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

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Message-ID: <ZuIgE2-GElzSGztH@google.com>
Date: Wed, 11 Sep 2024 15:56:19 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Colton Lewis <coltonlewis@...gle.com>
Cc: kvm@...r.kernel.org, Oliver Upton <oliver.upton@...ux.dev>, 
	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>, 
	Kan Liang <kan.liang@...ux.intel.com>, Will Deacon <will@...nel.org>, 
	Russell King <linux@...linux.org.uk>, Catalin Marinas <catalin.marinas@....com>, 
	Michael Ellerman <mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>, 
	Christophe Leroy <christophe.leroy@...roup.eu>, Naveen N Rao <naveen@...nel.org>, 
	Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>, 
	Alexander Gordeev <agordeev@...ux.ibm.com>, Christian Borntraeger <borntraeger@...ux.ibm.com>, 
	Sven Schnelle <svens@...ux.ibm.com>, Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H . Peter Anvin" <hpa@...or.com>, linux-perf-users@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org
Subject: Re: [PATCH v2 5/5] perf: Correct perf sampling with guest VMs

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.