On Tue, Dec 01, 2015 at 05:11:25PM +0100, Ingo Molnar wrote:
>
> * Peter Zijlstra <[email protected]> wrote:
>
> > > so I think the problem here is that the RSP does not match up to the RIP.
> > > We
> > > can either pass along the original RIP+RSP, or the fixed up one - but
> > > what we
> > > do currently is that we pass along only half of it - which corrupts dwarf
> > > unwinding state that doesn't tolerate such errors.
> >
> > Still not sure what that gets you. Then you get a sample at a known wrong
> > location, why would you want that?
>
> Well, we'd at least get a valid call trace - which the 'mixed' one isn't?
> I.e.
> this only matters with --call-graph.
>
> But yeah, with my suggestion we'd essentially fall back from cycles:pp to
> cycles:p, ideally we'd want to have real_rsp. Does the hardware provide that?
No, no real_rsp.
> User-space cannot compute that reliably I think, what if the 'real'
> instruction
> was manipulating RSP in more complex ways than doing a CALL?
I'm not really too aware of these asm details :/
Jiri, what is in PERF_SAMPLE_REGS_USER.IP ? from a quick reading that is
whatever is returned from task_pt_regs(current), not the
perf_data.regs.ip field which contains the corrected IP.
Should the uwinder then not use PERF_SAMPLE_REGS_USER.{IP,SP} for a
consistent unwind?
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