linux-kernel - Re: [Android-virt] [Embeddedxen-devel] [Xen-devel] [ANNOUNCE] Xen port to Cortex-A15 / ARMv7 with virt extensions

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Message-ID: <20111201160241.GH27394@arm.com>
Date:	Thu, 1 Dec 2011 16:02:41 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Ian Campbell <Ian.Campbell@...rix.com>,
	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>,
	Pawel Moll <Pawel.Moll@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	"android-virt@...ts.cs.columbia.edu" 
	<android-virt@...ts.cs.columbia.edu>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"embeddedxen-devel@...ts.sourceforge.net" 
	<embeddedxen-devel@...ts.sourceforge.net>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [Android-virt] [Embeddedxen-devel] [Xen-devel] [ANNOUNCE] Xen
 port to Cortex-A15 / ARMv7 with virt extensions

On Thu, Dec 01, 2011 at 03:42:19PM +0000, Arnd Bergmann wrote:
> On Thursday 01 December 2011, Catalin Marinas wrote:
> > Given the way register banking is done on AArch64, issuing an HVC on a
> > 32-bit guest OS doesn't require translation on a 64-bit hypervisor. We
> > have a similar implementation at the SVC level (for 32-bit user apps on
> > a 64-bit kernel), the only modification was where a 32-bit SVC takes a
> > 64-bit parameter in two separate 32-bit registers, so packing needs to
> > be done in a syscall wrapper.
> 
> How do you deal with signed integer arguments passed into SVC or HVC from
> a caller? If I understand the architecture correctly, the upper
> halves of the argument register end up zero-padded, while the callee
> expects sign-extension.

If you treat it as an "int" (32-bit) and function prototype defined
accordingly, then the generated code only accesses it as a W (rather
than X) register and the top 32-bit part is ignored (no need for
sign-extension). If it is defined as a "long" in the 32-bit world, then
it indeed needs explicit conversion given the different sizes for long
(for example sys_lseek, the second argument is a 'long' and we do
explicit sign extension in the wrapper).

-- 
Catalin
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