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: <20120301101258.GC7363@n2100.arm.linux.org.uk>
Date:	Thu, 1 Mar 2012 10:12:58 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Ian Campbell <Ian.Campbell@...rix.com>
Cc:	Dave Martin <dave.martin@...aro.org>,
	Peter Maydell <peter.maydell@...aro.org>,
	"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>,
	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"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, Feb 29, 2012 at 02:44:24PM +0000, Ian Campbell wrote:
> > If you need a specific register, this means that you must set up that
> > register explicitly inside the asm if you want a guarantee that the
> > code will work:
> > 
> > 	asm volatile (
> > 		"movw	r12, %[hvc_num]\n\t"
> 
> Is gcc (or gas?) smart enough to optimise this away if it turns out that
> %[hvc_num] == r12?

No, and it won't do, because %[hvc_num] is specified in these operands:

> > 		...
> > 		"hvc	#0"
> > 		:: [hvc_num] "i" (NUMBER) : "r12"

to be an integer, not a register.

> How are system calls implemented on the userspace side? I confess I
> don't know what the ARM syscall ABI looks like -- is it all registers or
> is some of it on the stack? It sounds like the solution ought to be
> pretty similar though.

All registers.  We have a few which take a pointer to an in memory array,
but those are for some old multiplexed syscalls.
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