linux-kernel - Re: [RFC][PATCH 1/5] hw-breakpoints: Make kernel breakpoints API truly generic

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Date:	Mon, 20 Jul 2009 13:27:48 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...uxtronix.de>,
	Mike Galbraith <efault@....de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Anton Blanchard <anton@...ba.org>,
	Li Zefan <lizf@...fujitsu.com>,
	Zhaolei <zhaolei@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	"K . Prasad" <prasad@...ux.vnet.ibm.com>,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: [RFC][PATCH 1/5] hw-breakpoints: Make kernel breakpoints API
	truly generic

* Frederic Weisbecker (fweisbec@...il.com) wrote:
[...]
> diff --git a/kernel/hw_breakpoint.c b/kernel/hw_breakpoint.c
> index c1f64e6..015fec6 100644
> --- a/kernel/hw_breakpoint.c
> +++ b/kernel/hw_breakpoint.c
> @@ -297,15 +297,21 @@ EXPORT_SYMBOL_GPL(unregister_user_hw_breakpoint);
>  /**
>   * register_kernel_hw_breakpoint - register a hardware breakpoint for kernel space
>   * @bp: the breakpoint structure to register
> - *
> - * @bp.info->name or @bp.info->address, @bp.info->len, @bp.info->type and
> + * @addr: the address where we want to set the breakpoint
> + * @len: length of the value in memory to break in
> + * @type: the type of the breakpoint (read/write/execute)
>   * @bp->triggered must be set properly before invocation

Hi Frederic,

I think one of the great addition in this patchset is to allow using
breakpoints from arch-agnostic code.

It becomes important to document the error values which can be returned
by register_kernel_hw_breakpoint, so this will serve as guidelines for
architecture-specific arch_fill_hw_breakpoint() implementation. This
will become increasingly important, as this abstraction layer will
basically be responsible for either:

- Finding the best support the architecture can provide for a given hw
  breakpoint.
- Failing with an explicit error value telling the in-kernel user why it
  failed (e.g. if it must use a fallback, or return the error to the
  user).

Maybe we should think of a more flexible breakpoint type mapping too,
e.g.:

 monitor _strictly_ execute operation on address 0x...
   -> would fail if the architecture does not support execution access
      monitoring
 monitor (at least) execute operations on address 0x...
   -> would be allowed to use a more general monitor (e.g. RWX) if the
      architecture does not support "execute only" monitor.

(same for read and write)

Mathieu

>   *
>   */
> -int register_kernel_hw_breakpoint(struct hw_breakpoint *bp)
> +int register_kernel_hw_breakpoint(struct hw_breakpoint *bp, unsigned long addr,
> +				  int len, enum breakpoint_type type)
>  {
>  	int rc;
>  
> +	rc = arch_fill_hw_breakpoint(bp, addr, len, type);
> +	if (rc)
> +		return rc;
> +
>  	rc = arch_validate_hwbkpt_settings(bp, NULL);
>  	if (rc)
>  		return rc;
-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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