Hi Leo,

On 08/02/2022 13:00, Leo Yan wrote:

Hi German,

On Mon, Feb 07, 2022 at 12:06:14PM +0000, German Gomez wrote:

[...]
Indeed!  I can reproduce the issue now.  And I can capture backtrace
for arm_spe_pmu_start() with below commands:

# cd /home/leoy/linux/tools/perf
# ./perf probe --add "arm_spe_pmu_start" -s /home/leoy/linux/ -k /home/leoy/linux/vmlinux
# echo 1 > /sys/kernel/debug/tracing/events/probe/arm_spe_pmu_start/enable
# echo stacktrace > /sys/kernel/debug/tracing/events/probe/arm_spe_pmu_start/trigger

... run your commands with non-root user ...

# cat /sys/kernel/debug/tracing/trace

             dd-7697    [000] d.h2.   506.068700: arm_spe_pmu_start: (arm_spe_pmu_start+0x8/0xe0)
             dd-7697    [000] d.h3.   506.068701: <stack trace>
=> kprobe_dispatcher
=> kprobe_breakpoint_handler
=> call_break_hook
[...]
=> do_el0_svc
=> el0_svc
=> el0t_64_sync_handler
=> el0t_64_sync

The backtrace clearly shows the function arm_spe_pmu_start() is
invoked in the 'dd' process (dd-7697); the flow is:
- perf program sends IPI to CPU0;
- 'dd' process is running on CPU0 and it's interrupted to handle IPI;
- 'dd' process has root capabilities, so it will enable context
  tracing for non-root perf session.

Thanks for testing, and sharing the commands in your replies!

quoted

quoted

quoted

One way to fix this is by caching the value of the CX bit during the
initialization of the PMU event, so that it remains consistent for the
duration of the session.

[...]

So the patch makes sense to me.  Just a minor comment:

Here we can define a u64 for recording pmscr value rather than a
bool value.

struct arm_spe_pmu {
    ...
    u64 pmscr;
};

I agree with the comment from Will that it makes more sense to store the
value of the register in the perf_event somehow (due to misunderstanding
from my side, I thought arm_spe_pmu struct was local to the session).

What about perf_event's void *pmu_private?

Thanks,
German

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