From: Nicolas Saenz Julienne <nsaenzju@redhat.com>
To: Marc Zyngier <maz@kernel.org>, Marcelo Tosatti <mtosatti@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 14:26:03 +0100 [thread overview]
Message-ID: <f83be3c5b582d1d25a4d3337e1d2971b48e9a83d.camel@redhat.com> (raw)
In-Reply-To: <87h7c873u0.wl-maz@kernel.org>
On Fri, 2021-11-19 at 12:00 +0000, Marc Zyngier wrote:
> 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.
To complete Marc's reply, here's an example of how CNTVCT_EL0 is being used in
rt-tests' oslat:
https://git.kernel.org/pub/scm/utils/rt-tests/rt-tests.git/tree/src/oslat/oslat.c#n87
--
Nicolás Sáenz
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next prev parent reply other threads:[~2021-11-19 13:27 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
2021-11-19 13:26 ` Nicolas Saenz Julienne [this message]
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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