From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1754541AbcEWUTJ (ORCPT <rfc822;w@1wt.eu>);
	Mon, 23 May 2016 16:19:09 -0400
Received: from gate.crashing.org ([63.228.1.57]:60120 "EHLO gate.crashing.org"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1751413AbcEWUTG (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Mon, 23 May 2016 16:19:06 -0400
Date: Mon, 23 May 2016 15:17:47 -0500
From: Segher Boessenkool <segher@kernel.crashing.org>
To: Christophe Leroy <christophe.leroy@c-s.fr>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>, Scott Wood <oss@buserror.net>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] powerpc32: use stmw/lmw for non volatile registers save/restore
Message-ID: <20160523201747.GA11583@gate.crashing.org>
References: <20160523084637.063611A239A@localhost.localdomain>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20160523084637.063611A239A@localhost.localdomain>
User-Agent: Mutt/1.4.2.3i
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Mon, May 23, 2016 at 10:46:36AM +0200, Christophe Leroy wrote:
> lmw/stmw have a 1 cycle (2 cycles for lmw on some ppc) in addition
> and implies serialising, however it reduces the amount of instructions
> hence the amount of instruction fetch compared to the equivalent
> operation with several lzw/stw. It means less pressure on cache and
> less fetching delays on slow memory.

lmw/stmw do not work at all in LE mode, on most processors.  This is a
supported configuration.  NAK.

> When we transfer 20 registers, it is worth it.
> gcc uses stmw/lmw at function entry/exit to save/restore non
> volatile register, so lets also do it that way.

No, C code is compiled with -mno-multiple for LE configs.  Saving a few
bytes of code is not "worth it", anyway.

> --- a/arch/powerpc/kernel/misc_32.S
> +++ b/arch/powerpc/kernel/misc_32.S
> @@ -1086,3 +1086,25 @@ relocate_new_kernel_end:
>  relocate_new_kernel_size:
>  	.long relocate_new_kernel_end - relocate_new_kernel
>  #endif
> +
> +_GLOBAL(setjmp)
> +	mflr	r0
> +	li	r3, 0
> +	stw	r0, 0(r3)
> +	stw	r1, 4(r3)
> +	stw	r2, 8(r3)
> +	mfcr	r12
> +	stmw	r12, 12(r3)
> +	blr

This code has been tested?  I very much doubt it.


Segher