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: <4F5A401E.3070008@arm.com>
Date:	Fri, 9 Mar 2012 17:38:38 +0000
From:	Richard Earnshaw <Richard.Earnshaw@....com>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
CC:	Dave Martin <dave.martin@...aro.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linaro-dev@...ts.linaro.org" <linaro-dev@...ts.linaro.org>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	"arnd@...db.de" <arnd@...db.de>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	David Vrabel <david.vrabel@...rix.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"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 09/03/12 16:20, Nicolas Pitre wrote:
> On Fri, 9 Mar 2012, Dave Martin wrote:
>
>> Register variables feel like a red herring though.  We're only using
>> those because we can't do the needful thing and actually desscribe
>> these constraints in the asm constraints (which would seem to be the
>> right place).  We specifically don't care where those values are
>> except at the boundaries of the asm block itself.
>
> Absolutely.
>
>> Is there a reason why ARM gcc doesn't provide the ability to specify
>> such exact-register constraints, or is this more for historical
>> reasons?  It is possible?
>
> I don't know how much things have changed since I last looked at the gcc
> code, but implementing this seemed to be pretty trivial at the time.
> The problem would be to determine a good letter scheme to map to actual
> registers.
>
>
> Nicolas
>

While it is technically possible, it is likely to end up hurting overall
compiler performance as we'll then have to define the machine as having
small register classes.

--
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