Re: [Xen-devel] [PATCH 09/13] x86/time.c: Scale host TSC in pvclock properly

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 10/22/2015 12:44 PM, Haozhong Zhang wrote:
> On Thu, Oct 22, 2015 at 11:50:18AM -0400, Boris Ostrovsky wrote:
> > On 09/28/2015 03:13 AM, 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>
> > > ---
> > >   xen/arch/x86/time.c | 15 ++++++++++++---
> > >   1 file changed, 12 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
> > > index 4b5402c..54eab6e 100644
> > > --- a/xen/arch/x86/time.c
> > > +++ b/xen/arch/x86/time.c
> > > @@ -832,10 +832,19 @@ static void __update_vcpu_system_time(struct vcpu *v, int 
> > > force)
> > >       }
> > >       else
> > >       {
> > > -        _u.tsc_timestamp     = t->local_tsc_stamp;
> > > +        if ( is_hvm_domain(d) && hvm_funcs.tsc_scaling_supported )
> > > +        {
> > > +            _u.tsc_timestamp     = hvm_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;
> > > +        }
> > > +        else
> > > +        {
> > > +            _u.tsc_timestamp     = t->local_tsc_stamp;
> > > +            _u.tsc_to_system_mul = t->tsc_scale.mul_frac;
> > > +            _u.tsc_shift         = (s8)t->tsc_scale.shift;
> > > +        }
> > >           _u.system_time       = t->stime_local_stamp;
> > > -        _u.tsc_to_system_mul = t->tsc_scale.mul_frac;
> > > -        _u.tsc_shift         = (s8)t->tsc_scale.shift;
> > >       }
> > >       if ( is_hvm_domain(d) )
> > >           _u.tsc_timestamp += v->arch.hvm_vcpu.cache_tsc_offset;
>               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>              the offset is subtract here
Ah, I missed this. Thanks.

-boris


> > So this is not directly related to this series but when we calculate
> > tsc_timestamp --- shouldn't we subtract TSC offset? Otherwise we are
> > reporting (possibly scaled) host's TSC and this is supposed to be guest's
> > counter.
> >
> >
> > -boris

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