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. M. -- Without deviation from the norm, progress is not possible. _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
