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

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Date:	Mon, 27 Feb 2012 21:05:08 +0000
From:	Peter Maydell <peter.maydell@...aro.org>
To:	Ian Campbell <Ian.Campbell@...rix.com>
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 27 February 2012 16:27, Ian Campbell <Ian.Campbell@...rix.com> wrote:
> R12 is not accessible from the 16 bit "T1" Thumb encoding of mov
> immediate (which can only target r0..r7).
>
> Since we support only ARMv7+ there are "T2" and "T3" encodings available
> which do allow direct mov of an immediate into R12, but are 32 bit Thumb
> instructions.
>
> Should we use r7 instead to maximise instruction density for Thumb code?

r7 is (used by gcc as) the Thumb frame pointer; I don't know if this
makes it worth avoiding in this context.

-- PMM
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