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

[Haozhong Zhang <haozhong.zhang@xxxxxxxxx>]

On 12/07/15 13:14, Boris Ostrovsky wrote:
> 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.
>

Because guest TSC is synchronized among all vcpus of a domain, I think
it's safe to use d->arch.vtsc_to_ns here.

Haozhong

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