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

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Message-ID: <CAP-5=fWcf7F1QvWXzAD_KMpOnnKGw9PFM7mNtgzp_jh4Vi6V-w@mail.gmail.com>
Date: Wed, 17 Sep 2025 14:12:48 -0700
From: Ian Rogers <irogers@...gle.com>
To: Dapeng Mi <dapeng1.mi@...ux.intel.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, 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 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.

Thanks,
Ian

> Since the change "KVM: x86/pmu: Add basic support to enable guest PEBS
>  via DS"[1] starts, host loses the capability to sample guest with PEBS
> since all PEBS related MSRs are switched to guest value after vm-entry,
> like IA32_DS_AREA MSR is switched to guest GVA at vm-entry. This leads
> to PEBS events can't be used to sample guest by host, otherwise no guest
> PEBS records can be really sampled. The patches 5-6/6 fix this issue by
> using "cycles" event instead of PEBS event "cycles:P" to sample guest on
> Intel platforms.
>
> Changes:
>   v1 -> v2:
>   * Free memory allocated by strdup().
>   * Check "--pfm-events" in kvm_add_default_arch_event() as well.
>   * Opportunistically fix the missed memory allocation and free issue in
>     builtin-kwork.
>   * Comments refine.
>
>
> Tests:
>   * Run command "perf kvm record -a && perf kvm report" and "perf kvm
>     top" on Intel Sapphire Rapids platform, guest records can be
>     captured successfully.
>   * Since no powerpc platforms on hand, doesn't check the patches on
>     powerpc. Any test on powerpc is appreciated.
>
> Ref:
>   [1] https://lore.kernel.org/all/20220411101946.20262-1-likexu@tencent.com/
>
>
> Dapeng Mi (6):
>   perf tools kvm: Add missed memory allocation check and free
>   perf tools kwork: Add missed memory allocation check and free
>   perf tools kvm: Fix the potential out of range memory access issue
>   perf tools: Add helper x86__is_intel_cpu()
>   perf tools kvm: Use "cycles" to sample guest for "kvm record" on Intel
>   perf tools kvm: Use "cycles" to sample guest for "kvm top" on Intel
>
>  tools/perf/arch/x86/util/kvm-stat.c |  51 +++++++++++
>  tools/perf/builtin-kvm.c            | 130 ++++++++++++++++++++--------
>  tools/perf/builtin-kwork.c          |  27 ++++--
>  tools/perf/util/env.c               |  22 +++++
>  tools/perf/util/env.h               |   2 +
>  tools/perf/util/kvm-stat.h          |  10 +++
>  6 files changed, 203 insertions(+), 39 deletions(-)
>
>
> base-commit: 6235ce77749f45cac27f630337e2fdf04e8a6c73
> --
> 2.34.1
>