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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 12/07/2015 08:52 PM, Haozhong Zhang wrote:
> 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.
This is only guaranteed if we have constant/reliable TSC. So perhaps you 
should set tsc_scaling_supported only when either (or both?) of these 
TSC properties are true. Which is probably the case anyway but may be 
worth explicitly checking in start_svm/vmx? (and use 
tsc_scaling_supported instead of cpu_has_tsc_ratio in the 'if' statement)
And just like I asked in the previous email --- should we then use the 
same scaler (which would be vtsc_to_ns) in both cases? At least for 
guests in HVM containers (it may work for PV guests as well, but it 
would need to be confirmed).
Also, I noticed that this routine uses is_hvm_domain(). I think it 
should be has_hvm_container_domain().
-boris



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