From: Marc Zyngier <maz@kernel.org>
To: Marcelo Tosatti <mtosatti@redhat.com>,
Nicolas Saenz Julienne <nsaenzju@redhat.com>
Cc: linux-arm-kernel@lists.infradead.org, rostedt@goodmis.org,
james.morse@arm.com, alexandru.elisei@arm.com,
suzuki.poulose@arm.com, catalin.marinas@arm.com, will@kernel.org,
linux-kernel@vger.kernel.org, kvmarm@lists.cs.columbia.edu,
mingo@redhat.com, nilal@redhat.com
Subject: Re: [RFC PATCH 1/2] arm64/tracing: add cntvct based trace clock
Date: Fri, 19 Nov 2021 12:00:23 +0000 [thread overview]
Message-ID: <87h7c873u0.wl-maz@kernel.org> (raw)
In-Reply-To: <20211119112624.GA51423@fuller.cnet>
On Fri, 19 Nov 2021 11:26:24 +0000,
Marcelo Tosatti <mtosatti@redhat.com> wrote:
>
> On Fri, Nov 19, 2021 at 11:21:17AM +0100, Nicolas Saenz Julienne wrote:
> > Add a new arm64-specific trace clock using the cntvct register, similar
> > to x64-tsc. This gives us:
> > - A clock that is relatively fast (1GHz on armv8.6, 1-50MHz otherwise),
> > monotonic, and resilient to low power modes.
> > - It can be used to correlate events across cpus as well as across
> > hypervisor and guests.
> >
> > By using arch_timer_read_counter() we make sure that armv8.6 cpus use
> > the less expensive CNTVCTSS_EL0, which cannot be accessed speculatively.
>
> Can this register be read by userspace ? (otherwise it won't be possible
> to correlate userspace events).
Yes. That's part of the userspace ABI. Although this particular
accessor is only available from ARMv8.6 and is advertised via a hwcap
to userspace.
For currently existing implementations, userspace will use the
CNTVCT_EL0 accessor, which requires extra synchronisation as it can be
speculated.
M.
--
Without deviation from the norm, progress is not possible.
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next prev parent reply other threads:[~2021-11-19 12:02 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-19 10:21 [RFC PATCH 0/2] KVM: arm64: Host/Guest trace syncronization Nicolas Saenz Julienne
2021-11-19 10:21 ` [RFC PATCH 1/2] arm64/tracing: add cntvct based trace clock Nicolas Saenz Julienne
2021-11-19 11:26 ` Marcelo Tosatti
2021-11-19 12:00 ` Marc Zyngier [this message]
2021-11-19 13:26 ` Nicolas Saenz Julienne
2021-11-22 14:57 ` Steven Rostedt
2021-11-24 9:45 ` Nicolas Saenz Julienne
2021-11-19 10:21 ` [RFC PATCH 2/2] KVM: arm64: export cntvoff in debugfs Nicolas Saenz Julienne
2021-11-19 11:11 ` Marcelo Tosatti
2021-11-19 12:17 ` Marc Zyngier
2021-11-19 12:59 ` Marcelo Tosatti
2021-11-19 13:31 ` Marc Zyngier
2021-11-22 20:40 ` Nicolas Saenz Julienne
2021-11-23 11:09 ` Marc Zyngier
2021-11-29 12:47 ` Marcelo Tosatti
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