Re: [PATCH 1/3] perf/x86/amd: AMD support for bp_len > HW_BREAKPOINT_LEN_8

Frederic Weisbecker
2013-12-02T23:12:30Z

2013/11/11 Oleg Nesterov <oleg@redhat.com>:
> On 11/11, Frederic Weisbecker wrote:
>>
>> On Sat, Nov 09, 2013 at 04:54:28PM +0100, Oleg Nesterov wrote:
>> >
>> > Up to you and Suravee, but can't we cleanup this later?
>> >
>> > This series was updated many times to address a lot of (sometimes
>> > contradictory) complaints.
>>
>> Sure. But I'm confident that we can solve the conflicting mask / len issue easily beside.
>> I mean, I don't feel confident with merging things as is, otoh it should be easy to fix up.
>
> I do not really understand where do you see the conflict...
>
> I can be easily wrong, but afaics currently mask / len issue is simply
> the implementation detail.

I think it's like we have an object that has a length, and to create
this object we pass both kilometers and miles. Ok it's a bit different
here because a mask can apply on top of a len. But here it's used to
define essentially the same thing (ie: a range of address)

>
>> > > I mean, that doesn't look right to me,
>> > > it's two units basically measuring the same thing, so that's asking for conflicting troubles.
>> >
>> > Yes. And we can kill either _len or _mask, not sure what would be more
>> > clean.
>> >
>> > At least with the current implementation (again, iirc) mask == len -1.
>> > Although amd supports the more generic masks (but I can't recall the
>> > details).
>>
>> Right. I think len is probably more powerful. Unless the mask could be used to define
>> multiple ranges of breakpoints, not sure it's ever going to be used that way though.
>
> Actually, mask is more powerfull. And initial versions of this patches
> (iirc) tried to use mask as an argument which comes from the userspace
> (tools/perf, perf_event_attr, etc). But one of reviewers nacked this
> interfacer, so we still use len.

Well, we can still reconsider it if needed but to me it seems that
mask is only interesting if we may deal with non contiguous range of
addresses. It doesn't appear to be the case, but I don't have much
tricky usecases in mind.

>
>> But we
>> can still extend the interface if we need a mask later.
>
> Yes, agreed.

Great.

>
>> > > I'm just not sure how to reuse the len to express breakpoint ranges (that was in fact the
>> > > initial purpose of it) without breaking the tools.
>> >
>> > Confused... user-space still uses len to express the range? Just
>> > the kernel "switches" to mask if len > 8 ?
>>
>> Right but what if we want breakpoints having a size below 8? Like break on instructions
>> from 0x1000 to 0x1008 ?
>>
>> Or should we ignore range instruction breakpoints when len < 8?
>
> In this case the new code has no effect (iirc), we simply use
> X86_BREAKPOINT_LEN_* and "tell the hardware about extended range/mask"
> code is never called. IIRC, currently we simply check bp_mask != 0
> to distinguish.

I'm not sure I understand correctly. Do you mean that range below 8
don't rely on extended breakpoint range?

Ideally it would be nice if we drop bp_mask and use extended ranges
only when len > 8. How does that sound?