[RFC PATCH v3 0/3] perf: sparc64 user regs and stack dump, with arch hooks

[Stian Halseth <stian@itx.no>]

This adds HAVE_PERF_REGS and HAVE_PERF_USER_STACK_DUMP to sparc64, so
that perf record --call-graph dwarf and elfutils' eu-stackprof work
there.

Two things about the user stack dump do not fit in arch code. The
sampled register window's %l/%i registers, which hold the frame pointer
and return address the unwinder starts from, stay in the register file
until a window spills. The kernel already flushes them wherever it
exposes user stack memory (perf_callchain_user(), ptrace), but the stack
dump has no arch entry point where that could happen. And a 64-bit
sparc stack pointer is biased by 2047, so a dump that starts at the
register value begins 2047 bytes below the frame, and is empty when that
page has never been touched.

Patch 1 therefore adds two hooks in the style of perf_arch_misc_flags(),
both no-ops by default. Patch 2 is the sparc64 implementation and their
user. Patch 3 teaches tools/perf the sparc registers and DWARF unwinding
from the dump. The matching elfutils changes are attached to the
tracking issue:

  https://github.com/sparclinux/issues/issues/99

Tested on an UltraSPARC T4-1 on 7.3-rc4: register values check out
against known contents, --call-graph dwarf unwinds correctly for both
cycles and cpu-clock, as does eu-stackprof, and perf stat/record/record
-g are unchanged. Starting the dump at the stack's actual address
matters in practice: with the dump at the biased register value, 94% of
the user samples of xz -T4 had an empty stack dump; with this series,
none do.

Changes since v2:
- Patch 1: add perf_arch_user_stack_pointer(), so an arch can start the
  dump at the stack's actual address (sashiko review).
- Patch 2: drop %g0 from the uapi, as trap entry does not save it; the
  PC takes its slot, as on mips and loongarch (sashiko review).
- Patch 2: start the dump at %sp + 2047 for a 64-bit stack (sashiko
  review).
- Patch 3: follow both, and comment the max_dwarf_reg adjustment (Ian).
  Drop the memcpy() change to memory_read(), which the unbiased dump
  start makes unnecessary.

v2: https://lore.kernel.org/all/20260922201507.1719668-1-stian@itx.no/
v1: https://lore.kernel.org/all/20260922135653.1622301-1-stian@itx.no/

Stian Halseth (3):
  perf/core: Let an arch prepare and locate the user stack dump
  sparc64: Support PERF_SAMPLE_REGS_USER and PERF_SAMPLE_STACK_USER
  perf tools: Support sparc user register samples and dwarf unwinding

 .../features/perf/perf-regs/arch-support.txt  |  2 +-
 .../perf/perf-stackdump/arch-support.txt      |  2 +-
 arch/sparc/Kconfig                            |  2 +
 arch/sparc/include/asm/perf_event.h           |  5 ++
 arch/sparc/include/uapi/asm/perf_regs.h       | 34 +++++++++
 arch/sparc/kernel/Makefile                    |  2 +-
 arch/sparc/kernel/perf_regs.c                 | 74 +++++++++++++++++++
 include/linux/perf_event.h                    | 11 +++
 kernel/events/core.c                          |  2 +
 kernel/events/internal.h                      |  2 +-
 tools/arch/sparc/include/uapi/asm/perf_regs.h | 34 +++++++++
 tools/perf/arch/sparc/include/perf_regs.h     | 18 +++++
 tools/perf/check-headers.sh                   |  1 +
 tools/perf/util/dwarf-regs-arch/Build         |  1 +
 .../util/dwarf-regs-arch/dwarf-regs-sparc.c   | 12 +++
 tools/perf/util/dwarf-regs.c                  |  4 +
 tools/perf/util/include/dwarf-regs.h          |  1 +
 tools/perf/util/perf-regs-arch/Build          |  1 +
 .../util/perf-regs-arch/perf_regs_sparc.c     | 67 +++++++++++++++++
 tools/perf/util/perf_regs.c                   | 18 +++++
 tools/perf/util/perf_regs.h                   |  5 ++
 tools/perf/util/unwind-libdw.c                | 37 ++++++++++
 22 files changed, 331 insertions(+), 4 deletions(-)
 create mode 100644 arch/sparc/include/uapi/asm/perf_regs.h
 create mode 100644 arch/sparc/kernel/perf_regs.c
 create mode 100644 tools/arch/sparc/include/uapi/asm/perf_regs.h
 create mode 100644 tools/perf/arch/sparc/include/perf_regs.h
 create mode 100644 tools/perf/util/dwarf-regs-arch/dwarf-regs-sparc.c
 create mode 100644 tools/perf/util/perf-regs-arch/perf_regs_sparc.c

-- 
2.55.0