On Mon, 22 Dec 2014 16:06:04 +1100 Cyril Bur <cyril...@gmail.com> wrote:
> On POWER8 virtualised kernels the VTB register can be read to have a view of
> time that only increases while the guest is running. This will prevent guests
> from seeing time jump if a guest is paused for significant amounts of time.
>
> On POWER7 and below virtualised kernels stolen time is subtracted from
> sched_clock as a best effort approximation. This will not eliminate spurious
> warnings in the case of a suspended guest but may reduce the occurance in the
> case of softlockups due to host over commit.
>
> Bare metal kernels should avoid reading the VTB as KVM does not restore sane
> values when not executing. sched_clock is returned in this case.
>
> --- a/arch/powerpc/kernel/time.c
> +++ b/arch/powerpc/kernel/time.c
> @@ -621,6 +621,30 @@ unsigned long long sched_clock(void)
> return mulhdu(get_tb() - boot_tb, tb_to_ns_scale) << tb_to_ns_shift;
> }
>
> +unsigned long long running_clock(void)
Non-kvm kernels don't need this code. Is there some appropriate
"#ifdef CONFIG_foo" we can wrap this in?
> +{
> + /*
> + * Don't read the VTB as a host since KVM does not switch in host
> timebase
> + * into the VTB when it takes a guest off the CPU, reading the VTB would
> + * result in reading 'last switched out' guest VTB.
> + */
> +
> + if (firmware_has_feature(FW_FEATURE_LPAR)) {
> + if (cpu_has_feature(CPU_FTR_ARCH_207S))
> + return mulhdu(get_vtb() - boot_tb, tb_to_ns_scale) <<
> tb_to_ns_shift;
> +
> + /* This is a next best approximation without a VTB. */
> + return sched_clock() -
> cputime_to_nsecs(kcpustat_this_cpu->cpustat[CPUTIME_STEAL]);
Why is this result dependent on FW_FEATURE_LPAR? It's all generic code.
In fact the kernel/sched/clock.c default implementation of
running_clock() could use this expression. Would that be good or bad? :)
> + }
> +
> + /*
> + * On a host which doesn't do any virtualisation TB *should* equal VTB
> so
> + * it makes no difference anyway.
> + */
> +
> + return sched_clock();
> +}
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
