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Date:   Tue, 31 Oct 2017 23:48:53 +0000
From:   James Hogan <james.hogan@mips.com>
To:     Corey Minyard <cminyard@mvista.com>
CC:     Matt Redfearn <matt.redfearn@mips.com>,
        Ralf Baechle <ralf@linux-mips.org>,
        Matthew Fortune <matthew.fortune@mips.com>,
        <linux-mips@linux-mips.org>, <linux-kernel@vger.kernel.org>,
        "Jason A. Donenfeld" <jason@zx2c4.com>,
        Paul Burton <paul.burton@imgtec.com>
Subject: Re: [PATCH] MIPS: Fix exception entry when CONFIG_EVA enabled
Message-ID: <20171031234853.GD15260@jhogan-linux>
References: <1507712360-20657-1-git-send-email-matt.redfearn@mips.com>
 <605f6a96-a843-085c-efc6-a2c0f2afd84a@mvista.com>
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On Wed, Oct 11, 2017 at 08:12:31AM -0500, Corey Minyard wrote:
> On 10/11/2017 03:59 AM, Matt Redfearn wrote:
> > Commit 9fef68686317b ("MIPS: Make SAVE_SOME more standard") made several
> > changes to the order in which registers are saved in the SAVE_SOME
> > macro, used by exception handlers to save the processor state. In
> > particular, it removed the
> > move   k1, sp
> > in the delay slot of the branch testing if the processor is already in
> > kernel mode. This is replaced later in the macro by a
> > move   k0, sp
> > When CONFIG_EVA is disabled, this instruction actually appears in the
> > delay slot of the branch. However, when CONFIG_EVA is enabled, instead
> > the RPS workaround of
> > MFC0	k0, CP0_ENTRYHI
> > appears in the delay slot. This results in k0 not containing the stack
> > pointer, but some unrelated value, which is then saved to the kernel
> > stack. On exit from the exception, this bogus value is restored to the
> > stack pointer, resulting in an OOPS.
> >
> > Fix this by moving the save of SP in k0 explicitly in the delay slot of
> > the branch, outside of the CONFIG_EVA section, restoring the expected
> > instruction ordering when CONFIG_EVA is active.
> >
> > Fixes: 9fef68686317b ("MIPS: Make SAVE_SOME more standard")
> > Signed-off-by: Matt Redfearn <matt.redfearn@mips.com>
> > Reported-by: Vladimir Kondratiev <vladimir.kondratiev@intel.com>
>=20
> I looked this over pretty carefully and it looks correct to me.=C2=A0 It=
=20
> makes no difference
> in the instructions generated by the non-EVA case.=C2=A0 I shouldn't have=
=20
> missed this :(.
>=20
> Reviewed-by: Corey Minyard <cminyard@mvista.com>

Yeh, having stared at it for a little while it looks correct to me too.

Reviewed-by: James Hogan <jhogan@kernel.org>

Cheers
James

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