Re: [PATCH bpf-next] perf, amd: support capturing LBR from software events

[Ingo Molnar <mingo@kernel.org>]

* Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:

> On Thu, Mar 28, 2024 at 4:21 AM Ingo Molnar <mingo@kernel.org> wrote:
> >
> >
> > * Andrii Nakryiko <andrii@kernel.org> wrote:
> >
> > > [0] added ability to capture LBR (Last Branch Records) on Intel CPUs
> > > from inside BPF program at pretty much any arbitrary point. This is
> > > extremely useful capability that allows to figure out otherwise
> > > hard-to-debug problems, because LBR is now available based on some
> > > application-defined conditions, not just hardware-supported events.
> > >
> > > retsnoop ([1]) is one such tool that takes a huge advantage of this
> > > functionality and has proved to be an extremely useful tool in
> > > practice.
> > >
> > > Now, AMD Zen4 CPUs got support for similar LBR functionality, but
> > > necessary wiring inside the kernel is not yet setup. This patch seeks to
> > > rectify this and follows a similar approach to the original patch [0]
> > > for Intel CPUs.
> > >
> > > Given LBR can be set up to capture any indirect jumps, it's critical to
> > > minimize indirect jumps on the way to requesting LBR from BPF program,
> > > so we split amd_pmu_lbr_disable_all() into a wrapper with some generic
> > > conditions vs always-inlined __amd_pmu_lbr_disable() called directly
> > > from BPF subsystem (through perf_snapshot_branch_stack static call).
> > >
> > > Now that it's possible to capture LBR on AMD CPU from BPF at arbitrary
> > > point, there is no reason to artificially limit this feature to sampling
> > > events. So corresponding check is removed. AFAIU, there is no
> > > correctness implications of doing this (and it was possible to bypass
> > > this check by just setting perf_event's sample_period to 1 anyways, so
> > > it doesn't guard all that much).
> > >
> > > This was tested on AMD Bergamo CPU and worked well when utilized from
> > > the aforementioned retsnoop tool.
> > >
> > >   [0] https://lore.kernel.org/bpf/20210910183352.3151445-2-songliubraving@fb.com/
> > >   [1] https://github.com/anakryiko/retsnoop
> > >
> > > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> > > ---
> > >  arch/x86/events/amd/core.c   | 29 ++++++++++++++++++++++++++++-
> > >  arch/x86/events/amd/lbr.c    | 11 +----------
> > >  arch/x86/events/perf_event.h | 11 +++++++++++
> > >  3 files changed, 40 insertions(+), 11 deletions(-)
> >
> > Please do not queue these up in the BPF tree, all similar changes to
> > perf code should go through the perf tree.
> >
> 
> Absolutely, I rebased on top of tip's perf/core branch and sent it as
> v2. Thanks!

Thanks, much appreciated!

	Ingo