Re: [PATCH v3 5/6] pvclock: add clocksource change notification on changing of tsc stable bit

[Paolo Bonzini <pbonzini@redhat.com>]

On 29/07/2017 14:35, Denis Plotnikov wrote:
> Now, L2 guests can't have a stable kvmclock while working over L1 with
> a stable kvmclock. It is so just because of lack of functionality.
> 
> The patch is a part of work making kvmclock stable in L2 if it works
> over a stable kvmclock.
> 
> Signed-off-by: Denis Plotnikov <dplotnikov@virtuozzo.com>
> ---
>  arch/x86/kernel/pvclock.c | 19 ++++++++++++++++---
>  1 file changed, 16 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/pvclock.c b/arch/x86/kernel/pvclock.c
> index a2c554a..5b0bb2e 100644
> --- a/arch/x86/kernel/pvclock.c
> +++ b/arch/x86/kernel/pvclock.c
> @@ -22,6 +22,7 @@
>  #include <linux/gfp.h>
>  #include <linux/bootmem.h>
>  #include <linux/nmi.h>
> +#include <linux/cs_notifier.h>
>  
>  #include <asm/fixmap.h>
>  #include <asm/pvclock.h>
> @@ -73,6 +74,8 @@ u8 pvclock_read_flags(struct pvclock_vcpu_time_info *src)
>  	return flags & valid_flags;
>  }
>  
> +static atomic64_t clocksource_stable = ATOMIC64_INIT(0);

No need for atomic64.  Otherwise looks good.

Paolo

>  u64 pvclock_clocksource_read(struct pvclock_vcpu_time_info *src, u64 *cycles)
>  {
>  	unsigned version;
> @@ -96,10 +99,20 @@ u64 pvclock_clocksource_read(struct pvclock_vcpu_time_info *src, u64 *cycles)
>  		pvclock_touch_watchdogs();
>  	}
>  
> -	if ((valid_flags & PVCLOCK_TSC_STABLE_BIT) &&
> -		(flags & PVCLOCK_TSC_STABLE_BIT))
> -		return ret;
> +	if (likely(valid_flags & PVCLOCK_TSC_STABLE_BIT)) {
> +		bool stable_now = !!(flags & PVCLOCK_TSC_STABLE_BIT);
> +		bool stable_last = (bool) atomic64_read(&clocksource_stable);
> +
> +		if (unlikely(stable_now != stable_last)) {
> +			/* send notification once */
> +			if (stable_last == atomic64_cmpxchg(
> +				&clocksource_stable, stable_last, stable_now))
> +				clocksource_changes_notify();
> +		}
>  
> +		if (stable_now)
> +			return ret;
> +	}
>  	/*
>  	 * Assumption here is that last_value, a global accumulator, always goes
>  	 * forward. If we are less than that, we should not be much smaller.
>