Re: [POC 5/5] x86_64: invoke SFrame based stack tracer for user space

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On Tue, May 16, 2023 at 10:38 AM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> On Tue, 16 May 2023 10:25:52 -0700
> Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote:
>
> > As discussed in the halls of LSF/MM2023, such lazily mapped .sframe in
> > approach won't work with BPF's bpf_get_stack() approach, which expects
> > stack trace to be grabbed and returned synchronously from NMI context.
> > But we can probably retrofit it into bpf_get_stackid()+STACK_TRACE BPF
> > map API.
>
> Note that we will likely not be able to use sframe in NMI context, and if
> that's a requirement, then BPF will need to continue using the method it is
> currently using.

Yes, unfortunately. We should give users a way to use sframes, but
transition might be slow.

>
> The best way to access sframe reliable is in normal context. NMI is
> special, and really should never had been used to access user space
> addresses. That was just a simple solution but not a good one. There's a
> lot of hacks just to allow a page fault in NMI context.
> See https://lwn.net/Articles/484932/
>
> >
> > Indu, please cc bpf@xxxxxxxxxxxxxxx for future revisions so we can
> > track and plan accordingly. Thank you!
>
> I'll likely be taking over the kernel side of sframes. I'll be happy to
> Cc that work to the bpf mailing list.

Thanks!


>
> -- Steve





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