Re: [Xen-devel] [PATCH v2 06/14] x86/time.c: Scale host TSC in pvclock properly

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 12/06/2015 03:58 PM, Haozhong Zhang wrote:
> This patch makes the pvclock return the scaled host TSC and
> corresponding scaling parameters to HVM domains if guest TSC is not
> emulated and TSC scaling is enabled.
>
> Signed-off-by: Haozhong Zhang <haozhong.zhang@xxxxxxxxx>
+Joao who has been staring at this code.

Joao, can you run this series through your test with non-native 
frequency? 
(http://lists.xenproject.org/archives/html/xen-devel/2015-12/msg00683.html 
provides an interface to set it in config file).

> ---
>   xen/arch/x86/time.c | 16 ++++++++++++----
>   1 file changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
> index 95df4f1..732d1e9 100644
> --- a/xen/arch/x86/time.c
> +++ b/xen/arch/x86/time.c
> @@ -815,10 +815,18 @@ static void __update_vcpu_system_time(struct vcpu *v, int 
> force)
>       }
>       else
>       {
> -        tsc_stamp = t->local_tsc_stamp;
> -
> -        _u.tsc_to_system_mul = t->tsc_scale.mul_frac;
> -        _u.tsc_shift         = (s8)t->tsc_scale.shift;
> +        if ( is_hvm_domain(d) && cpu_has_tsc_ratio )
> +        {
> +            tsc_stamp            = hvm_funcs.scale_tsc(v, t->local_tsc_stamp);
> +            _u.tsc_to_system_mul = d->arch.vtsc_to_ns.mul_frac;
> +            _u.tsc_shift         = d->arch.vtsc_to_ns.shift;
I am not sure this is correct (which is why I asked Joao to look at this 
and test). The scaler below is calculated as result of TSC calibration 
across physical CPUs and what you use above (vtsc_to_ns) is an 
uncalibrated value.
-boris

> +        }
> +        else
> +        {
> +            tsc_stamp            = t->local_tsc_stamp;
> +            _u.tsc_to_system_mul = t->tsc_scale.mul_frac;
> +            _u.tsc_shift         = (s8)t->tsc_scale.shift;
> +        }
>       }
>   
>       _u.tsc_timestamp = tsc_stamp;

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