linux-kernel - Re: [PATCH] MIPS: Fix exception entry when CONFIG

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Date:   Wed, 18 Oct 2017 18:34:49 +0100
From:   "Maciej W. Rozycki" <macro@...s.com>
To:     Matt Redfearn <matt.redfearn@...s.com>
CC:     Ralf Baechle <ralf@...ux-mips.org>,
        James Hogan <james.hogan@...s.com>,
        Matthew Fortune <matthew.fortune@...s.com>,
        <linux-mips@...ux-mips.org>, Corey Minyard <cminyard@...sta.com>,
        <linux-kernel@...r.kernel.org>,
        "Jason A. Donenfeld" <jason@...c4.com>,
        Paul Burton <paul.burton@...tec.com>
Subject: Re: [PATCH] MIPS: Fix exception entry when CONFIG_EVA enabled

On Wed, 11 Oct 2017, Matt Redfearn wrote:

> diff --git a/arch/mips/include/asm/stackframe.h b/arch/mips/include/asm/stackframe.h
> index 5d3563c55e0c..2161357cc68f 100644
> --- a/arch/mips/include/asm/stackframe.h
> +++ b/arch/mips/include/asm/stackframe.h
> @@ -199,6 +199,10 @@
>  		sll	k0, 3		/* extract cu0 bit */
>  		.set	noreorder
>  		bltz	k0, 8f
> +		 move	k0, sp
> +		.if \docfi
> +		.cfi_register sp, k0
> +		.endif

 Using $k1 for the Status.CU0 check would let us get rid of the 
`noreorder' block, making this code less fragile at no run-time cost, 
i.e.:

		mfc0	k1, CP0_STATUS
		sll	k1, 3
		move	k0, sp
		bltz	k1, 8f

(unfortunately I can't see a way to usefully fill the coprocessor move 
delay slot automatically scheduled by GAS here between MFC0 and SLL for 
processors that require it).

  Maciej