Date:   Fri, 19 Nov 2021 08:26:24 -0300
From:   Marcelo Tosatti <mtosatti@redhat.com>
To:     Nicolas Saenz Julienne <nsaenzju@redhat.com>
Cc:     linux-arm-kernel@lists.infradead.org, maz@kernel.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
Message-ID: <20211119112624.GA51423@fuller.cnet>
References: <20211119102117.22304-1-nsaenzju@redhat.com>
 <20211119102117.22304-2-nsaenzju@redhat.com>
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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).