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

[Mirela Simonovic <mirela.simonovic@xxxxxxxxxx>]

Hi Julien,


On Thu, Apr 12, 2018 at 3:31 PM, Julien Grall <julien.grall@xxxxxxx> wrote:
>
>
> 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.
>

Even if PSCI cpu_off call fails, what is unlikely to happen, the
system is still functional. Enabling that pCPU later will fail, but
Xen can handle this error and continue running properly on the boot
pCPU (I've tested this in 2 pCPUs config).
Therefore, I believe panic may not be necessary in this case. I
suggest that we dump the error message and continue to run. Please let
me know if you disagree.

Thanks,
Mirela

> Cheers,
>
> --
> Julien Grall

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