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

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Date:	Wed, 16 Sep 2015 20:56:00 -0700 (PDT)
From:	Palmer Dabbelt <palmer@...belt.com>
To:	peterz@...radead.org
CC:	fweisbec@...il.com
Subject:     Re: [PATCH 09/13] Move bp_type_idx to kernel/event/hw_breakpoint.c

On Tue, 15 Sep 2015 12:39:10 PDT (-0700), peterz@...radead.org wrote:
> On Tue, Sep 15, 2015 at 11:40:11AM -0700, Palmer Dabbelt wrote:
>> On Tue, 15 Sep 2015 01:06:07 PDT (-0700), peterz@...radead.org wrote:
>> > On Mon, Sep 14, 2015 at 03:50:43PM -0700, Palmer Dabbelt wrote:
>> >> This has a "#ifdef CONFIG_*" that used to be exposed to userspace.
>> >>
>> >> The names in here are so generic that I don't think it's a good idea
>> >> to expose them to userspace (or even the rest of the kernel).  Since
>> >> there's only one kernel user, it's been moved to that file.
>> >>
>> >> Signed-off-by: Palmer Dabbelt <palmer@...belt.com>
>> >> Reviewed-by: Andrew Waterman <waterman@...s.berkeley.edu>
>> >> Reviewed-by: Albert Ou <aou@...s.berkeley.edu>
>> >> ---
>> >>  include/uapi/linux/hw_breakpoint.h | 10 ----------
>> >>  kernel/events/hw_breakpoint.c      | 10 ++++++++++
>> >>  2 files changed, 10 insertions(+), 10 deletions(-)
>> >>
>> >> 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?
>>
>> perf_event_open(2) says
>>
>>        bp_type (since Linux 2.6.33)
>>               This chooses the breakpoint type.  It is one of:
>>
>>               HW_BREAKPOINT_EMPTY
>>                      No breakpoint.
>>
>>               HW_BREAKPOINT_R
>>                      Count when we read the memory location.
>>
>>               HW_BREAKPOINT_W
>>                      Count when we write the memory location.
>>
>>               HW_BREAKPOINT_RW
>>                      Count when we read or write the memory location.
>>
>>               HW_BREAKPOINT_X
>>                      Count when we execute code at the memory location.
>>
>>               The values can be combined via a bitwise or, but the combination
>>               of HW_BREAKPOINT_R or HW_BREAKPOINT_W  with  HW_BREAKPOINT_X  is
>>               not allowed.
>>
>> so I think removing this enum from userspace is OK.  Did I miss
>> something?
>
> Nah, could've just been me not being awake. Unless Frederic says
> otherwise I'll chalk it up to not having drank enough morning juice.

Well, I'm going to leave this alone, then -- as Arnd pointed out in a
later email that got mis-threaded, this would be super tricky to fix
if userspace actually relied on it (which is why I was scared I was
wrong).

v4 (which is hopefully the final version of the patch set) will leave
this as it is.
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