linux-kernel - Re: [RFC PATCH 1/2] arm64/tracing: add cntvct based trace clock

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Date:   Fri, 19 Nov 2021 12:00:23 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Marcelo Tosatti <mtosatti@...hat.com>,
        Nicolas Saenz Julienne <nsaenzju@...hat.com>
Cc:     linux-arm-kernel@...ts.infradead.org, rostedt@...dmis.org,
        james.morse@....com, alexandru.elisei@....com,
        suzuki.poulose@....com, catalin.marinas@....com, will@...nel.org,
        linux-kernel@...r.kernel.org, kvmarm@...ts.cs.columbia.edu,
        mingo@...hat.com, nilal@...hat.com
Subject: Re: [RFC PATCH 1/2] arm64/tracing: add cntvct based trace clock

On Fri, 19 Nov 2021 11:26:24 +0000,
Marcelo Tosatti <mtosatti@...hat.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.