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

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
don't spread BUG_ON()/panic/stop_cpu everywhere. The benefit is
only one place to fix over multiple one because very likely the
is the same everywhere.

I agree that today it will end up to crashing the system because of
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.


Julien Grall

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