hwpmc(4): stop overwalking past _start on amd64 user callchainsHEADmain

pmc_save_user_callchain() emits the pc it just loaded before checking
whether fp is the ABI's zero frame-chain terminator. At the bottom of
a well-formed chain under _start, fp comes back 0 as expected, but the
paired pc is stale rtld data left on the stack -- a legal userspace VA
that still passes PMC_IN_USERSPACE(), so it gets emitted as a bogus
extra frame. This shows up in flame graphs as a spurious hex-valued
root frame below _start.

Check fp == 0 alongside the existing checks before emitting, matching
how arm/arm64/powerpc already load the next fp before their check.

Measured via 1kHz hwpmc sampling on an OCA: stacks with any unresolved
hex frame drop from 23.9% to 1.3%, and stacks with hex at the root drop
from 5.5% to 0.3%.

Reviewed by:	mhorne, Ali Mashtizadeh <ali@mashtizadeh.com>, gallatin
MFC after:	3 days
Sponsored by:	Netflix
Differential Revision:	https://reviews.freebsd.org/D59229

0 files changed, 0 insertions, 0 deletions