linux-kernel - Re: [PATCH 09/13] Move bp_type_idx to kernel/event/hw

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Date:	Thu, 24 Sep 2015 14:15:25 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	mathieu.desnoyers@...icios.com,
	Palmer Dabbelt <palmer@...belt.com>, dhowells@...hat.com,
	viro@...iv.linux.org.uk, ast@...mgrid.com,
	aishchuk@...ux.vnet.ibm.com, aarcange@...hat.com,
	akpm@...ux-foundation.org, luto@...nel.org, acme@...nel.org,
	bhe@...hat.com, 3chas3@...il.com, chris@...kel.net, dave@...1.net,
	dyoung@...hat.com, drysdale@...gle.com, davem@...emloft.net,
	ebiederm@...ssion.com, geoff@...radead.org,
	gregkh@...uxfoundation.org, hpa@...or.com, mingo@...nel.org,
	iulia.manda21@...il.com, plagnioj@...osoft.com, jikos@...nel.org,
	josh@...htriplett.org, kexec@...ts.infradead.org,
	linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	jcmvbkbc@...il.com, paulmck@...ux.vnet.ibm.com, tglx@...utronix.de,
	tomi.valkeinen@...com, vgoyal@...hat.com, x86@...nel.org
Subject: Re: [PATCH 09/13] Move bp_type_idx to kernel/event/hw_breakpoint.c

On Tue, Sep 15, 2015 at 11:15:29PM +0200, Arnd Bergmann wrote:
> On Tuesday 15 September 2015 10:06:07 Peter Zijlstra wrote:
> > > diff --git a/include/uapi/linux/hw_breakpoint.h b/include/uapi/linux/hw_breakpoint.h
> > > index b04000a2296a..7a6a5a7f9511 100644
> > > --- a/include/uapi/linux/hw_breakpoint.h
> > > +++ b/include/uapi/linux/hw_breakpoint.h
> > > @@ -17,14 +17,4 @@ enum {
> > >       HW_BREAKPOINT_INVALID   = HW_BREAKPOINT_RW | HW_BREAKPOINT_X,
> > >  };
> > >  
> > > -enum bp_type_idx {
> > > -     TYPE_INST       = 0,
> > > -#ifdef CONFIG_HAVE_MIXED_BREAKPOINTS_REGS
> > > -     TYPE_DATA       = 0,
> > > -#else
> > > -     TYPE_DATA       = 1,
> > > -#endif
> > > -     TYPE_MAX
> > > -};
> > 
> > This is rather unfortunate; you are correct that the naming is too
> > generic (and I tend to agree), but I think these values are required by
> > userspace to fill out:
> > 
> >   perf_event_attr::bp_type
> > 
> > So removing them will break things.
> > 
> > Frederic?
> 
> If user space actually relies on the definition from this header file,
> then it will use the wrong one on x86 and get 'TYPE_DATA = 1', while the
> kernel uses 'TYPE_DATA = 0'.
> 
> That seems unlikely to work, so I suspect it gets a different definition.
> If it uses this definition and it does work, we can probably use
> 
> #if defined(__KERNEL__) && defined(CONFIG_HAVE_MIXED_BREAKPOINTS_REGS)
> 
> but that requires a comment explaining exactly why that works.

I think this TYPE_DATA/TYPE_INST can be safely removed from uapi. This is only
about internal kernel code. Userspace only relies on HW_BREAKPOINT_[R/W/X]
to tell about the nature of the breakpoint.

If userspace ever relies on it, which I have no idea why, it even needs to define
CONFIG_HAVE_MIXED_BREAKPOINTS_REGS. No really there shouldn't be any user of that outside
the kernel.

Thanks.

> 
> 	Arnd
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