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

[Pratyush Anand]

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@xxxxxxxxxx>
> > ---
> >   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