From: Marc Zyngier <maz@kernel.org>
To: Reiji Watanabe <reijiw@google.com>
Cc: Alexandru Elisei <alexandru.elisei@arm.com>,
james.morse@arm.com, suzuki.poulose@arm.com, will@kernel.org,
mark.rutland@arm.com, linux-arm-kernel@lists.infradead.org,
kvmarm@lists.cs.columbia.edu, tglx@linutronix.de,
mingo@redhat.com
Subject: Re: [PATCH v2 0/4] KVM: arm64: Improve PMU support on heterogeneous systems
Date: Tue, 14 Dec 2021 11:56:57 +0000 [thread overview]
Message-ID: <874k7bjt6e.wl-maz@kernel.org> (raw)
In-Reply-To: <CAAeT=FxK2F+GUdJ_0z6rb4F5ttjW5ZyD7Sa_eyiaKc=xxDdMkg@mail.gmail.com>
On Tue, 14 Dec 2021 06:24:38 +0000,
Reiji Watanabe <reijiw@google.com> wrote:
>
> Hi Alex,
>
> On Mon, Dec 13, 2021 at 3:14 AM Alexandru Elisei
> <alexandru.elisei@arm.com> wrote:
> >
> > Also, as VCPUs get migrated from one physical CPU to the other, the
> > semantics of the microarchitectural events change, even if the event ID is
> > the same.
>
> Yes, I understand. As mentioned, this can work only when the
> CPU affinity is set for vCPU threads appropriately (, which could
> be done even without changing userspace).
Implicit bindings to random PMUs based on the scheduling seems a
pretty fragile API to me, and presents no real incentive for userspace
to start doing the right thing.
I'd prefer not counting events at all when on the wrong CPU (for some
definition of 'wrong'), rather than accumulating unrelated events.
Both are admittedly wrong, but between two evils, I'd rather stick
with the one I know (and that doesn't require any change)...
Alex's series brings a way to solve this by allowing userspace to pick
a PMU and make sure userspace is aware of the consequences. It puts
userspace in charge, and doesn't leave space for ambiguous behaviours.
I definitely find value in this approach.
M.
--
Without deviation from the norm, progress is not possible.
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next prev parent reply other threads:[~2021-12-14 12:06 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-06 17:02 [PATCH v2 0/4] KVM: arm64: Improve PMU support on heterogeneous systems Alexandru Elisei
2021-12-06 17:02 ` [PATCH v2 1/4] perf: Fix wrong name in comment for struct perf_cpu_context Alexandru Elisei
2021-12-06 17:02 ` [PATCH v2 2/4] KVM: arm64: Keep a list of probed PMUs Alexandru Elisei
2021-12-06 17:02 ` [PATCH v2 3/4] KVM: arm64: Add KVM_ARM_VCPU_PMU_V3_SET_PMU attribute Alexandru Elisei
2021-12-08 3:13 ` Reiji Watanabe
2021-12-08 12:23 ` Alexandru Elisei
2021-12-08 12:43 ` Alexandru Elisei
2021-12-08 14:25 ` Marc Zyngier
2021-12-08 15:20 ` Alexandru Elisei
2021-12-08 15:44 ` Marc Zyngier
2021-12-08 16:11 ` Alexandru Elisei
2021-12-08 16:21 ` Marc Zyngier
2021-12-06 17:02 ` [PATCH v2 4/4] KVM: arm64: Refuse to run VCPU if the PMU doesn't match the physical CPU Alexandru Elisei
2021-12-07 14:17 ` Alexandru Elisei
2021-12-08 7:54 ` Reiji Watanabe
2021-12-08 10:38 ` Alexandru Elisei
2021-12-13 7:40 ` Reiji Watanabe
2021-12-08 9:56 ` Marc Zyngier
2021-12-08 11:18 ` Alexandru Elisei
2021-12-08 2:36 ` [PATCH v2 0/4] KVM: arm64: Improve PMU support on heterogeneous systems Reiji Watanabe
2021-12-08 8:05 ` Marc Zyngier
2021-12-13 6:36 ` Reiji Watanabe
2021-12-13 11:14 ` Alexandru Elisei
2021-12-14 6:24 ` Reiji Watanabe
2021-12-14 11:56 ` Marc Zyngier [this message]
2021-12-15 6:47 ` Reiji Watanabe
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