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

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Date:   Fri, 19 Nov 2021 08:26:24 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Nicolas Saenz Julienne <nsaenzju@...hat.com>
Cc:     linux-arm-kernel@...ts.infradead.org, maz@...nel.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, 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).