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SubjectRe: [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).

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