Re: [PATCH v2 06/18] perf/hw_breakpoint: add arch-independent hw_breakpoint_modify_local()

[Jinchao Wang <wangjinchao600@gmail.com>]

On Thu, Sep 04, 2025 at 08:44:48AM +0200, Peter Zijlstra wrote:
> On Thu, Sep 04, 2025 at 08:21:03AM +0800, Jinchao Wang wrote:
> > Introduce hw_breakpoint_modify_local() as a generic helper to modify an
> > existing hardware breakpoint. The function invokes
> > hw_breakpoint_arch_parse() and delegates the reinstall step to the
> > architecture via arch_reinstall_hw_breakpoint().
> > 
> > A weak default implementation of arch_reinstall_hw_breakpoint() is
> > provided, returning -EOPNOTSUPP on architectures without support.
> > 
> > This makes the interface arch-independent while allowing x86 (and others)
> > to provide their own implementation.
> > 
> > Signed-off-by: Jinchao Wang <wangjinchao600@gmail.com>
> > ---
> >  include/linux/hw_breakpoint.h |  1 +
> >  kernel/events/hw_breakpoint.c | 18 ++++++++++++++++++
> >  2 files changed, 19 insertions(+)
> > 
> > diff --git a/include/linux/hw_breakpoint.h b/include/linux/hw_breakpoint.h
> > index db199d653dd1..9453b5bdb443 100644
> > --- a/include/linux/hw_breakpoint.h
> > +++ b/include/linux/hw_breakpoint.h
> > @@ -67,6 +67,7 @@ extern int
> >  modify_user_hw_breakpoint_check(struct perf_event *bp, struct perf_event_attr *attr,
> >  				bool check);
> >  
> > +int hw_breakpoint_modify_local(struct perf_event *bp, struct perf_event_attr *attr);
> >  /*
> >   * Kernel breakpoints are not associated with any particular thread.
> >   */
> > diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
> > index 8ec2cb688903..ff428739f71e 100644
> > --- a/kernel/events/hw_breakpoint.c
> > +++ b/kernel/events/hw_breakpoint.c
> > @@ -983,6 +983,24 @@ static void hw_breakpoint_del(struct perf_event *bp, int flags)
> >  	arch_uninstall_hw_breakpoint(bp);
> >  }
> >  
> > +int hw_breakpoint_modify_local(struct perf_event *bp, struct perf_event_attr *attr)
> > +{
> > +	int err;
> > +
> > +	err = hw_breakpoint_arch_parse(bp, attr, counter_arch_bp(bp));
> > +	if (err)
> > +		return err;
> > +
> > +	return arch_reinstall_hw_breakpoint(bp);
> > +}
> > +EXPORT_SYMBOL(hw_breakpoint_modify_local);
> > +
> > +/* weak fallback for arches without support */
> > +__weak int arch_reinstall_hw_breakpoint(struct perf_event *bp)
> > +{
> > +	return -EOPNOTSUPP;
> > +}
> 
> Again, so much fail :/
> 

> So we have:
> 
> {register,modify,unregister}_user_hw_breakpoint()
> 
> and
> 
> {register,unregister}_wide_hw_breakpoint()
> 
> And you choose to extend this latter with hw_breakpoint_modify_local()
> instead of sticking with the naming scheme and say adding:
> 
> modify_wide_hw_breakpoint_local().
> 
> Also, again, that EXPORT is a fail, these other interfaces are all
> EXPORT_SYMBOL_GPL().
> 
Thanks for your patience.

I was misled by another family:
  - hw_breakpoint_add
  - hw_breakpoint_del
  - hw_breakpoint_start
  - hw_breakpoint_stop

Since this logic was also added in the wprobe series by Masami, I will
adopt his version instead.
> Also note that modify_user_hw_breakpoint() doesn't seem to need new arch
> hooks. Yet you fail to explain why you think you do.
Thanks for feedback, I will study existing code to better handle the arch
dependencies.