Messages in this thread | | | Date | Thu, 26 Jun 2014 10:00:45 +0100 | From | Will Deacon <> | Subject | Re: [PATCH] ARM: perf: allow tracing with kernel tracepoints events |
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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@arm.com> wrote: > > On Fri, Jun 20, 2014 at 09:10:35AM +0100, Jean Pihet wrote: > >> On 18 June 2014 14:53, Will Deacon <will.deacon@arm.com> 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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