Re: [Xen-devel] [PATCH v3 04/13] x86/time.c: Scale host TSC in pvclock properly

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 01/04/2016 07:59 PM, Haozhong Zhang wrote:
> On 01/04/16 13:09, Boris Ostrovsky wrote:
> > On 12/30/2015 10:03 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>
> > > ---
> > > Changes in v3:
> > >   (addressing Boris Ostrovsky's comments)
> > >   * No changes in fact. tsc_set_info() does not set d->arch.vtsc to 0
> > >     if host_tsc_is_safe() is not satisfied, so it is safe to use
> > >     d->arch.vtsc_to_ns here.
> > I don't think I understand your argument here. I think last time we decided
> > that vtsc_to_ns can be used only if TSC is constant.
> >
> > -boris
> In tsc_set_info(), if TSC scaling is available and host has
> X86_FEATURE_TSC_RELIABLE (checked by host_tsc_is_safe()), vtsc is left
> to 0. And looking at init_amd() and init_intel(),
> X86_FEATURE_CONSTANT_TSC is available whenever
> X86_FEATURE_TSC_RELIABLE is available as well. Therefore, when
> vtsc_to_ns is used in __update_vcpu_system_time(), we can always
> ensure that host has X86_FEATURE_CONSTANT_TSC and do not need to
> recheck it there.
OK --- I was looking at tsc_set_info()'s TSC_MODE_NEVER_EMULATE path but 
now I realize that we don't use scaling in that mode (right?).
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>


> Haozhong
>
> > >   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 d83f068..b31634a 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 ( has_hvm_container_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;
> > > +        }
> > > +        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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