linux-kernel - Re: [PATCH-WIP 01/13] xen/arm: use r12 to pass the hypercall number to the hypervisor

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Message-ID: <1330509362.4270.20.camel@zakaz.uk.xensource.com>
Date:	Wed, 29 Feb 2012 09:56:02 +0000
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Dave Martin <dave.martin@...aro.org>,
	Peter Maydell <peter.maydell@...aro.org>
CC:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linaro-dev@...ts.linaro.org" <linaro-dev@...ts.linaro.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"arnd@...db.de" <arnd@...db.de>,
	"catalin.marinas@....com" <catalin.marinas@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	David Vrabel <david.vrabel@...rix.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH-WIP 01/13] xen/arm: use r12 to pass the hypercall number
 to the hypervisor

On Wed, 2012-02-29 at 09:34 +0000, Dave Martin wrote:
> On Tue, Feb 28, 2012 at 12:28:29PM +0000, Stefano Stabellini wrote:

> > I don't have a very strong opinion on which register we should use, but
> > I would like to avoid r7 if it is already actively used by gcc.
> 
> But there is no framepointer for Thumb-2 code (?)

Peter Maydell suggested there was:
> r7 is (used by gcc as) the Thumb frame pointer; I don't know if this
> makes it worth avoiding in this context.

Sounds like it might be a gcc-ism, possibly a non-default option?

Anyway, I think r12 will be fine for our purposes so the point is rather
moot.

Ian.

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