Re: [PATCH 2/2] perf/x86: Disable precise sampling for PERF_SAMPLE_STACK_USER

[Ian Rogers]

On Tue, Sep 8, 2026 at 8:10 AM Andi Kleen <ak@xxxxxxxxxxxxxxx> wrote:
>
> > That's what we already do, no? I have distinct memories of making the
> > stack unwind use the NMI regs rather then the PEBS regs.
> >
> > > In my opinion, it could even make the thing worse. User
> > > requires to get precise samplings, but perf silently returns imprecise
> > > records, this would mislead user.
> >
> > Mostly just the unwind might be off a little, the rest is accurate. This
> > has been the case 'forever'. Performance analysis isn't for silly
> > people, if they can't deal with a little fuzz then perhaps they're in
> > the wrong business.
>
> Is the main problem that the stack doesn't agree? Perhaps there
> could be a check for regs->rsp == pebs->user rsp (if in user space)
> to detect problematic samples.
>
> The question is how to report it and who should do the checking.
>
> It may need new fields in the ABI either to communicate the extra PEBS RSP
> or a bit to indicate that there might be a mismatch.
>
> I guess checking in the kernel and reporting an error might be simpler
> and maybe cleaner, but it would likely limit more advanced recovery
> possibilities.
>
> Are there other mismatches that break the unwinding? Perhaps the same
> for RBP?

For DWARF unwinding any register may be the source of a frame pointer
(e.g. the OpenSSL library would use R11 rather than RBP).

There is redundancy on x86 you can sample the PERF_REG_X86_IP register
in the user register and there is PERF_SAMPLE_IP in the sample event
itself.

My understanding is that IBS can only sample IP and so for precise
samples we can use PERF_SAMPLE_IP as the precise location and the user
register PERF_REG_X86_IP as the interrupt IP - this would match the
other register values in the interrupt.

In DWARF unwinding, we initialize the register state using the sampled
user registers:
https://web.git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/unwind-libdw.c?h=perf-tools-next#n270
and on x86 we sample all registers for DWARF unwinding:
https://web.git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/arch/x86/include/perf_regs.h?h=perf-tools-next#n20
https://web.git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/perf-regs-arch/perf_regs_x86.c?h=perf-tools-next#n238

Having PERF_SAMPLE_IP be precise and the user registers from the
interrupt I believe works for AMD IBS and ARM SPE, but for Intel PEBS
there is the ability to use the PEBS register samples for other
non-redundant registers. In the x86 driver could we disable PEBS
sampling for these registers when doing user stack sampling, so that
the sampled user registers match the stack sample? We can keep the
PERF_SAMPLE_IP precise, and make all the registers precise when there
is no stack sampling.

Thanks,
Ian