Re: bpf_get_branch_snapshot on qemu-kvm

[Song Liu <songliubraving@fb.com>]

> On Oct 9, 2021, at 2:03 AM, Like Xu <like.xu.linux@gmail.com> wrote:
> 
> On 9/10/2021 1:08 am, Song Liu wrote:
>>> 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.
> 
> Hi Song,
> 
> Thanks for your detailed input. I saw your workaround "if (is_hypervisor())" on the tree.
> 
> Even when the guest supports PEBS, this use case fails and the root cause is still
> playing hide-and-seek with me. Just check with you to see if you get similar results
> when the guest LBR behavior makes the test case fail like this:
> 
> serial_test_get_branch_snapshot:FAIL:find_looptest_in_lbr unexpected find_looptest_in_lbr: actual 0 <= expected 6
> serial_test_get_branch_snapshot:FAIL:check_wasted_entries unexpected check_wasted_entries: actual 32 >= expected 10
> #52 get_branch_snapshot:FAIL
> 
> Also, do you know or rough guess about how extable code relates to the test case ?

Sorry for the delayed response. I finally got some time to look into 
this again. After disabling most debug configs, I managed to get it 
work in the VM with a simple change as 

diff --git i/arch/x86/events/intel/core.c w/arch/x86/events/intel/core.c
index 1248fc1937f82..3887b579297d7 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);


(of course we also need to remove the is_hypervisor() check.). 

But I am not sure whether this is the best fix. 

I pushed all the change and debug code I used to 

https://git.kernel.org/pub/scm/linux/kernel/git/song/linux.git/log/?h=get_branch_snapshot_in_vm

Could you please take a look at it and share your feedback on this?
Specifically, can we fix intel_pmu_snapshot_branch_stack in vm with the
change above?

Thanks,
Song