Re: bpf_get_branch_snapshot on qemu-kvm

[Song Liu <songliubraving@xxxxxx>]

Hi Kan,

> On Sep 29, 2021, at 9:35 AM, Liang, Kan <kan.liang@xxxxxxxxx> 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 */
-       __intel_pmu_pebs_disable_all();
        __intel_pmu_lbr_disable();
        /*            ... 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