Re: [Xen-devel] [PATCH v3 12/13] xen/arm: start the vtimer Xen timers on the processor they should be running on

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 25 Apr 2013, Ian Campbell wrote:
> On Wed, 2013-04-24 at 20:07 +0100, Stefano Stabellini wrote:
> > The Xen physical timer emulator and virtual timer driver use two
> > internal Xen timers: initialize them on the processor the vcpu is
> > going to be running on, rather than the processor that it's creating the
> > vcpu.
> 
> But this CPU can change as the vcpu migrates around the pcpus, do we not
> need to handle that case too?
>
> Surely we need some code in virt_timer_restore at least? Possibly a call
> to migrate_timer? Although perhaps there is a less expensive way when we
> know the timer isn't running?

I think that the comment above is misleading: the issue is that they
need to be both initialized with the same starting time and because
vcpu_vtimer_init is a vcpu initialization function they are called at
different times, therefore they are going to have a different starting
time on the two vcpus.
So the problem is not the physical cpu time but the virtual cpu time.


> > 
> > Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> > ---
> >  xen/arch/arm/vtimer.c |    4 ++--
> >  1 files changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/xen/arch/arm/vtimer.c b/xen/arch/arm/vtimer.c
> > index 1cb365e..2444851 100644
> > --- a/xen/arch/arm/vtimer.c
> > +++ b/xen/arch/arm/vtimer.c
> > @@ -48,7 +48,7 @@ int vcpu_vtimer_init(struct vcpu *v)
> >  {
> >      struct vtimer *t = &v->arch.phys_timer;
> >  
> > -    init_timer(&t->timer, phys_timer_expired, t, smp_processor_id());
> > +    init_timer(&t->timer, phys_timer_expired, t, v->processor);
> >      t->ctl = 0;
> >      t->offset = NOW();
> >      t->cval = NOW();
> > @@ -56,7 +56,7 @@ int vcpu_vtimer_init(struct vcpu *v)
> >      t->v = v;
> >  
> >      t = &v->arch.virt_timer;
> > -    init_timer(&t->timer, virt_timer_expired, t, smp_processor_id());
> > +    init_timer(&t->timer, virt_timer_expired, t, v->processor);
> >      t->ctl = 0;
> >      t->offset = READ_SYSREG64(CNTVCT_EL0) + READ_SYSREG64(CNTVOFF_EL2);
> >      t->cval = 0;
> 
> 
> 

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