From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Martin Subject: Re: [PATCH-WIP 01/13] xen/arm: use r12 to pass the hypercall number to the hypervisor Date: Mon, 27 Feb 2012 18:03:55 +0000 Message-ID: <20120227180355.GB2023@linaro.org> References: <1330019314-20865-1-git-send-email-stefano.stabellini@eu.citrix.com> <1330360043.8557.302.camel@zakaz.uk.xensource.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <1330360043.8557.302.camel-o4Be2W7LfRlXesXXhkcM7miJhflN2719@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: linaro-dev-bounces-cunTk1MwBs8s++Sfvej+rw@public.gmane.org Errors-To: linaro-dev-bounces-cunTk1MwBs8s++Sfvej+rw@public.gmane.org To: Ian Campbell Cc: "xen-devel-GuqFBffKawuULHF6PoxzQEEOCMrvLtNR@public.gmane.org" , "linaro-dev-cunTk1MwBs8s++Sfvej+rw@public.gmane.org" , "kvm-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , "arnd-r2nGTMty4D4@public.gmane.org" , "catalin.marinas-5wv7dgnIgG8@public.gmane.org" , "linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , David Vrabel , "linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org" List-Id: xen-devel@lists.xenproject.org On Mon, Feb 27, 2012 at 04:27:23PM +0000, Ian Campbell wrote: > On Thu, 2012-02-23 at 17:48 +0000, Stefano Stabellini wrote: > > We need a register to pass the hypercall number because we might not > > know it at compile time and HVC only takes an immediate argument. > > > > Among the available registers r12 seems to be the best choice because it > > is defined as "intra-procedure call scratch register". > > R12 is not accessible from the 16 bit "T1" Thumb encoding of mov > immediate (which can only target r0..r7). This is untrue. The important instructions, like MOV Rd, Rn can access all the regs. But anyway, there is no such thing as a Thumb-1 kernel, so we won't really care. > Since we support only ARMv7+ there are "T2" and "T3" encodings available > which do allow direct mov of an immediate into R12, but are 32 bit Thumb > instructions. > > Should we use r7 instead to maximise instruction density for Thumb code? The difference seems trivial when put into context, even if you code a special Thumb version of the code to maximise density (the Thumb-2 code which gets built from assembler in the kernel is very suboptimal in size, but there simply isn't a high proportion of asm code in the kernel anyway.) I wouldn't consider the ARM/Thumb differences as an important factor when deciding on a register. One argument for _not_ using r12 for this purpose is that it is then harder to put a generic "HVC" function (analogous to the "syscall" syscall) out-of-line, since r12 could get destroyed by the call. If you don't think you will ever care about putting HVC out of line though, it may not matter. Cheers ---Dave