Re: [PATCH v2] perf/x86/amd: support capturing LBR from software events

[Andrii Nakryiko]

On Fri, Mar 29, 2024 at 2:29 AM Sandipan Das <sandipan.das@xxxxxxx> wrote:
>
> On 3/28/2024 7:03 PM, Andrii Nakryiko 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@xxxxxx/
> >   [1] https://github.com/anakryiko/retsnoop
> >
> > Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
>
> Changes look good to me.
>
> Reviewed-by: Sandipan Das <sandipan.das@xxxxxxx>

Thanks for your review! Will split out the check removal, as Ingo
asked, and will post v3, thanks!

>
> > ---
> >  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(-)
> >

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