Re: [Xen-devel] [PATCH 3/7] xen/arm/psci: Implement CPU_OFF PSCI call (physical interface)

[Julien Grall <julien.grall@xxxxxxx>]

On 12/04/18 12:33, Mirela Simonovic wrote:
> On Wed, Apr 11, 2018 at 4:46 PM, Julien Grall <julien.grall@xxxxxxx> wrote:
> > On 11/04/18 14:19, Mirela Simonovic wrote:
> >
> > >        local_irq_disable();
> > >        cpu_is_dead = true;
> > >        /* Make sure the write happens before we sleep forever */
> > >        dsb(sy);
> > >        isb();
> > > +    /* PSCI cpu off call will return only in case of an error */
> > > +    errno = call_psci_cpu_off();
> > > +    printk(XENLOG_DEBUG "PSCI cpu off call failed for CPU#%d err=%d\n",
> > > +           get_processor_id(), errno);
> > > +    isb();
> >
> > What are you trying to achieve with the isb() here?
> I use to have a problem that the wfi below gets executed before the
> call_psci_cpu_off(). Adding isb() fixed the issue. However, I tried
> now to reproduce the problem and it doesn't show up. I still believe
> isb() should be here, please let me know if you disagree (I obviously
> can't prove the claim now).
The problem you describe can't be possible with the code you have 
because call_psci_cpu_off() is issuing a SMC. SMC will lead to change 
exception level and therefore have a context-synchronization barrier.
This is obviously based on the assumption you don't have an errata on 
your CPU exposing the behavior you describe. For that you would need to 
check errata notice for your CPU and/or try to reproduce.
However, what you would need is a dsb(sy); isb(); to drain the write 
buffer if you print a message.
Furthermore, now on platform without CPU off support (e.g non-PSCI 
platform and PSCI 0.1) you will log an error message that may worry 
people. In reality, PSCI cpu_off will unlikely fail, so you probably 
want to add a panic in call_psci_cpu_off instead.
Cheers,

--
Julien Grall

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