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 _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
