linux-kernel - Re: [Patch v2 0/6] Perf kvm commands bug fix

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Message-ID: <aMxkqnXdrvv9BN8s@x1>
Date: Thu, 18 Sep 2025 16:59:38 -0300
From: Arnaldo Carvalho de Melo <acme@...nel.org>
To: "Mi, Dapeng" <dapeng1.mi@...ux.intel.com>
Cc: Ian Rogers <irogers@...gle.com>, Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Namhyung Kim <namhyung@...nel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Kan Liang <kan.liang@...ux.intel.com>,
	Kevin Tian <kevin.tian@...el.com>, linux-perf-users@...r.kernel.org,
	linux-kernel@...r.kernel.org, Dapeng Mi <dapeng1.mi@...el.com>
Subject: Re: [Patch v2 0/6] Perf kvm commands bug fix

On Thu, Sep 18, 2025 at 07:52:43AM +0800, Mi, Dapeng wrote:
> On 9/18/2025 5:12 AM, Ian Rogers wrote:
> > On Sun, Aug 10, 2025 at 10:56 PM Dapeng Mi <dapeng1.mi@...ux.intel.com> wrote:
> >> his patch-set fixes perf kvm commands issues, like missed memory
> >> allocation check/free, out of range memory access and especially the
> >> issue that fails to sample guest with "perf kvm record/top" commands on
> >> Intel platforms.
> >>
> >> The commit 634d36f82517 ("perf record: Just use "cycles:P" as the
> >>  default event") changes to use PEBS event to do sampling by default
> >> including guest sampling. This breaks host to sample guest with commands
> >> "perf kvm record/top" on Intel platforms.
> > Huh? That change is:
> > ```
> > $ git show 634d36f82517
> > commit 634d36f82517eb5c6a9b9ec7fe3ba19dbbcb7809
> > Author: Namhyung Kim <namhyung@...nel.org>
> > Date:   Tue Oct 15 23:23:58 2024 -0700
> >
> >     perf record: Just use "cycles:P" as the default event
> >
> >     The fallback logic can add ":u" modifier if needed.
> > ...
> > -               bool can_profile_kernel = perf_event_paranoid_check(1);
> > -
> > -               err = parse_event(rec->evlist, can_profile_kernel ?
> > "cycles:P" : "cycles:Pu");
> > +               err = parse_event(rec->evlist, "cycles:P");
> > ...
> > ```
> > isn't the precision the same before and after? I think you've blamed
> > the wrong patch.
> >
> > The change to use cycles:P looks to come from commit 7b100989b4f6
> > ("perf evlist: Remove __evlist__add_default") but the old code was
> > doing things like "evsel->precise_max = true;" so I think I was just
> > carrying forward behavior. The use of precise_max comes from commit
> > 4e8a5c155137 ("perf evsel: Fix max perf_event_attr.precise_ip
> > detection") from over 6 years ago, and the behavior before that also
> > appears to have been to use the maximum supported precision.
> >
> > Apart from the blame and commit message being off I think the change
> > is okay, delta my usual complaint that weak symbols are the devil's
> > work.
> 
> Hmm, yeah, you're right. Thanks for correcting this. 

Hi Dapeng,

	Can you please fix the patch descriptions and Fixes references
and resubmit?

- Arnaldo