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

[Julien Grall <julien.grall@xxxxxxx>]

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?
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.
Cheers,

--
Julien Grall

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