linux-kernel - Re: [PATCH 5/5] perf: Correct perf sampling with guest VMs

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Message-ID: <gsntv7z2cck0.fsf@coltonlewis-kvm.c.googlers.com>
Date: Wed, 11 Sep 2024 17:42:07 +0000
From: Colton Lewis <coltonlewis@...gle.com>
To: Mark Rutland <mark.rutland@....com>
Cc: kvm@...r.kernel.org, oliver.upton@...ux.dev, seanjc@...gle.com, 
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Subject: Re: [PATCH 5/5] perf: Correct perf sampling with guest VMs

Mark Rutland <mark.rutland@....com> writes:

> On Wed, Sep 04, 2024 at 08:41:33PM +0000, Colton Lewis wrote:
>> Previously any PMU overflow interrupt that fired while a VCPU was
>> loaded was recorded as a guest event whether it truly was or not. This
>> resulted in nonsense perf recordings that did not honor
>> perf_event_attr.exclude_guest and recorded guest IPs where it should
>> have recorded host IPs.

>> Reorganize that plumbing to record perf events correctly even when
>> VCPUs are loaded.

> It'd be good if we could make that last bit a little more explicit,
> e.g.

>    Rework the sampling logic to only record guest samples for events with
>    exclude_guest clear. This way any host-only events with exclude_guest
>    set will never see unexpected guest samples. The behaviour of events
>    with exclude_guest clear is unchanged.

> [...]

Done

>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>> index 4384f6c49930..e1a66c9c3773 100644
>> --- a/kernel/events/core.c
>> +++ b/kernel/events/core.c
>> @@ -6915,13 +6915,26 @@ void perf_unregister_guest_info_callbacks(struct  
>> perf_guest_info_callbacks *cbs)
>>   EXPORT_SYMBOL_GPL(perf_unregister_guest_info_callbacks);
>>   #endif

>> -unsigned long perf_misc_flags(unsigned long pt_regs *regs)
>> +static bool is_guest_event(struct perf_event *event)
>>   {
>> +	return !event->attr.exclude_guest && perf_guest_state();
>> +}

> Could we name this something like "should_sample_guest()"? Calling this
> "is_guest_event()" makes it should like it's checking a static property
> of the event (and not other conditions like perf_guest_state()).

> Otherwise this all looks reasonable to me, modulo Ingo's comments. I'll
> happily test a v2 once those have been addressed.

Done

> Mark.