Re: bpf_get_branch_snapshot on qemu-kvm

[Like Xu <like.xu.linux@gmail.com>]

On 30/9/2021 4:05 am, Song Liu wrote:
> Hi Kan,
> 
>> On Sep 29, 2021, at 9:35 AM, Liang, Kan <kan.liang@intel.com> wrote:
>>
>>>>> - get confirmation that clearing GLOBAL_CTRL is suffient to supress
>>>>>   PEBS, in which case we can simply remove the PEBS_ENABLE clear.
>>>>
>>>> How should we confirm this? Can we run some tests for this? Or do we
>>>> need hardware experts' input for this?
>>>
>>> I'll put it on the list to ask the hardware people when I talk to them next. But
>>> maybe Kan or Andi know without asking.
>>
>> If the GLOBAL_CTRL is explicitly disabled, the counters do not count anymore.
>> It doesn't matter if PEBS is enabled or not.
>>
>> See 6c1c07b33eb0 ("perf/x86/intel: Avoid unnecessary PEBS_ENABLE MSR
>> access in PMI "). We optimized the PMU handler base on it.
> 
> Thanks for these information!
> 
> IIUC, all we need is the following on top of bpf-next/master:
> 
> diff --git i/arch/x86/events/intel/core.c w/arch/x86/events/intel/core.c
> index 1248fc1937f82..d0d357e7d6f21 100644
> --- i/arch/x86/events/intel/core.c
> +++ w/arch/x86/events/intel/core.c
> @@ -2209,7 +2209,6 @@ intel_pmu_snapshot_branch_stack(struct perf_branch_entry *entries, unsigned int
>          /* must not have branches... */
>          local_irq_save(flags);
>          __intel_pmu_disable_all(false); /* we don't care about BTS */

If the value passed in is true, does it affect your use case?

> -       __intel_pmu_pebs_disable_all();

In that case, we can reuse "static __always_inline void intel_pmu_disable_all(void)"
regardless of whether PEBS is supported or enabled inside the guest and the host ?

>          __intel_pmu_lbr_disable();

How about using intel_pmu_lbr_disable_all() to cover Arch LBR?

>          /*            ... until here */
>          return __intel_pmu_snapshot_branch_stack(entries, cnt, flags);
> @@ -2223,7 +2222,6 @@ intel_pmu_snapshot_arch_branch_stack(struct perf_branch_entry *entries, unsigned
>          /* must not have branches... */
>          local_irq_save(flags);
>          __intel_pmu_disable_all(false); /* we don't care about BTS */
> -       __intel_pmu_pebs_disable_all();
>          __intel_pmu_arch_lbr_disable();
>          /*            ... until here */
>          return __intel_pmu_snapshot_branch_stack(entries, cnt, flags);
> 
> 
> In the test, this does eliminate the warning.
> 
> Thanks,
> Song
>