linux-kernel - Re: [PATCH] arm64: mm: support instruction SETEND

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Date:	Wed, 7 Jan 2015 18:58:24 +0800
From:	Leo Yan <leo.yan@...aro.org>
To:	"Suzuki K. Poulose" <Suzuki.Poulose@....com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Will Deacon <Will.Deacon@....com>,
	Xiaolong Ye <yexl@...vell.com>
Subject: Re: [PATCH] arm64: mm: support instruction SETEND

On Wed, Jan 07, 2015 at 10:10:34AM +0000, Suzuki K. Poulose wrote:
> On 07/01/15 05:52, Leo Yan wrote:
> >Currently kernel has set the bit SCTLR_EL1.SED, so the SETEND
> >instruction will be treated as UNALLOCATED; this error can be
> >reproduced when ARMv8 cpu runs with EL1/aarch64 and EL0/aarch32
> >mode, finally kernel will trap the exception if the userspace
> >libs use SETEND instruction.
> >
> >So this patch clears bit SCTLR_EL1.SED to support SETEND instruction.
> >
> The best way to do this, is via the instruction emulation framework
> added by Punit, which handles the armv8 deprecated/obsoleted
> instructions. This is now queued for 3.19.
> I have a patchset which adds the 'SETEND' emulation support to the
> framework. This will enable better handling of the feature,
> including finding out the users of the deprecated instruction (when
> we switch to the emulation mode).
> 

i'm a little confuse for this point:

if the deprecated instructions cannot be supported by CPU, then only
can use emulation; on the other hand, if CPU can natively support the
deprecated instruction, why we cannot directly enable this h/w feature?
if use the emulation mode, suppose here will have performance penalty.

how about u think for this? :-)

> >Signed-off-by: Leo Yan <leo.yan@...aro.org>
> >Signed-off-by: Xiaolong Ye <yexl@...vell.com>
> >---
> >  arch/arm64/mm/proc.S | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> >diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
> >index 4e778b1..66a7363 100644
> >--- a/arch/arm64/mm/proc.S
> >+++ b/arch/arm64/mm/proc.S
> >@@ -249,9 +249,9 @@ ENDPROC(__cpu_setup)
> >  	 *       CE0      XWHW CZ     ME TEEA S
> >  	 * .... .IEE .... NEAI TE.I ..AD DEN0 ACAM
> >  	 * 0011 0... 1101 ..0. ..0. 10.. .... .... < hardware reserved
> >-	 * .... .1.. .... 01.1 11.1 ..01 0001 1101 < software settings
> >+	 * .... .1.. .... 01.1 11.1 ..00 0001 1101 < software settings
> >  	 */
> >  	.type	crval, #object
> >  crval:
> >-	.word	0x000802e2			// clear
> >-	.word	0x0405d11d			// set
> >+	.word	0x000803e2			// clear
> >+	.word	0x0405d01d			// set
> >
> 
> 
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