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Date:   Fri, 2 Sep 2022 12:10:51 -0500
From:   Segher Boessenkool <segher@kernel.crashing.org>
To:     Peter Bergner <bergner@linux.ibm.com>
Cc:     Christophe Leroy <christophe.leroy@csgroup.eu>,
        "linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Nicholas Piggin <npiggin@gmail.com>,
        Zhouyi Zhou <zhouzhouyi@gmail.com>
Subject: Re: [PATCH v2] powerpc: Fix irq_soft_mask_set() and irq_soft_mask_return() with sanitizer
Message-ID: <20220902171051.GU25951@gate.crashing.org>
References: <c0b486e782b6695092dcdb2cd340a3d44c8c266d.1661272738.git.christophe.leroy@csgroup.eu> <CMJ3VICKD1CI.SVFJOKYJPKZQ@bobo> <e022754d-b4d3-bc9f-cc79-2cf556180459@csgroup.eu> <CMJ8P06JA9OY.1S8VDV2XRU3W5@bobo> <7c11b659-5b8e-256c-508e-39395041fccb@csgroup.eu> <20220831224522.GX25951@gate.crashing.org> <bd00d8c2-663d-cb35-c1c3-4fa9bd4cb4ad@linux.ibm.com>
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On Fri, Sep 02, 2022 at 10:57:27AM -0500, Peter Bergner wrote:
> On 8/31/22 5:45 PM, Segher Boessenkool wrote:
> > Yes, this is guaranteed.
> 
> Agree with Segher here.  That said, there was a gcc bug a looooong time
> ago where gcc copied r13 into a temporary register and used it from there.

r13 is a fixed register on most of our ABIs (everything that is not AIX
or Darwin, even), so this can never happen.  Except if there are bugs,
of course ;-)

> That's ok (correctness wise, but not ideal) from user land standpoint,
> but we took a context switch after the reg copy and it was restarted on
> a different cpu, so differnt local_paca and r13 value.  We went boom
> because the copy wasn't pointing to the correct local_paca anymore.
> So it is very important the compiler always use r13 when accessing
> the local_paca.

Yes.  So we either whould use -ffixed-r13, or just not use unsupported
compilers.  powerpc*-linux and powerpc*-elf work fine for example :-)


Segher