On Fri, Sep 18, 2015 at 05:54:29PM +0200, Radim Krčmář wrote:
> Newer KVM won't be exposing PVCLOCK_COUNTS_FROM_ZERO anymore.
> The purpose of that flags was to start counting system time from 0 when
> the KVM clock has been initialized.
> We can achieve the same by selecting one read as the initial point.
>
> A simple subtraction will work unless the KVM clock count overflows
> earlier (has smaller width) than scheduler's cycle count. We should be
> safe till x86_128.
>
> Because PVCLOCK_COUNTS_FROM_ZERO was enabled only on new hypervisors,
> setting sched clock as stable based on PVCLOCK_TSC_STABLE_BIT might
> regress on older ones.
>
> I presume we don't need to change kvm_clock_read instead of introducing
> kvm_sched_clock_read. A problem could arise in case sched_clock is
> expected to return the same value as get_cycles, but we should have
> merged those clocks in that case.
>
> Signed-off-by: Radim Krčmář <rkrcmar@xxxxxxxxxx>
> ---
> arch/x86/kernel/kvmclock.c | 46 +++++++++++++++++++++++++++++++++++-----------
> 1 file changed, 35 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
> index 2c7aafa70702..ef5b3d2cecce 100644
> --- a/arch/x86/kernel/kvmclock.c
> +++ b/arch/x86/kernel/kvmclock.c
> @@ -32,6 +32,7 @@
> static int kvmclock = 1;
> static int msr_kvm_system_time = MSR_KVM_SYSTEM_TIME;
> static int msr_kvm_wall_clock = MSR_KVM_WALL_CLOCK;
> +static cycle_t kvm_sched_clock_offset;
>
> static int parse_no_kvmclock(char *arg)
> {
> @@ -92,6 +93,29 @@ static cycle_t kvm_clock_get_cycles(struct clocksource *cs)
> return kvm_clock_read();
> }
>
> +static cycle_t kvm_sched_clock_read(void)
> +{
> + return kvm_clock_read() - kvm_sched_clock_offset;
> +}
> +
> +static inline void kvm_sched_clock_init(bool stable)
> +{
> + if (!stable) {
> + pv_time_ops.sched_clock = kvm_clock_read;
> + return;
> + }
> +
> + kvm_sched_clock_offset = kvm_clock_read();
> + pv_time_ops.sched_clock = kvm_sched_clock_read;
> + set_sched_clock_stable();
> +
> + printk("kvm-clock: using sched offset of %llu cycles\n",
> + kvm_sched_clock_offset);
> +
> + BUILD_BUG_ON(sizeof(kvm_sched_clock_offset) >
> + sizeof(((struct pvclock_vcpu_time_info *)NULL)->system_time));
> +}
> +
> /*
> * If we don't do that, there is the possibility that the guest
> * will calibrate under heavy load - thus, getting a lower lpj -
> @@ -248,7 +272,17 @@ void __init kvmclock_init(void)
> memblock_free(mem, size);
> return;
> }
> - pv_time_ops.sched_clock = kvm_clock_read;
> +
> + if (kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE_STABLE_BIT))
> + pvclock_set_flags(PVCLOCK_TSC_STABLE_BIT);
> +
> + cpu = get_cpu();
> + vcpu_time = &hv_clock[cpu].pvti;
> + flags = pvclock_read_flags(vcpu_time);
> +
> + kvm_sched_clock_init(flags & PVCLOCK_TSC_STABLE_BIT);
> + put_cpu();
> +
> x86_platform.calibrate_tsc = kvm_get_tsc_khz;
> x86_platform.get_wallclock = kvm_get_wallclock;
> x86_platform.set_wallclock = kvm_set_wallclock;
> @@ -265,16 +299,6 @@ void __init kvmclock_init(void)
> kvm_get_preset_lpj();
> clocksource_register_hz(&kvm_clock, NSEC_PER_SEC);
> pv_info.name = "KVM";
> -
> - if (kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE_STABLE_BIT))
> - pvclock_set_flags(~0);
> -
> - cpu = get_cpu();
> - vcpu_time = &hv_clock[cpu].pvti;
> - flags = pvclock_read_flags(vcpu_time);
> - if (flags & PVCLOCK_COUNTS_FROM_ZERO)
> - set_sched_clock_stable();
> - put_cpu();
> }
>
> int __init kvm_setup_vsyscall_timeinfo(void)
> --
> 2.5.2
ACK
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