Re: [PATCH V2 1/4] hw_breakpoint: Add step_needed event attribute

From: Pratyush Anand <panand@redhat.com>
To: Will Deacon <will.deacon@arm.com>,
	mark.rutland@arm.com, Peter Zijlstra <peterz@infradead.org>
Cc: linux-arm-kernel@lists.infradead.org,
	Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	huawei.libin@huawei.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH V2 1/4] hw_breakpoint: Add step_needed event attribute
Date: Wed, 26 Jul 2017 11:12:03 +0530	[thread overview]
Message-ID: <70b51184-dbe0-6d0b-c853-be5b556927f1@redhat.com> (raw)
In-Reply-To: <20170725132737.GC28815@arm.com>

On Tuesday 25 July 2017 06:57 PM, Will Deacon wrote:
> On Fri, Jul 07, 2017 at 05:33:57PM +0530, Pratyush Anand wrote:
>> Architecture like ARM64 currently allows to use default hw breakpoint
>> single step handler only to perf. However, some other users like few
>> systemtap tests or kernel test in
>> samples/hw_breakpoint/data_breakpoint.c can also work with default step
>> handler implementation. At the same time, some other like GDB/ptrace may
>> implement their own step handler.
>>
>> Therefore, this patch introduces a new perf_event_attr bit field, so
>> that arch specific code(specially on arm64) can make a decision to
>> enable single stepping.
>>
>> Any architecture which is not using this field will not have any
>> side effect.
>>
>> Signed-off-by: Pratyush Anand <panand@redhat.com>
>> ---
>>   include/linux/hw_breakpoint.h         | 6 ++++++
>>   include/uapi/linux/perf_event.h       | 3 ++-
>>   kernel/events/core.c                  | 2 ++
>>   tools/include/uapi/linux/perf_event.h | 3 ++-
>>   4 files changed, 12 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/hw_breakpoint.h b/include/linux/hw_breakpoint.h
>> index 0464c85e63fd..6173ae048cbc 100644
>> --- a/include/linux/hw_breakpoint.h
>> +++ b/include/linux/hw_breakpoint.h
>> @@ -38,6 +38,12 @@ static inline int hw_breakpoint_type(struct perf_event *bp)
>>   	return bp->attr.bp_type;
>>   }
>>   
>> +static inline bool
>> +hw_breakpoint_needs_single_step(struct perf_event *bp)
>> +{
>> +	return bp->attr.step_needed;
>> +}
>> +
>>   static inline unsigned long hw_breakpoint_len(struct perf_event *bp)
>>   {
>>   	return bp->attr.bp_len;
>> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
>> index b1c0b187acfe..00935808de0d 100644
>> --- a/include/uapi/linux/perf_event.h
>> +++ b/include/uapi/linux/perf_event.h
>> @@ -345,7 +345,8 @@ struct perf_event_attr {
>>   				context_switch :  1, /* context switch data */
>>   				write_backward :  1, /* Write ring buffer from end to beginning */
>>   				namespaces     :  1, /* include namespaces data */
>> -				__reserved_1   : 35;
>> +				step_needed    :  1, /* Use arch step handler */
>> +				__reserved_1   : 34;
> 
> This needs documenting properly, as I really have no idea how userspace is
> going to use it sensibley, especially as you silently overwrite it in some
> cases below.

I too had thought to put it under include/linux/perf_event.h : struct 
perf_event. But, see hw_break_module_init() which does not have knowledge of 
this structure, and we need to have some way so that none-perf kernel module 
implementation can tell that it needs default arch step handler.

Do you see any alternative?

-- 
Regards
Pratyush