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

[James Clark]

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.

> > +
> >   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