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

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Date: Tue, 20 Feb 2024 08:51:01 +0000
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Michael Ellerman <mpe@...erman.id.au>, Aneesh Kumar K.V
	<aneesh.kumar@...nel.org>, Luming Yu <luming.yu@...ngroup.cn>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"npiggin@...il.com" <npiggin@...il.com>
CC: "luming.yu@...il.com" <luming.yu@...il.com>, "ke.zhao@...ngroup.cn"
	<ke.zhao@...ngroup.cn>, "dawei.li@...ngroup.cn" <dawei.li@...ngroup.cn>,
	"shenghui.qu@...ngroup.cn" <shenghui.qu@...ngroup.cn>
Subject: Re: [PATCH v1 2/2] powerpc/debug: hook to user return notifier
 infrastructure



Le 19/12/2023 à 07:33, Michael Ellerman a écrit :
> Aneesh Kumar K.V <aneesh.kumar@...nel.org> writes:
>> Luming Yu <luming.yu@...ngroup.cn> 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