Hi,
In regards to the instruction tracing probe points that were added to SystemTap
last year, Frank had asked whether the block-trace functionality (.insn.block)
is working. I tested this on x86_64/Fedora 10 and, indeed, it does work.
However, when testing on a ppc64 system, it failed terribly -- "kernel BUG at
include/linux/ptrace.h:299!" Here's the stack trace from the system log:
finish_resume_report
utrace_resume
do_signal
do_work
In finish_resume_report, user_enable_block_step() is called if
utrace_report->action==UTRACE_BLOCKSTEP. user_enable_block_step() is defined
in include/linux/ptrace.h, and if arch_has_block_step is not defined, its
implementation is a simple call to BUG().
Apparently, arch_has_block_step is not defined on ppc64, although the hardware
is physically capable of branch exceptions using the MSR_BE bit. Is there a
reason why this has not been defined on ppc64 architecture? Or is it simply
that no one has gotten around to it yet. Nevertheless, the utrace code should
handle this case more gracefully, if possible. Can we check for
action==UTRACE_BLOCKSTEP earlier and bail out gracefully instead of blindly
calling user_enable_block_step()?
Once this issue is resolved, I will add a testcase to the itrace.exp in the
testsuite to test the insn.block probe.
Thanks.
-Maynard Johnson
