a70b1d8724
Ran the freeze under the corpus's gdb wrapper (ptrace_scope is 1, so a debugger
must launch rather than attach, and its handle lines are needed because Xenia
uses SIGSEGV for guest memory watches).
One thread of 80 is in guest code: thread 50, Main XThread, at rip a05be939 in
JIT output. All others are in a futex or clock_nanosleep.
Xenia's x64 backend keeps PPCContext in rsi, with r[32] at +0x20. The guest
GPRs there give r8 = a3ac0000 (the 64 MB buffer from the doubling sequence),
r12 = a3ac0a18 inside it, and r13 = 82457864 -- guest code, sub_82457780.
That address sits in a grow-and-copy: size = count*8 clamped to 0x1FFFFFFF, a
call to 0x824F7240, then a loop copying halfwords eight bytes at a time. The
host instruction it is stopped on is that copy's store:
mov %r12w,(%rdi,%rax,1) rdi = membase, rax = 0x701d0000
Sampled three times seconds apart with continue in between: rip, r13, r11 and
r31 identical every time. No progress.
The loop's exit test is beq -- equality, not >= -- so an inconsistent start/end
pair never terminates it. Recorded as amber: that is a reading of the
disassembly, not a demonstration.
Two of my own readings corrected:
* "the guest spins at ~400% CPU" was ps's CUMULATIVE AVERAGE since process
start, not an instantaneous rate. Per-thread sampling puts Main XThread
nowhere near the top.
* a SIGSEGV fault storm fitted the constant rip nicely and is refuted: 1500
minor faults in 5 s, zero major.
Caveat kept on the page: these are gdb-hosted observations and gdb intercepts
every SIGSEGV, so absolute timings are not the ungoverned ones. The freeze is
not a gdb artefact -- it reproduces in every non-gdb run.