Re: [RFC:PATCH 00/03] powerpc: Expose BookE debug registers through extended ptrace interface

Re: [RFC:PATCH 00/03] powerpc: Expose BookE debug registers through extended ptrace interface

[Frederic Weisbecker]

On Mon, Jan 25, 2010 at 07:32:00AM +1100, Benjamin Herrenschmidt wrote:
> On Mon, 2010-01-25 at 00:48 +0530, K.Prasad wrote:
> > 
> > Some of the benefits of using these generic interfaces include:
> > - Interoperability with other users of debug register (such as
> > parallel
> >   kernel requests) i.e. non-exclusive use of debug registers.
> > - Enables debugging/tracing tools such as perf-events and ftrace to
> > make
> >   use of debug registers.
> > - Re-use of common code available in kernel (kernel/hw_breakpoint.c).
> > 
> > Let me know what you think.
> 
> This might have changed but last I looked the "generic" breakpoint
> interface was still too x86-centric and wasn't capable of expressing
> some of the features of the BookE debug register set such as the data
> value compare, the ranged breakpoints, etc...



We have one field for addr, one for len and one for the memory access
type.

I think that those three are enough to express breakpoint ranges.
Basically a breakpoint range is a breakpoint that can have a high
len.

I've looked at the G2 PowerPc core breakpoint implementation, just to
face one of such tricky examples.

We have DABR and DABR2 for watchpoint (and respectively IABR and IABR2
for ins breakpoints). Those host the addresses to target, or well,
let's rather call them "address operand" registers. To generalize, I
call these xABR and xABR2 as it seems instruction and data breakpoint
work the same, they just have their own dedicated registers.

Then you have DBCR/IBCR (let's call them xBCR) which control the
breakpoints, with two fields in each that detail the operators to affect
in the address operands registers. You can choose betwen ==, <, or >= .

Another field in the operator is the SIG_TYPE, which describes the
combination, either "matches xABR AND xABR2" or "matches xABR OR xABR2".

If you choose the "OR" SIG_TYPE, it makes no sense to use the < or >=
operators on the addresses operands in practice. Who needs the following
matches?

	addr < xABR || addr >= xABR2
	addr < xABR || addr < xABR2
	addr >= xABR || addr >= xABR2

The only operator that makes sense in a OR type is ==, which
basically provides you two breakpoints:

	addr == xABR || addr == xABR2

Now if you choose the "AND" SIG_TYPE, the following matches
make no sense:

	addr < xABR && addr < xABR2
	addr < xABR && addr >= xABR2 (if xABR < xABR2)
	addr == xABR && addr (>=,<) xABR2

Basically, it's only usable for delimited ranges:

	addr >= xABR && addr < xABR2 (xABR < xABR2)

So the comparison is a trick that can actually only have
a practical use to define two exact matching breakpoints
or a delimited breakpoint range.

Hence, unless I'm missing something obvious, the current
generic interface is sufficient to express that.

I may also miss other kind of implementation that could
have other requirements.

 
> I'd rather have this more dedicated and more complete interface merged
> for gdb's sake, and in a second step look at unifying.


Perhaps. Supporting ptrace breakpoints should be an easy first
step as it's basically the responsibility of the user to fill
the registers, but it's a pretty limited scope work, especially you
won't have perf support.


> I believe that the generic breakpoint infrastructure should not be the
> mid-layer. IE. It cannot be made in any clean shape of form, to express
> all of the subtle features that a given architecture or platform can
> support and as such would always be inferior to a dedicated one.


Actually I think the current interface already does, as I explained
above.

What is broken for any other architectures than x86 is the set
of constraints, but I can easily move it to the arch, unless
I find a good generic solution (or a cool combination between
both).



> I can see the interest in exposing some kind of generic API that
> implements a common subset of breakpoint or watchpoint facilities to
> generic code such as the event tracer. This could be layered on top of
> an arch specific mechanism
> 
> But having the generic mechanism at the core for everybody is another
> attempt of "make everybody look like x86" which I believe in this case
> is sub optimal.


Not at all. It's an attempt to make a generic interface that can
exploit at best _each_ arch specific features. Other than the set
of constraints that I'm going to rework, the generic interface is powerful
enough to host what I've seen in term of cpu breakpoints implementations
for now. But if it's actually not and I'm missing other cases, please
report it to me.

The reason that makes the current generic constraints x86
oriented only is that I've only x86 boxes at home and I needed
to make a first shot without knowing anything about other archs
constraints, but in the long term, our motivations (Prasad's and mines)
are definetely not archX-centric.

Re: [RFC:PATCH 00/03] powerpc: Expose BookE debug registers through extended ptrace interface

[Benjamin Herrenschmidt]

> We have one field for addr, one for len and one for the memory access
> type.
> 
> I think that those three are enough to express breakpoint ranges.
> Basically a breakpoint range is a breakpoint that can have a high
> len.
> 
> I've looked at the G2 PowerPc core breakpoint implementation, just to
> face one of such tricky examples.

BookE has a richer semantic. We have watchpoints with data value compare
for example, we also have instruction value compare for breakpoints, and
a few other niceties. There's also subtle differences between what
processor variants support.

 .../...

> > I'd rather have this more dedicated and more complete interface merged
> > for gdb's sake, and in a second step look at unifying.
> 
> 
> Perhaps. Supporting ptrace breakpoints should be an easy first
> step as it's basically the responsibility of the user to fill
> the registers, but it's a pretty limited scope work, especially you
> won't have perf support.

But we can add it later. 

> > I believe that the generic breakpoint infrastructure should not be the
> > mid-layer. IE. It cannot be made in any clean shape of form, to express
> > all of the subtle features that a given architecture or platform can
> > support and as such would always be inferior to a dedicated one.
> 
> 
> Actually I think the current interface already does, as I explained
> above.
> 
> What is broken for any other architectures than x86 is the set
> of constraints, but I can easily move it to the arch, unless
> I find a good generic solution (or a cool combination between
> both).

Again, this is all "can do" vs. "already done and working". May I point
you to Linus recent rant against magic infrastructures that try to do
everything and do nothing right ? :-) I much prefer starting with
something dedicated that does exactly what is expected, have that
upstream (and in that regard the patches are good for the next merge
window) and -then- maybe look at how some of it could be re-used for
perf.

> Not at all. It's an attempt to make a generic interface that can
> exploit at best _each_ arch specific features. 

That reminds me of the justifications for utrace :-) It might well be
but I very much doubt that is possible. In any case, it doesn't appear
to be there yet. So let's just get that stuff in so we have our
interface finally working, and we can look at doing fancy things with
perf in a second pass.

> Other than the set
> of constraints that I'm going to rework, the generic interface is powerful
> enough to host what I've seen in term of cpu breakpoints implementations
> for now. But if it's actually not and I'm missing other cases, please
> report it to me.
> 
> The reason that makes the current generic constraints x86
> oriented only is that I've only x86 boxes at home and I needed
> to make a first shot without knowing anything about other archs
> constraints, but in the long term, our motivations (Prasad's and mines)
> are definetely not archX-centric.

Cheers,
Ben.

Re: [RFC:PATCH 00/03] powerpc: Expose BookE debug registers through extended ptrace interface

[Frederic Weisbecker]

On Sun, Jan 31, 2010 at 08:33:25AM +1100, Benjamin Herrenschmidt wrote:
> 
> > We have one field for addr, one for len and one for the memory access
> > type.
> > 
> > I think that those three are enough to express breakpoint ranges.
> > Basically a breakpoint range is a breakpoint that can have a high
> > len.
> > 
> > I've looked at the G2 PowerPc core breakpoint implementation, just to
> > face one of such tricky examples.
> 
> BookE has a richer semantic. We have watchpoints with data value compare
> for example, we also have instruction value compare for breakpoints, and
> a few other niceties. There's also subtle differences between what
> processor variants support.
> 
>  .../...



Ah indeed, I missed the data value compare thing. Especially how
it is implemented won't make things easy.

This is basically a comparison against chosen bytes of the data,
with or/and patterns.

Not sure what the "or" can be useful for.

That won't be easy to implement in the generic interface, looking
at how it is done in the BookE.

There is also the address comparison by mask.

Anyway, I think we can add fields in the interface to provide
such features, but we can't support all of them given, as you
said, the subtle differences between different cpu.

For example I think it can be useful to implement support
for data comparison, by mask for example. But I don't imagine
useful usecases to compare byte 4 and byte1 and trigger an event
if one OR other match.

I think we are going to implement what has obvious usecases
(parts of such data comparisons, parts of address mask
comparison) in the generic interface: the fields in perf_attr
that can be filled by perf in userspace.
And the rest can be implemented from the hw_perf_event structure
which contains the arch structure and can then be filled by ptrace at
will.


> 
> > > I'd rather have this more dedicated and more complete interface merged
> > > for gdb's sake, and in a second step look at unifying.
> > 
> > 
> > Perhaps. Supporting ptrace breakpoints should be an easy first
> > step as it's basically the responsibility of the user to fill
> > the registers, but it's a pretty limited scope work, especially you
> > won't have perf support.
> 
> But we can add it later. 



Yeah you're right. Having a raw ptrace support is a first
useful step that won't be a barrier to enhance it further
through the generic API.


> > > I believe that the generic breakpoint infrastructure should not be the
> > > mid-layer. IE. It cannot be made in any clean shape of form, to express
> > > all of the subtle features that a given architecture or platform can
> > > support and as such would always be inferior to a dedicated one.
> > 
> > 
> > Actually I think the current interface already does, as I explained
> > above.
> > 
> > What is broken for any other architectures than x86 is the set
> > of constraints, but I can easily move it to the arch, unless
> > I find a good generic solution (or a cool combination between
> > both).
> 
> Again, this is all "can do" vs. "already done and working". May I point
> you to Linus recent rant against magic infrastructures that try to do
> everything and do nothing right ? :-) I much prefer starting with
> something dedicated that does exactly what is expected, have that
> upstream (and in that regard the patches are good for the next merge
> window) and -then- maybe look at how some of it could be re-used for
> perf.


Sure I'm not against a first raw ptrace support. As I said,
this is not a barrier for what comes next.

Now for the rest, I don't think this is the same story than
utrace.

Trying to have a generic layer for a hardware feature implemented
differently across archs can't be called something that tries to do
everything. Otherwise you can oppose these arguments to everything
that is not in the arch/ directories. No generic irq layer, no
generic timer, etc...

We need this generic layer because we want the breakpoints
to be available for wider uses than ptrace. If there was only
ptrace, I would really agree with you that it's not worth the
generic layer.

It's just that breakpoints are a set of possible features, but each
archs implement its own subset among the possible features. x86
is one of the weakest there, and since this generic layer has been
first x86 oriented, it looks too weak to host the most interesting
possibilities.

Let it grow a bit, it's still young.


> > Not at all. It's an attempt to make a generic interface that can
> > exploit at best _each_ arch specific features. 
> 
> That reminds me of the justifications for utrace :-) It might well be
> but I very much doubt that is possible. In any case, it doesn't appear
> to be there yet.


You are too pessimistic ;-)

I don't think we can express every possibilities through the generic
interface. But we can express the most interesting ones for profiling
uses.

The rest (ptrace) can still be expressed through the arch part of the perf
events.


> So let's just get that stuff in so we have our
> interface finally working, and we can look at doing fancy things with
> perf in a second pass.


Sure.