Re: [PATCH v1 2/2] powerpc/debug: hook to user return notifier infrastructure

[Christophe Leroy]

Le 19/12/2023 à 07:33, Michael Ellerman a écrit :
> Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxx> writes:
>> Luming Yu <luming.yu@xxxxxxxxxxxx> writes:
>>
>>> Before we have powerpc to use the generic entry infrastructure,
>>> the call to fire user return notifier is made temporarily in powerpc
>>> entry code.
>>>
>>
>> It is still not clear what will be registered as user return notifier.
>> Can you summarize that here?
> 
> fire_user_return_notifiers() is defined in kernel/user-return-notifier.c
> 
> That's built when CONFIG_USER_RETURN_NOTIFIER=y.
> 
> That is not user selectable, it's only enabled by:
> 
> arch/x86/kvm/Kconfig:        select USER_RETURN_NOTIFIER
> 
> So it looks to me like (currently) it's always a nop and does nothing.
> 
> Which makes me wonder what the point of wiring this feature up is :)
> Maybe it's needed for some other feature I don't know about?
> 
> Arguably we could just enable it because we can, and it currently does
> nothing so it's unlikely to break anything. But that also makes it
> impossible to test the implementation is correct, and runs the risk that
> one day in the future when it does get enabled only then do we discover
> it doesn't work.

Opened an "issue" for the day we need it: 
https://github.com/KSPP/linux/issues/348