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

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Date:	Fri, 25 Oct 2013 16:22:36 +0100
From:	Will Deacon <will.deacon@....com>
To:	Sandeepa Prabhu <sandeepa.prabhu@...aro.org>
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 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).

> +		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.

Will
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