Re: [Xen-devel] [PATCH v3 10/10] xen/arm: Enable errata for secondary CPU on hotplug after the boot

[Mirela Simonovic <mirela.simonovic@xxxxxxxxxx>]

Hi Stefano,

On Mon, May 14, 2018 at 6:59 PM, Stefano Stabellini
<sstabellini@xxxxxxxxxx> wrote:
> On Mon, 14 May 2018, Julien Grall wrote:
>> On 11/05/18 22:47, Stefano Stabellini wrote:
>> > On Fri, 11 May 2018, Dario Faggioli wrote:
>> > > On Fri, 2018-05-11 at 14:08 +0100, Julien Grall wrote:
>> > > > The whole idea here is we have only one place taking the decision and
>> > > > we
>> > > > don't spread BUG_ON()/panic/stop_cpu everywhere. The benefit is
>> > > > having
>> > > > only one place to fix over multiple one because very likely the
>> > > > decision
>> > > > is the same everywhere.
>> > > >
>> > > > I agree that today it will end up to crashing the system because of
>> > > > the
>> > > > BUG_ON. But that's a separate topic.
>> > > >
>> > > Yes!!! :-D
>> > >
>> > > I.e., as I've said countless times, I would think that a series which
>> > > introduces a CPU_STARTING notifier that fails, should also deal with
>> > > adjusting the CPU process accordingly.
>> > >
>> > > *BUT* if you ARM people are ok with arch/arm/ code that does that,
>> > > perhaps with a comment saying something like:
>> > >
>> > > "This will cause us to hit the BUG_ON() in notify_cpu_starting(). To
>> > > fix that, we need to properly change the CPU bringup code, which will
>> > > happen in a leter series."
>> > >
>> > > that would also work, I guess. :-)
>> >
>> > Yes, I think that returning error with an in-code comment on top is the
>> > best solution.
>>
>> It is the second best solution ;). If we consider the notifier can return an
>> error, then best solution is to fix notify_cpu_starting().
>>
>> I would be ok with the second best solution if we have someone to fix it for
>> Xen 4.12. Per my understanding, Mirela is not going to do it. So what's the
>> plan here?
>
> I can look at fixing notify_cpu_starting(). I am also OK with you
> reworking the vmap code as you suggested below. Regardless, I think
> Mirela should go ahead with the comment now. Then, either you or me are
> going to come in and remove the comment one way or another (either
> fixing notify_cpu_starting or imposing all the callbacks to never return
> an error).
>

Thanks, I submitted v4.

Regards,
Mirela

>
>> Another solution is to impose all the enable callbacks to never return an
>> error (AFAICT Linux is just ignoring the return of the callback)).
>>
>> Today, we happen to return an error only in the case vmap is failing (used to
>> remapped vector table read-write). It might be possible to avoid the 
>> potential
>> re-mapping failure by reworking the code.
>>
>> I could explore that solution if we prefer going towards imposing all the
>> enable callbacks to never return an error.

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