linux-kernel - Re: [PATCH v7 bpf-next 1/3] perf: enable branch record for software events

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Message-ID: <20210910192845.GT4323@worktop.programming.kicks-ass.net>
Date:   Fri, 10 Sep 2021 21:28:45 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Song Liu <songliubraving@...com>
Cc:     bpf@...r.kernel.org, linux-kernel@...r.kernel.org, acme@...nel.org,
        mingo@...hat.com, kjain@...ux.ibm.com, kernel-team@...com,
        John Fastabend <john.fastabend@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>
Subject: Re: [PATCH v7 bpf-next 1/3] perf: enable branch record for software
 events

On Fri, Sep 10, 2021 at 11:33:50AM -0700, Song Liu wrote:
> The typical way to access branch record (e.g. Intel LBR) is via hardware
> perf_event. For CPUs with FREEZE_LBRS_ON_PMI support, PMI could capture
> reliable LBR. On the other hand, LBR could also be useful in non-PMI
> scenario. For example, in kretprobe or bpf fexit program, LBR could
> provide a lot of information on what happened with the function. Add API
> to use branch record for software use.
> 
> Note that, when the software event triggers, it is necessary to stop the
> branch record hardware asap. Therefore, static_call is used to remove some
> branch instructions in this process.
> 
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Acked-by: John Fastabend <john.fastabend@...il.com>
> Acked-by: Andrii Nakryiko <andrii@...nel.org>
> Signed-off-by: Song Liu <songliubraving@...com>

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>