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

[Nicolas Saenz Julienne]

On Fri, 2021-11-19 at 12:00 +0000, Marc Zyngier wrote:
> On Fri, 19 Nov 2021 11:26:24 +0000,
> Marcelo Tosatti <mtosatti@xxxxxxxxxx> 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