Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758111Ab2B2Opx (ORCPT ); Wed, 29 Feb 2012 09:45:53 -0500 Received: from smtp.eu.citrix.com ([62.200.22.115]:52161 "EHLO SMTP.EU.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758059Ab2B2Opv (ORCPT ); Wed, 29 Feb 2012 09:45:51 -0500 X-IronPort-AV: E=Sophos;i="4.73,502,1325462400"; d="scan'208";a="11012105" Date: Wed, 29 Feb 2012 14:52:38 +0000 From: Stefano Stabellini X-X-Sender: sstabellini@kaball-desktop To: Dave Martin CC: Ian Campbell , Peter Maydell , Stefano Stabellini , "xen-devel@lists.xensource.com" , "linaro-dev@lists.linaro.org" , "kvm@vger.kernel.org" , "arnd@arndb.de" , "catalin.marinas@arm.com" , "linux-kernel@vger.kernel.org" , David Vrabel , "linux-arm-kernel@lists.infradead.org" Subject: Re: [PATCH-WIP 01/13] xen/arm: use r12 to pass the hypercall number to the hypervisor In-Reply-To: <20120229125826.GC2077@linaro.org> Message-ID: References: <1330019314-20865-1-git-send-email-stefano.stabellini@eu.citrix.com> <1330360043.8557.302.camel@zakaz.uk.xensource.com> <20120227180355.GB2023@linaro.org> <1330371219.10008.34.camel@dagon.hellion.org.uk> <20120228102040.GB2063@linaro.org> <1330426133.31269.70.camel@zakaz.uk.xensource.com> <20120229093436.GA2077@linaro.org> <1330509362.4270.20.camel@zakaz.uk.xensource.com> <20120229125826.GC2077@linaro.org> User-Agent: Alpine 2.00 (DEB 1167 2008-08-23) MIME-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2813 Lines: 66 On Wed, 29 Feb 2012, Dave Martin wrote: > On Wed, Feb 29, 2012 at 09:56:02AM +0000, Ian Campbell wrote: > > On Wed, 2012-02-29 at 09:34 +0000, Dave Martin wrote: > > > On Tue, Feb 28, 2012 at 12:28:29PM +0000, Stefano Stabellini wrote: > > > > > > I don't have a very strong opinion on which register we should use, but > > > > I would like to avoid r7 if it is already actively used by gcc. > > > > > > But there is no framepointer for Thumb-2 code (?) > > > > Peter Maydell suggested there was: > > > r7 is (used by gcc as) the Thumb frame pointer; I don't know if this > > > makes it worth avoiding in this context. > > > > Sounds like it might be a gcc-ism, possibly a non-default option? > > > > Anyway, I think r12 will be fine for our purposes so the point is rather > > moot. > > Just had a chat with some tools guys -- apparently, when passing register > arguments to gcc inline asms there really isn't a guarantee that those > variables will be in the expected registers on entry to the inline asm. > > If gcc reorders other function calls or other code around the inline asm > (which it can do, except under certain controlled situations), then > intervening code can clobber any registers in general. > > Or, to summarise another way, there is no way to control which register > is used to pass something to an inline asm in general (often we get away > with this, and there are a lot of inline asms in the kernel that assume > it works, but the more you inline the more likely you are to get nasty > surprises). There is no workaroud, except on some architectures where > special asm constraints allow specific individual registers to be > specified for operands (i386 for example). > > 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" > ... > "hvc #0" > :: [hvc_num] "i" (NUMBER) : "r12" > ); > OK, we can arrange the hypercall code to be like that. Also with your patch series it would be "_hvc" because of the .macro, right? > This is the kind of problem which goes away when out-of-lining the > hvc wrapper behind a C function interface, since the ABI then provides > guarantees about how values are mershaled into and out of that code. Do you mean implementing the entire HYPERVISOR_example_op in assembly and calling it from C? Because I guess that gcc would still be free to mess with the registers between the C function entry point and any inline assembly code. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/