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SubjectRe: [PATCH 1/3] powerpc: Emulation support for load/store instructions on LE
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On Sunday 06 November 2016 01:01 AM, Anton Blanchard wrote:
> Hi,
>
>> kprobe, uprobe, hw-breakpoint and xmon are the only user of
>> emulate_step.
>>
>> Kprobe / uprobe single-steps instruction if they can't emulate it, so
>> there is no problem with them. As I mention, hw-breakpoint is broken.
>> However I'm not sure about xmon, I need to check that.
> I was mostly concerned that it would impact kprobes. Sounds like we are
> ok there.
>
>> So yes, there is no user-visible feature that depends on this.
> Aren't hardware breakpoints exposed via perf? I'd call perf
> user-visible.


Thanks Anton, That's a good catch. I tried this on ppc64le:

$ sudo cat /proc/kallsyms | grep pid_max
c00000000116998c D pid_max

$ sudo ./perf record -a --event=mem:0xc00000000116998c sleep 10


Before patch:
It does not record any data and throws below warning.

$ dmesg
[ 817.895573] Unable to handle hardware breakpoint. Breakpoint at 0xc00000000116998c will be disabled.
[ 817.895581] ------------[ cut here ]------------
[ 817.895588] WARNING: CPU: 24 PID: 2032 at arch/powerpc/kernel/hw_breakpoint.c:277 hw_breakpoint_handler+0x124/0x230
...

After patch:
It records data properly.

$ sudo ./perf report --stdio
...
# Samples: 36 of event 'mem:0xc00000000116998c'
# Event count (approx.): 36
#
# Overhead Command Shared Object Symbol
# ........ ............. ................ .............
#
63.89% kdumpctl [kernel.vmlinux] [k] alloc_pid
27.78% opal_errd [kernel.vmlinux] [k] alloc_pid
5.56% kworker/u97:4 [kernel.vmlinux] [k] alloc_pid
2.78% systemd [kernel.vmlinux] [k] alloc_pid


-Ravi

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