linux-kernel - Re: [PATCH 1/3] powerpc: Emulation support for load/store instructions on LE

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Date:   Mon, 7 Nov 2016 00:29:04 +0530
From:   Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com>
To:     Anton Blanchard <anton@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     Andrew Donnellan <andrew.donnellan@....ibm.com>,
        chris@...troguy.com, aneesh.kumar@...ux.vnet.ibm.com,
        ast@...nel.org, linux-kernel@...r.kernel.org, oss@...error.net,
        paul.gortmaker@...driver.com, duwe@....de,
        lsorense@...lub.uwaterloo.ca, bauerman@...ux.vnet.ibm.com,
        oohall@...il.com, naveen.n.rao@...ux.vnet.ibm.com,
        linuxppc-dev@...ts.ozlabs.org, paulus@...ba.org,
        viro@...iv.linux.org.uk,
        Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com>
Subject: Re: [PATCH 1/3] powerpc: Emulation support for load/store
 instructions on LE



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