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

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Date:	Fri, 9 Mar 2012 11:20:20 -0500 (EST)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Dave Martin <dave.martin@...aro.org>
cc:	Richard Earnshaw <Richard.Earnshaw@....com>,
	"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 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
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