linux-kernel - Re: [PATCH v4 0/4] perf/x86/amd: add LBR capture support outside of hardware events

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Date: Mon, 1 Apr 2024 11:29:57 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Andrii Nakryiko <andrii@...nel.org>
Cc: x86@...nel.org, peterz@...radead.org, mingo@...hat.com,
	tglx@...utronix.de, bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org, jolsa@...nel.org, song@...nel.org,
	kernel-team@...a.com
Subject: Re: [PATCH v4 0/4] perf/x86/amd: add LBR capture support outside of
 hardware events


* Andrii Nakryiko <andrii@...nel.org> wrote:

> Add AMD-specific implementation of perf_snapshot_branch_stack static call that
> allows LBR capture from arbitrary points in the kernel. This is utilized by
> BPF programs. See patch #3 for all the details.
> 
> Patches #1 and #2 are preparatory steps to ensure LBR freezing is completely
> inlined and have no branches, to minimize LBR snapshot contamination.
> 
> Patch #4 removes an artificial restriction on perf events with LBR enabled.
> 
> Andrii Nakryiko (4):
>   perf/x86/amd: ensure amd_pmu_core_disable_all() is always inlined
>   perf/x86/amd: avoid taking branches before disabling LBR
>   perf/x86/amd: support capturing LBR from software events
>   perf/x86/amd: don't reject non-sampling events with configured LBR
> 
>  arch/x86/events/amd/core.c   | 37 +++++++++++++++++++++++++++++++++++-
>  arch/x86/events/amd/lbr.c    | 11 +----------
>  arch/x86/events/perf_event.h | 11 +++++++++++
>  3 files changed, 48 insertions(+), 11 deletions(-)

So there's a new conflict with patch #2, probably due to interaction 
with this recent fix that is now upstream:

   598c2fafc06f ("perf/x86/amd/lbr: Use freeze based on availability")

I don't think it should change the logic of the snapshot feature 
materially, X86_FEATURE_AMD_LBR_PMC_FREEZE should be orthogonal to it, 
as the LBR snapshot isn't taken from a PMI.

Thanks,

	Ingo