linux-kernel - Re: [PATCH v2 5/5] perf: Correct perf sampling with guest VMs

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Message-ID: <gsntldzwd37f.fsf@coltonlewis-kvm.c.googlers.com>
Date: Thu, 12 Sep 2024 20:31:00 +0000
From: Colton Lewis <coltonlewis@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: kvm@...r.kernel.org, oliver.upton@...ux.dev, peterz@...radead.org, 
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	linux-s390@...r.kernel.org
Subject: Re: [PATCH v2 5/5] perf: Correct perf sampling with guest VMs

Sean Christopherson <seanjc@...gle.com> writes:

> On Wed, Sep 11, 2024, 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.

>> 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.

> Nit, "with exclude_guest clear" is easy to misread as simply "with  
> exclude_guest"
> (I did so at least three times).  Maybe

>    The behavior of exclude_guest=0 events is unchanged.

> or

>    The behavior of events without exclude_guest is unchanged.

> I think it's also worth explicitly calling out that events that are  
> configured
> to sample both host and guest may still be prone to misattributing a PMI  
> that
> arrived in the host as a guest event, depending on the KVM arch and/or  
> vendor
> behavior.

Done