linux-kernel - Re: [PATCH v2 05/20] ARM: LPAE: Introduce L_PTE_NOEXEC and L_PTE

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Message-ID: <20101116181906.GA21926@n2100.arm.linux.org.uk>
Date:	Tue, 16 Nov 2010 18:19:06 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 05/20] ARM: LPAE: Introduce L_PTE_NOEXEC and
	L_PTE_NOWRITE

On Tue, Nov 16, 2010 at 03:18:03PM +0000, Catalin Marinas wrote:
> On 15 November 2010 18:30, Russell King - ARM Linux
> <linux@....linux.org.uk> wrote:
> > Let's not make this more complicated than it has to be.  If we need the
> > inverse of WRITE and EXEC, then that's what we should change everyone to,
> > not invent a new system to work along side the old system.
> 
> This adds an additional instruction in set_pte_ext, unless you can
> write the bit checking in a better way:

It actually results in the same number of instructions.  From memory:

ARMv3-ARMv5:
-	eor	r3, r1, #L_PTE_PRESENT | L_PTE_YOUNG | L_PTE_WRITE | L_PTE_DIRTY
-	tst	r3, #L_PTE_WRITE | L_PTE_DIRTY	@ write and dirty?
-	orreq	r2, r2, #PTE_SMALL_AP_UNO_SRW
+	eor	r3, r1, #L_PTE_PRESENT | L_PTE_YOUNG | L_PTE_DIRTY
+	tst	r3, #L_PTE_RDONLY | L_PTE_DIRTY	@ write and dirty?
+	orreq	r2, r2, #PTE_SMALL_AP_UNO_SRW

and for ARMv6+:

-	tst	r1, #L_PTE_WRITE
-	tstne	r1, #L_PTE_DIRTY
-	orreq	r3, r3, #PTE_EXT_APX
+	eor	r1, r1, #L_PTE_DIRTY
+	tst	r1, #L_PTE_RDONLY | L_PTE_DIRTY
+	orrne	r3, r3, #PTE_EXT_APX
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