Re: [PATCH 1/2] x86: kvmclock: abolish PVCLOCK_COUNTS_FROM_ZERO

[Marcelo Tosatti <mtosatti@redhat.com>]

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@redhat.com>
> ---
>  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