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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Christophe Leroy <christophe.leroy@csgroup.eu>,
        Liu Shixin <liushixin2@huawei.com>,
        Marco Elver <elver@google.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>
Cc:     linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] powerpc: don't select KFENCE on platform PPC_FSL_BOOK3E
In-Reply-To: <9c1ee778-b38b-3d41-37f3-5ea22dca063b@csgroup.eu>
References: <20210924063927.1341241-1-liushixin2@huawei.com>
 <77ce95e4-1af1-6536-5f0c-a573c648801a@huawei.com>
 <87bl3u7oay.fsf@mpe.ellerman.id.au>
 <9c1ee778-b38b-3d41-37f3-5ea22dca063b@csgroup.eu>
Date:   Wed, 13 Oct 2021 11:48:20 +1100
Message-ID: <8735p57nsb.fsf@mpe.ellerman.id.au>
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Christophe Leroy <christophe.leroy@csgroup.eu> writes:
> Le 12/10/2021 =C3=A0 08:24, Michael Ellerman a =C3=A9crit=C2=A0:
>> Liu Shixin <liushixin2@huawei.com> writes:
>>> kindly ping.
>>=20
>> I was under the impression you were trying to debug why it wasn't
>> working with Christophe.
>
> The investigation was a bit dormant to be honest since Liu confirmed=20
> that neither KFENCE not DEBUG_PAGEALLOC works.

No worries. Sorry it fell to you to do the investigation.

> I now looked at the effort to make it work, and it is not trivial.
> At the time being, all linear space is mapped with pinned TLBs and=20
> everything is setup for space 0, with space 1 being used temporarily=20
> when doing heavy changes to space 0.
>
> We can't use standard pages for linear space on space 0 because we need=20
> memory mapped at all time for exceptions (on booke exception run with=20
> MMU on in space 0).
>
> In order to use standard pages, we'd need to reorganise the kernel to=20
> have it run mostly in space 1 (for data at least) where we would map=20
> almost everything with standard pages, and keep pinned TLB to map linear=
=20
> space on space 0 for TLB miss exceptions. Then we'd do more or less like=
=20
> book3s/32 and switch back into space 1 into other exceptions prolog.
>
> That could be good to do it as we could maybe have more code in common=20
> with non booke 32 bits, but it is not a trivial job.
>
> So I suggest that for now, we just make KFENCE and DEBUG_PAGEALLOC=20
> unselectable for booke/32 (e500 and 44x).

Yep seems reasonable.

cheers