Re: [PATCH] perf/amd/ibs: Clear stale IBS_{FETCH|OP}_CTL in perf_ibs_start() with CTL2[Dis]

[sashiko-bot@kernel.org]

> Commit 1b044ff3c17e ("perf/amd/ibs: Avoid race between event add and
> NMI") makes perf_ibs_start() reset IBS_{FETCH|OP}_CTL before it sets
> IBS_STARTED. Thus, an NMI from another source that arrives before the
> event is enabled finds Val=0 and does not process a stale sample or
> enable the event too early.
> 
> The reset is a call to perf_ibs_disable_event() with config=0. A later
> commit changed perf_ibs_disable_event():
> 
>   commit efa5700ec0da ("perf/amd/ibs: Support IBS_{FETCH|OP}_CTL2[Dis] to eliminate RMW race")
> 
> When IBS_CAPS_DIS is set, the function now writes only
> IBS_{FETCH|OP}_CTL2 and returns. It does not write IBS_{FETCH|OP}_CTL.
> On hardware with IBS_CAPS_DIS, the reset does nothing, and the race is
> possible again: CTL still holds the previous sample, with Val=1, when
> IBS_STARTED is set.
> 
> Clear IBS_{FETCH|OP}_CTL explicitly when IBS_CAPS_DIS is set, as
> perf_ibs_disable_event() did before CTL2 support. Everything in CTL is
> stale at this point, so it does not matter if this write discards a
> Val that the hardware sets late.
> 
> Fixes: efa5700ec0da ("perf/amd/ibs: Support IBS_{FETCH|OP}_CTL2[Dis] to eliminate RMW race")
> Assisted-by: LLM
> Signed-off-by: Jim Mattson <jmattson@google.com>

Sashiko has reviewed this patch and found no issues. It looks great!

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Sashiko AI review · https://sashiko.dev/#/patchset/20261007234815.4026028-1-jmattson@google.com?part=1