >> +               .ev_strings = strings,  \
> >> +       }
> >> +
> >> +static const struct midr_range n1_event_encoding_cpus[] = {
> >> +       MIDR_ALL_VERSIONS(MIDR_NEOVERSE_N1),
> >> +       {},
> >> +};
> >> +
> >> +static const struct event_print_handle event_print_handles[] = {
> >> +       EV_PRINT(n1_event_encoding_cpus, n1_event_strings),
> >> +};
> >> +
> >>   static int arm_spe_pkt_desc_event(const struct arm_spe_pkt *packet,
> >> -                                 char *buf, size_t buf_len)
> >> +                                 char *buf, size_t buf_len, u64 midr)
> >>   {
> >>          u64 payload = packet->payload;
> >>          int err = 0;
> >>
> >>          arm_spe_pkt_out_string(&err, &buf, &buf_len, "EV");
> >> -       print_event_list(&err, &buf, &buf_len, common_ev_strings, payload);
> >> +       payload = print_event_list(&err, &buf, &buf_len, common_ev_strings,
> >> +                                  payload);
> >> +
> >> +       /* Try to decode IMPDEF bits for known CPUs */
> >> +       for (unsigned int i = 0; i < ARRAY_SIZE(event_print_handles); i++) {
> >> +               if (is_midr_in_range_list(midr,
> >> +                                         event_print_handles[i].midr_ranges))
> >> +                       payload = print_event_list(&err, &buf, &buf_len,
> >> +                                                  event_print_handles[i].ev_strings,
> >> +                                                  payload);
> >> +       }
> >>
> >>          return err;
> >>   }
> >> @@ -506,7 +543,7 @@ static int arm_spe_pkt_desc_counter(const struct arm_spe_pkt *packet,
> >>   }
> >>
> >>   int arm_spe_pkt_desc(const struct arm_spe_pkt *packet, char *buf,
> >> -                    size_t buf_len)
> >> +                    size_t buf_len, u64 midr)
> >>   {
> >>          int idx = packet->index;
> >>          unsigned long long payload = packet->payload;
> >> @@ -522,7 +559,7 @@ int arm_spe_pkt_desc(const struct arm_spe_pkt *packet, char *buf,
> >>                  arm_spe_pkt_out_string(&err, &buf, &blen, "%s", name);
> >>                  break;
> >>          case ARM_SPE_EVENTS:
> >> -               err = arm_spe_pkt_desc_event(packet, buf, buf_len);
> >> +               err = arm_spe_pkt_desc_event(packet, buf, buf_len, midr);
> >>                  break;
> >>          case ARM_SPE_OP_TYPE:
> >>                  err = arm_spe_pkt_desc_op_type(packet, buf, buf_len);
> >> diff --git a/tools/perf/util/arm-spe-decoder/arm-spe-pkt-decoder.h b/tools/perf/util/arm-spe-decoder/arm-spe-pkt-decoder.h
> >> index adf4cde320aa..17b067fe3c87 100644
> >> --- a/tools/perf/util/arm-spe-decoder/arm-spe-pkt-decoder.h
> >> +++ b/tools/perf/util/arm-spe-decoder/arm-spe-pkt-decoder.h
> >> @@ -11,7 +11,7 @@
> >>   #include <stddef.h>
> >>   #include <stdint.h>
> >>
> >> -#define ARM_SPE_PKT_DESC_MAX           256
> >> +#define ARM_SPE_PKT_DESC_MAX           512
> >>
> >>   #define ARM_SPE_NEED_MORE_BYTES                -1
> >>   #define ARM_SPE_BAD_PACKET             -2
> >> @@ -186,5 +186,6 @@ const char *arm_spe_pkt_name(enum arm_spe_pkt_type);
> >>   int arm_spe_get_packet(const unsigned char *buf, size_t len,
> >>                         struct arm_spe_pkt *packet);
> >>
> >> -int arm_spe_pkt_desc(const struct arm_spe_pkt *packet, char *buf, size_t len);
> >> +int arm_spe_pkt_desc(const struct arm_spe_pkt *packet, char *buf, size_t len,
> >> +                    u64 midr);
> >>   #endif
> >> diff --git a/tools/perf/util/arm-spe.c b/tools/perf/util/arm-spe.c
> >> index 7447b000f9cd..46f0309c092b 100644
> >> --- a/tools/perf/util/arm-spe.c
> >> +++ b/tools/perf/util/arm-spe.c
> >> @@ -135,7 +135,7 @@ struct data_source_handle {
> >>          }
> >>
> >>   static void arm_spe_dump(struct arm_spe *spe __maybe_unused,
> >> -                        unsigned char *buf, size_t len)
> >> +                        unsigned char *buf, size_t len, u64 midr)
> >>   {
> >>          struct arm_spe_pkt packet;
> >>          size_t pos = 0;
> >> @@ -161,7 +161,7 @@ static void arm_spe_dump(struct arm_spe *spe __maybe_unused,
> >>                          color_fprintf(stdout, color, "   ");
> >>                  if (ret > 0) {
> >>                          ret = arm_spe_pkt_desc(&packet, desc,
> >> -                                              ARM_SPE_PKT_DESC_MAX);
> >> +                                              ARM_SPE_PKT_DESC_MAX, midr);
> >>                          if (!ret)
> >>                                  color_fprintf(stdout, color, " %s\n", desc);
> >>                  } else {
> >> @@ -174,10 +174,10 @@ static void arm_spe_dump(struct arm_spe *spe __maybe_unused,
> >>   }
> >>
> >>   static void arm_spe_dump_event(struct arm_spe *spe, unsigned char *buf,
> >> -                              size_t len)
> >> +                              size_t len, u64 midr)
> >>   {
> >>          printf(".\n");
> >> -       arm_spe_dump(spe, buf, len);
> >> +       arm_spe_dump(spe, buf, len, midr);
> >>   }
> >>
> >>   static int arm_spe_get_trace(struct arm_spe_buffer *b, void *data)
> >> @@ -1469,8 +1469,11 @@ static int arm_spe_process_auxtrace_event(struct perf_session *session,
> >>                  /* Dump here now we have copied a piped trace out of the pipe */
> >>                  if (dump_trace) {
> >>                          if (auxtrace_buffer__get_data(buffer, fd)) {
> >> +                               u64 midr = 0;
> >> +
> >> +                               arm_spe__get_midr(spe, buffer->cpu.cpu, &midr);
> >
> > Sashiko claims to have spotted an issue here:
> > """
> > Is it possible for arm_spe__get_midr() to cause a segmentation fault here?
> >
> > If the trace is from an older recording (metadata version 1) and the
> > environment lacks a CPUID string (such as during cross-architecture
> > analysis), perf_env__cpuid() returns NULL.
> >
> > It appears arm_spe__get_midr() then passes this NULL pointer to
> > strtol(cpuid, NULL, 16), which leads to undefined behavior.
> > """
> >
> > But this feels like, if this happens you're already having a bad time
> > and these changes aren't necessarily making things worse.
> >
> > Thanks,
> > Ian
> >
>
> Yeah I think it might be possible so I can add an error instead of a
> segfault. I'll check the rest of the Sashiko comments too.
>
> >>                                  arm_spe_dump_event(spe, buffer->data,
> >> -                                               buffer->size);
> >> +                                               buffer->size, midr);
> >>                                  auxtrace_buffer__put_data(buffer);
> >>                          }
> >>                  }
> >>
> >> --
> >> 2.34.1
> >>
>