linux-kernel - Re: [PATCH RFC v4 1/6] arm64: support single-step and breakpoint handler hooks

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Date:	Tue, 3 Dec 2013 20:03:17 +0530
From:	Sandeepa Prabhu <sandeepa.prabhu@...aro.org>
To:	Will Deacon <will.deacon@....com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"patches@...aro.org" <patches@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"steve.capper@...aro.org" <steve.capper@...aro.org>,
	"nico@...aro.org" <nico@...aro.org>,
	"srikar@...ux.vnet.ibm.com" <srikar@...ux.vnet.ibm.com>,
	"rostedt@...dmis.org" <rostedt@...dmis.org>,
	"masami.hiramatsu.pt@...achi.com" <masami.hiramatsu.pt@...achi.com>,
	"dsaxena@...aro.org" <dsaxena@...aro.org>,
	"jiang.liu@...wei.com" <jiang.liu@...wei.com>,
	"Vijaya.Kumar@...iumnetworks.com" <Vijaya.Kumar@...iumnetworks.com>
Subject: Re: [PATCH RFC v4 1/6] arm64: support single-step and breakpoint
 handler hooks

Hi Will,

Sorry for responding to this after long-time, I missed this review
during Linaro connect travels.

On 25 October 2013 20:52, Will Deacon <will.deacon@....com> wrote:
> Hi Sandeepa,
>
> This is getting there, thanks for persevering with it. I still have a few
> minor comments though.
>
> On Thu, Oct 17, 2013 at 12:17:46PM +0100, Sandeepa Prabhu wrote:
>> AArch64 Single Steping and Breakpoint debug exceptions will be
>> used by multiple debug framworks like kprobes & kgdb.
>>
>> This patch implements the hooks for those frameworks to register
>> their own handlers for handling breakpoint and single step events.
>>
>> Reworked the debug exception handler in entry.S: do_dbg to route
>> software breakpoint (BRK64) exception to do_debug_exception()
>>
>> Signed-off-by: Sandeepa Prabhu <sandeepa.prabhu@...aro.org>
>> Signed-off-by: Deepak Saxena <dsaxena@...aro.org>
>> ---
>>  arch/arm64/include/asm/debug-monitors.h | 21 ++++++++
>>  arch/arm64/kernel/debug-monitors.c      | 86 ++++++++++++++++++++++++++++++++-
>>  arch/arm64/kernel/entry.S               |  2 +
>>  3 files changed, 108 insertions(+), 1 deletion(-)
>
> [...]
>
>> @@ -215,7 +257,10 @@ static int single_step_handler(unsigned long addr, unsigned int esr,
>>                */
>>               user_rewind_single_step(current);
>>       } else {
>> -             /* TODO: route to KGDB */
>> +             /* call registered single step handlers */
>
> Don't bother with this comment (it's crystal clear from the code).
OK, I will remove this unnecessary print.
>
>> +             if (call_step_hook(regs, esr) == DBG_HOOK_HANDLED)
>> +                     return 0;
>> +
>>               pr_warning("Unexpected kernel single-step exception at EL1\n");
>>               /*
>>                * Re-enable stepping since we know that we will be
>> @@ -227,11 +272,50 @@ static int single_step_handler(unsigned long addr, unsigned int esr,
>>       return 0;
>>  }
>>
>> +
>> +static LIST_HEAD(break_hook);
>> +DEFINE_RWLOCK(break_hook_lock);
>
> This guy can be a plain old spinlock. That way, the readers have less
> overhead but things still work because we only call a single hook function.
well, kprobes need to support recursive breakpoints (i.e. breakpoint
handler executing BRK once again)
so I converted this lock to rw_lock.  I should put this info in commit
description to be more clearer.
Let me know if you find any issue with re-cursing in breakpoint exception?

Thanks,
Sandeepa
>
> Will
> --
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