alpha: add HAVE_PERF_REGS and HAVE_PERF_USER_STACK_DUMP support

Let perf sample registers and dump the user stack, which is what the
perf tool needs for --call-graph dwarf.

The register indices follow the hardware numbering, so r0-r30 map
directly to DWARF register numbers, with pc after them. r9-r15
are callee-saved and live in switch_stack rather than pt_regs, so they
cannot be sampled. Without r15 an unwinder cannot recover the CFA of a
first frame that uses $15 as its frame pointer.

sp is not in pt_regs either: for user frames it comes from
user_stack_pointer(), and for kernel frames (PERF_SAMPLE_REGS_INTR) it
is the end of the trap frame, as in stacktrace.c.

Software events such as context switches and tracepoints hand perf a
per-CPU pt_regs filled in by perf_fetch_caller_regs() instead of a
trap frame. It is not on the stack and has no room for sp, so report
sp as 0 there rather than the address of the per-CPU buffer.

Assisted-by: Claude:claude-opus-5-5
Acked-by: Ian Rogers <irogers@google.com>
Tested-by: Magnus Lindholm <linmag7@gmail.com>
Reviewed-by: Magnus Lindholm <linmag7@gmail.com>
Signed-off-by: Matt Turner <mattst88@gmail.com>
Link: https://lore.kernel.org/r/20261018-alpha-perf-regs-v3-v3-4-52be85332c57@gmail.com
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>

0 files changed, 0 insertions, 0 deletions