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

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Date: Tue, 20 Feb 2024 09:02:02 +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: "shenghui.qu@...ngroup.cn" <shenghui.qu@...ngroup.cn>,
	"dawei.li@...ngroup.cn" <dawei.li@...ngroup.cn>, "ke.zhao@...ngroup.cn"
	<ke.zhao@...ngroup.cn>, "luming.yu@...il.com" <luming.yu@...il.com>
Subject: Re: [PATCH v1 2/2] powerpc/debug: hook to user return notifier
 infrastructure



Le 20/02/2024 à 09:51, Christophe Leroy a écrit :
> 
> 
> 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

Correct one is https://github.com/linuxppc/issues/issues/477