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

["Mi, Dapeng" <dapeng1.mi@linux.intel.com>]

On 9/18/2025 5:12 AM, Ian Rogers wrote:
> On Sun, Aug 10, 2025 at 10:56 PM Dapeng Mi <dapeng1.mi@linux.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@kernel.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. 


>
> 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
>>