Re: bpf_get_branch_snapshot on qemu-kvm

[Song Liu <songliubraving@fb.com>]

> On Oct 7, 2021, at 11:36 PM, Like Xu <like.xu.linux@gmail.com> wrote:
> 
> On 8/10/2021 1:46 pm, Song Liu wrote:
>>> On Oct 7, 2021, at 8:34 PM, Like Xu <like.xu.linux@gmail.com> wrote:
>>> 
>>> 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?
>> We are using LBR without PMI, so there isn't any hardware mechanism to
>> stop the LBR, we have to stop it in software. There is always a delay
>> between the event triggers and the LBR is stopped. In this window,
> 
> Do you use counters for snapshot branch stack?
> 
> Can the assumption of "without PMI" be broken sine Intel does have
> the hardware mechanism like "freeze LBR on counter overflow
> (aka, DEBUGCTLMSR_FREEZE_LBRS_ON_PMI)" ?

We are capturing LBR on software events. For example, when a complex syscall,
such as sys_bpf() and sys_perf_event_open(), returns -EINVAL, it is not obvious
what wen wrong. The branch stack at the return (on a kretprobe or fexit) could 
give us additional information. 

> 
>> the LBR is still running and old entries are being replaced by new entries.
>> We actually need the old entries before the triggering event, so the key
>> design goal here is to minimize the number of branch instructions between
>> the event triggers and the LBR is stopped.
> 
> Yes, it makes sense.
> 
>> Here, both __intel_pmu_disable_all(false) and __intel_pmu_lbr_disable()
>> are used to optimize for this goal: the fewer branch instructions the
>> better.
> 
> Is it possible that we have another LBR in-kernel user in addition to the current
> BPF-LBR snapshot user, such as another BPF-LBR snapshot user or a LBR perf user ?

I think it is OK to have another user. We just need to capture the LBR entries. 
In fact, we simply enable LBR by opening a perf_event on each CPU. So from the 
kernel's point of view, the LBR is owned used by "another user". 

> 
> In the intel_pmu_snapshot_[arch]_branch_stack(), what if there is a PMI or NMI handler
> to be called before __intel_pmu_lbr_disable(), which means more branch instructions
> (assuming we don't use the FREEZE_LBRS_ON_xxx capability)?

If we are unlucky and hit an NMI, we may get garbage data. The user will run the 
test again. 

> How about try to disable LBR at the earliest possible time, before __intel_pmu_disable_all(false) ?

I am not sure which solution is the best here. On bare metal, current version works
fine (available in bpf-next tree). 

> 
>> After removing __intel_pmu_pebs_disable_all() from
>> intel_pmu_snapshot_branch_stack(), we found quite a few LBR entries in
>> extable related code. With these entries, snapshot branch stack is not
> 
> Are you saying that you still need to call
> __intel_pmu_pebs_disable_all() to maintain precision ?

I think we don't need pebs_disable_all. In the VM, pebs_disable_all will trigger
"unchecked MSR access error" warning. After removing it, the warning message is 
gone. However, after we remove pebs_disable_all, we still see too many LBR entries
are flushed before LBR is stopped. Most of these new entries are in extable code. 
I guess this is because the VM access these MSR differently. 

Thanks,
Song