Re: bpf_get_branch_snapshot on qemu-kvm

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

On 26/10/2021 3:09 pm, Song Liu wrote:
> 
> 
>> 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

Yes, most of the contaminated lbr records come from these guest symbols:

intel_pmu_snapshot_branch_stack
native_write_msr
trace_hardirqs_off
lockdep_hardirqs_off
__lock_acquire
mark_lock
migrate_disable
rcu_is_watching
bpf_get_branch_snapshot
__bpf_prog_enter

I think we're fine with the current guest LBR emulation, right?

> 
> 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);

LGTM.

> 
> 
> (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?

How do we inform the user of bpf_get_branch_snapshot in a reasonable way
that the lbr data will be inaccurate when using the debug kernel?

Is it better to check for mutual exclusion in code or to use the user documentation
to specify this part of the restriction? It affects the user experience.

> Specifically, can we fix intel_pmu_snapshot_branch_stack in vm with the
> change above?

At least it's a valid fix and we can start from this change.

> 
> Thanks,
> Song
> 
> 
> 
>