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

Liang, Kan
2023-12-01T14:38:22Z

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;

> 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.

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