Re: [PATCH 10/10] perf/doc: update design.txt for exclude_{host|guest} flags

From: Michael Ellerman <mpe@ellerman.id.au>
To: Andrew Murray <andrew.murray@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Sascha Hauer <s.hauer@pengutronix.de>,
	x86@kernel.org, Will Deacon <will.deacon@arm.com>,
	linuxppc-dev@lists.ozlabs.org,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
	Borislav Petkov <bp@alien8.de>,
	linux-alpha@vger.kernel.org,
	"paulus@samba.org" <paulus@samba.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Shawn Guo <shawnguo@kernel.org>, Joerg Roedel <joro@8bytes.org>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 10/10] perf/doc: update design.txt for exclude_{host|guest} flags
Date: Tue, 11 Dec 2018 22:06:53 +1100	[thread overview]
Message-ID: <87bm5sxqya.fsf@concordia.ellerman.id.au> (raw)
In-Reply-To: <20181120133202.GH35798@e119886-lin.cambridge.arm.com>

[ Reviving old thread. ]

Andrew Murray <andrew.murray@arm.com> writes:
> On Tue, Nov 20, 2018 at 10:31:36PM +1100, Michael Ellerman wrote:
>> Andrew Murray <andrew.murray@arm.com> writes:
>> 
>> > Update design.txt to reflect the presence of the exclude_host
>> > and exclude_guest perf flags.
>> >
>> > Signed-off-by: Andrew Murray <andrew.murray@arm.com>
>> > ---
>> >  tools/perf/design.txt | 4 ++++
>> >  1 file changed, 4 insertions(+)
>> >
>> > diff --git a/tools/perf/design.txt b/tools/perf/design.txt
>> > index a28dca2..7de7d83 100644
>> > --- a/tools/perf/design.txt
>> > +++ b/tools/perf/design.txt
>> > @@ -222,6 +222,10 @@ The 'exclude_user', 'exclude_kernel' and 'exclude_hv' bits provide a
>> >  way to request that counting of events be restricted to times when the
>> >  CPU is in user, kernel and/or hypervisor mode.
>> >  
>> > +Furthermore the 'exclude_host' and 'exclude_guest' bits provide a way
>> > +to request counting of events restricted to guest and host contexts when
>> > +using virtualisation.
>> 
>> How does exclude_host differ from exclude_hv ?
>
> I believe exclude_host / exclude_guest are intented to distinguish
> between host and guest in the hosted hypervisor context (KVM).

OK yeah, from the perf-list man page:

           u - user-space counting
           k - kernel counting
           h - hypervisor counting
           I - non idle counting
           G - guest counting (in KVM guests)
           H - host counting (not in KVM guests)

> Whereas exclude_hv allows to distinguish between guest and
> hypervisor in the bare-metal type hypervisors.

Except that's exactly not how we use them on powerpc :)

We use exclude_hv to exclude "the hypervisor", regardless of whether
it's KVM or PowerVM (which is a bare-metal hypervisor).

We don't use exclude_host / exclude_guest at all, which I guess is a
bug, except I didn't know they existed until this thread.

eg, in a KVM guest:

  $ perf record -e cycles:G /bin/bash -c "for i in {0..100000}; do :;done"
  $ perf report -D | grep -Fc "dso: [hypervisor]"
  16

> In the case of arm64 - if VHE extensions are present then the host
> kernel will run at a higher privilege to the guest kernel, in which
> case there is no distinction between hypervisor and host so we ignore
> exclude_hv. But where VHE extensions are not present then the host
> kernel runs at the same privilege level as the guest and we use a
> higher privilege level to switch between them - in this case we can
> use exclude_hv to discount that hypervisor role of switching between
> guests.

I couldn't find any arm64 perf code using exclude_host/guest at all?

And I don't see any x86 code using exclude_hv.

But maybe that's OK, I just worry this is confusing for users.

cheers

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