Re: [PATCH] KVM: x86/pmu: Prevent any host user from enabling PEBS for profiling guest

Liang, Kan
2023-12-04T15:19:59Z

On 2023-12-04 3:32 a.m., Like Xu wrote:
>
>
> On 1/12/2023 10:38 pm, Liang, Kan wrote:
>>
>>
>> On 2023-11-30 10:59 p.m., Like Xu wrote:
>>> On 30/11/2023 11:49 pm, Liang, Kan wrote:
>>>>
>>>>
>>>> On 2023-11-30 2:29 a.m., Like Xu wrote:
>>>>> On 29/11/2023 10:38 pm, Liang, Kan wrote:
>>>>>>
>>>>>>
>>>>>> On 2023-11-29 4:50 a.m., Like Xu wrote:
>>>>>>> From: Like Xu <likexu@tencent.com>
>>>>>>>
>>>>>>> Stop using PEBS counters on host to profiling guest. Limit the
>>>>>>> range of
>>>>>>> enabled PEBS counters to only those counters enabled from the guest
>>>>>>> PEBS
>>>>>>> emulation perspective.
>>>>>>>
>>>>>>> If there is a perf-record agent on host that uses perf-tools events
>>>>>>> like
>>>>>>> "cpu-cycles:GP" (G for attr.exclude_host, P for max precise event
>>>>>>> counter)
>>>>>>> to capture guest performance events, then the guest will be hanged.
>>>>>>> This is
>>>>>>> because Intel DS-based PEBS buffer is addressed using the 64-bit
>>>>>>> linear
>>>>>>> address of the current {p/v}CPU context based on MSR_IA32_DS_AREA.
>>>>>>>
>>>>>>> Any perf user using PEBS counters to profile guest on host is, in
>>>>>>> perf/core
>>>>>>> implementation details, trying to set bits on
>>>>>>> cpuc->intel_ctrl_guest_mask
>>>>>>> and arr[pebs_enable].guest, much like the guest PEBS emulation
>>>>>>> behaviour.
>>>>>>> But the subsequent PEBS memory write, regardless of whether guest
>>>>>>> PEBS is
>>>>>>> enabled, can overshoot guest entry and corrupt guest memory.
>>>>>>>
>>>>>>> Profiling guest via PEBS-DS buffer on host is not supported at this
>>>>>>> time.
>>>>>>> Fix this by filtering the real configured value of
>>>>>>> arr[pebs_enable].guest
>>>>>>> with the emulated state of guest enabled PEBS counters, under the
>>>>>>> condition
>>>>>>> of none cross-mapped PEBS counters.
>>>>>>
>>>>>> So the counter will be silently disabled. The user never knows why
>>>>>> nothing is sampled.
>>>>>> Since we don't support the case, profiling guest via PEBS-DS
>>>>>> buffer on
>>>>>> host. Maybe we should error out when creating the event. For example
>>>>>> (not tested),
>>>>>
>>>>> Test failed.
>>>>>
>>>>>>
>>>>>> diff --git a/arch/x86/events/intel/core.c
>>>>>> b/arch/x86/events/intel/core.c
>>>>>> index 3871267d3237..24b90c70737f 100644
>>>>>> --- a/arch/x86/events/intel/core.c
>>>>>> +++ b/arch/x86/events/intel/core.c
>>>>>> @@ -3958,6 +3958,10 @@ static int intel_pmu_hw_config(struct
>>>>>> perf_event
>>>>>> *event)
>>>>>>             if ((event->attr.config & INTEL_ARCH_EVENT_MASK) ==
>>>>>> INTEL_FIXED_VLBR_EVENT)
>>>>>>                 return -EINVAL;
>>>>>>
>>>>>> +        /* Profiling guest via PEBS-DS buffer on host is not
>>>>>> supported. */
>>>>>> +        if (event->attr.exclude_host)
>>>>>> +            return -EINVAL;
>>>>>> +
>>>>>
>>>>> Guest PEBS emulation also sets this bit, a typical call stack looks
>>>>> like:
>>>>>
>>>>>       intel_pmu_hw_config+0x441/0x4d0
>>>>>       hsw_hw_config+0x12/0xa0
>>>>>       x86_pmu_event_init+0x98/0x370
>>>>>       perf_try_init_event+0x47/0x130
>>>>>       perf_event_alloc+0x446/0xeb0
>>>>>       perf_event_create_kernel_counter+0x38/0x190
>>>>>       pmc_reprogram_counter.constprop.17+0xd9/0x230 [kvm]
>>>>>       kvm_pmu_handle_event+0x1a6/0x310 [kvm]
>>>>>       vcpu_enter_guest+0x1388/0x19b0 [kvm]
>>>>>       vcpu_run+0x117/0x6c0 [kvm]
>>>>>       kvm_arch_vcpu_ioctl_run+0x13d/0x4d0 [kvm]
>>>>>       kvm_vcpu_ioctl+0x301/0x6e0 [kvm]
>>>>>
>>>>
>>>> Oh right, the event from the KVM guest is also exclude_host.
>>>> So we should only error out with the non-KVM exclude_host PEBS event.
>>>>
>>>>> Alternatively, this path is taken when using PEBS-via-PT to profile
>>>>> guests on host.
>>>>
>>>> There is a is_pebs_pt(event), so we can skip the PEBS-via-PT.
>>>>
>>>> Seems we just need to distinguish a KVM event and a normal host event.
>>>> I don't have a better way to do it except using
>>>> event->overflow_handler_context, which is NULL for a normal host event.
>>>
>>> The assumption that event->overflow_handler_context == NULL is unsafe,
>>> considering .overflow_handler_context hook has its acclaimed generality.
>>>
>>
>> Yes, I agree it's very hacky.
>>
>> What we need here is a way to distinguish the KVM guest request from the
>> others. How about the PF_VCPU flag?
>>
>>       if (event->attr.exclude_host &&
>>           !is_pebs_pt(event) &&
>>           (!(event->attach_state & PERF_ATTACH_TASK) ||
>> !(current->flags &
>> PF_VCPU))
>>               return -EINVAL;
>
> Unfortunately, the tests are not passed w/ the above diff.

What's the exact failed perf command?

Is it because that the KVM guest is mistakenly killed or the host
command is not detected?

> But it's good to know that there is PF_VCPU and more things to play
> around with.
>
> The AMD IBS also takes the precise path, but it puts the recorded values
> on group of MSRs instead of the linear address-based memory.

The check is in the intel_pmu_hw_config(). So AMD isn't impacted.

>
> The root cause of this issue is the hardware limitation, just like what
> we did
> in the commit 26a4f3c08de4 ("perf/x86: disable PEBS on a guest entry"),
> a similar fix should also belong in the context of directly configuring
> VMX-switch related hardware configuration values.
>

The above fix is different than this case. IIRC, It's caused by a ucode
issue. So once there is a guest, SW has to explicitly disable the PEBS.
Otherwise, the guest crashes even the host doesn't intend to profile the
guest.

While the case in the patch is apparently a violation of the current
rules. I think it's better to detect it and error out early.

Thanks,
Kan

> I haven't find a better location than intel_guest_get_msrs().
>
>>
>>
>>> I understand your motivation very well, but this is not the right move
>>> (based on my previous history of being sprayed by Peter). For perf/core,
>>> in-kernel perf_events should be treated equally, and the semantics of
>>> kvm_pmu should only be accounted when a perf/core API is only used for
>>> guest-only path. In this case for KVM perf_events,
>>> intel_guest_get_msrs()
>>> and x86_pmu_handle_guest_pebs() have this context.
>>>
>>>>
>>>> diff --git a/arch/x86/events/intel/core.c
>>>> b/arch/x86/events/intel/core.c
>>>> index a968708ed1fb..c93a2aaff7c3 100644
>>>> --- a/arch/x86/events/intel/core.c
>>>> +++ b/arch/x86/events/intel/core.c
>>>> @@ -3958,6 +3958,16 @@ static int intel_pmu_hw_config(struct perf_event
>>>> *event)
>>>>            if ((event->attr.config & INTEL_ARCH_EVENT_MASK) ==
>>>> INTEL_FIXED_VLBR_EVENT)
>>>>                return -EINVAL;
>>>>
>>>> +        /*
>>>> +         * Profiling guest via PEBS-DS buffer on host is not
>>>> supported.
>>>> +         * The event->overflow_handler_context is to distinguish a KVM
>>>> +         * event and a normal host event.
>>>> +         */
>>>> +        if (event->attr.exclude_host &&
>>>> +            !is_pebs_pt(event) &&
>>>> +            !event->overflow_handler_context)
>>>> +            return -EINVAL;
>>>> +
>>>>            if (!(event->attr.freq || (event->attr.wakeup_events &&
>>>> !event->attr.watermark))) {
>>>>                event->hw.flags |= PERF_X86_EVENT_AUTO_RELOAD;
>>>>                if (!(event->attr.sample_type &
>>>>
>>>>>
>>>>> The status of the guest can only be queried in the NMI handler and
>>>>> the func
>>>>> intel_guest_get_msrs() in the perf/core context, where it's easier
>>>>> and more
>>>>> centrally to review this part of changes that affects vPMU for corner
>>>>> cases.
>>>>>
>>>>> Maybe adding print info on the perf-tool side would help.
>>>>>
>>>>> For perf-tool users, it will get 0 number of sample for
>>>>> "cpu-cycles:GP"
>>>>> events,
>>>>> just like other uncounted perf-tool events.
>>>>
>>>> perf-tool would never know such details, e.g., whether the platform
>>>> supports PEBS-DS or other PEBS method. It's hard to tell if the 0 is
>>>> because of an unsupported hardware or nothing sampled in the guest.
>>>
>>> It kind of looks like this use case to me:
>>>
>>>      perf record -e cycles -e cpu/event=0xf4,umask=0x10/ ./workload #
>>> ICX
>>>
>>> # Total Lost Samples: 0
>>> #
>>> # Samples: 0  of event 'cpu/event=0xf4,umask=0x10/'
>>> # Event count (approx.): 0
>>>
>>> A end-user has to check if the event-umask combination is supported
>>> or not,
>>> or nothing sampled for the workload. Is there any room for
>>> improvement in
>>> perf-tool to reduce the pain of this part ? If any, the same thing could
>>> be applied
>>> to cpu-cycles:GP, isn't it ?
>>
>> I don't think we can expect the end user knows such details. Most of
>> them may even don't know what's PEBS-via-DS.
>
> Maybe the following generic reminder helps:
>
> # Total Lost Samples: 0
> # Note: the event is not counted or unsupported.
> #
> # Samples: 0  of event 'cpu/event=0xf4,umask=0x10/'
> # Event count (approx.): 0
>
>>
>> Thanks,
>> Kan
>>
>>>
>>>>
>>>> Thanks,
>>>> Kan
>>>>>
>>>>>>             if (!(event->attr.freq || (event->attr.wakeup_events &&
>>>>>> !event->attr.watermark))) {
>>>>>>                 event->hw.flags |= PERF_X86_EVENT_AUTO_RELOAD;
>>>>>>                 if (!(event->attr.sample_type &
>>>>>>
>>>>>>
>>>>>> Thanks,
>>>>>> Kan
>>>>>>
>>>>>>>
>>>>>>> Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
>>>>>>> Fixes: c59a1f106f5c ("KVM: x86/pmu: Add IA32_PEBS_ENABLE MSR
>>>>>>> emulation for extended PEBS")
>>>>>>> Signed-off-by: Like Xu <likexu@tencent.com>
>>>>>>> ---
>>>>>>>     arch/x86/events/intel/core.c | 8 +++++++-
>>>>>>>     1 file changed, 7 insertions(+), 1 deletion(-)
>>>>>>>
>>>>>>> diff --git a/arch/x86/events/intel/core.c
>>>>>>> b/arch/x86/events/intel/core.c
>>>>>>> index a08f794a0e79..17afd504c35b 100644
>>>>>>> --- a/arch/x86/events/intel/core.c
>>>>>>> +++ b/arch/x86/events/intel/core.c
>>>>>>> @@ -4103,13 +4103,19 @@ static struct perf_guest_switch_msr
>>>>>>> *intel_guest_get_msrs(int *nr, void *data)
>>>>>>>             .guest = pebs_mask & ~cpuc->intel_ctrl_host_mask,
>>>>>>>         };
>>>>>>>     +    /* In any case, clear guest PEBS bits first. */
>>>>>>> +    arr[global_ctrl].guest &= ~arr[pebs_enable].guest;
>>>>>>> +
>>>>>>>         if (arr[pebs_enable].host) {
>>>>>>>             /* Disable guest PEBS if host PEBS is enabled. */
>>>>>>>             arr[pebs_enable].guest = 0;
>>>>>>>         } else {
>>>>>>>             /* Disable guest PEBS thoroughly for cross-mapped PEBS
>>>>>>> counters. */
>>>>>>>             arr[pebs_enable].guest &=
>>>>>>> ~kvm_pmu->host_cross_mapped_mask;
>>>>>>> -        arr[global_ctrl].guest &= ~kvm_pmu->host_cross_mapped_mask;
>>>>>>> +
>>>>>>> +        /* Prevent any host user from enabling PEBS for profiling
>>>>>>> guest. */
>>>>>>> +        arr[pebs_enable].guest &= (kvm_pmu->pebs_enable &
>>>>>>> kvm_pmu->global_ctrl);
>>>>>>> +
>>>>>>>             /* Set hw GLOBAL_CTRL bits for PEBS counter when it runs
>>>>>>> for guest */
>>>>>>>             arr[global_ctrl].guest |= arr[pebs_enable].guest;
>>>>>>>         }
>>>>>>>
>>>>>>> base-commit: 6803fb00772cc50cd59a66bd8caaee5c84b13fcf
>