Re: [PATCH] ARM: perf: allow tracing with kernel tracepoints events

[Will Deacon]

On Wed, Jun 25, 2014 at 03:54:14PM +0100, Jean Pihet wrote:
> Hi Will,

Hello,

> On 25 June 2014 11:01, Will Deacon <will.deacon@xxxxxxx> wrote:
> > On Fri, Jun 20, 2014 at 09:10:35AM +0100, Jean Pihet wrote:
> >> On 18 June 2014 14:53, Will Deacon <will.deacon@xxxxxxx> wrote:
> >> > On Tue, Jun 17, 2014 at 06:11:05PM +0100, Jean Pihet wrote:
> >> >> Tested with perf record and tracepoints filtering (-e <tracepoint>), with
> >> >> unwinding using fp (--call-graph fp) and dwarf info (--call-graph dwarf).
> >> >
> >> > Whilst the old ACPS unwinding only needs PC, FP and SP, is this definitely
> >> > true for exidx and DWARF-based unwinding? Given that libunwind ends up
> >> > running a state machine for the latter, can we guarantee that we won't hit
> >> > instructions that require access to other general purpose registers?
> >> Yes. dwarf unwinding does not need anything extra. Once seeded all the
> >> rest is extracted from the dwarf trace info.
> >
> > Ok, but what if the LR isn't saved on the stack, for example? What if the
> > code you're trying to unwind is hand-written assembly annotated with CFI
> > directives?
> Then in that case the unwinding is not possible unless the
> hand-crafted asm is compatible with the requested unwinding method
> (fp, dwarf etc.). Do you expect problems there, if so can you give
> more details?

To use a readily available AArch64 example, take a look at
__kernel_gettimeofday in arch/arm64/kernel/vdso/gettimeofday.S

It starts by moving the link register into x2, so that it can later call
__do_get_tspec without clobbering it. Furthermore, it doesn't make use of
the stack at all.

How can you unwind that using your current code?

Will
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