From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751820Ab2G0JA7 (ORCPT ); Fri, 27 Jul 2012 05:00:59 -0400 Received: from smtp.eu.citrix.com ([62.200.22.115]:32091 "EHLO SMTP.EU.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751113Ab2G0JA5 (ORCPT ); Fri, 27 Jul 2012 05:00:57 -0400 X-IronPort-AV: E=Sophos;i="4.77,666,1336348800"; d="scan'208";a="13729848" Message-ID: <1343379653.6812.92.camel@zakaz.uk.xensource.com> Subject: Re: [Xen-devel] [PATCH 02/24] xen/arm: hypercalls From: Ian Campbell To: David Vrabel CC: Stefano Stabellini , "linux-kernel@vger.kernel.org" , "xen-devel@lists.xensource.com" , "linaro-dev@lists.linaro.org" , "arnd@arndb.de" , "konrad.wilk@oracle.com" , "catalin.marinas@arm.com" , "Tim (Xen.org)" , "linux-arm-kernel@lists.infradead.org" Date: Fri, 27 Jul 2012 10:00:53 +0100 In-Reply-To: <501176AF.3000607@cantab.net> References: <1343316846-25860-2-git-send-email-stefano.stabellini@eu.citrix.com> <501176AF.3000607@cantab.net> Organization: Citrix Systems, Inc. Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.4.3-1 MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2012-07-26 at 17:56 +0100, David Vrabel wrote: > On 26/07/12 16:33, Stefano Stabellini wrote: > > > > + * The hvc ISS is required to be 0xEA1, that is the Xen specific ARM > > + * hypercall tag. > > Is this number, 0xea1, assigned to Xen by some external body? The value and semantics of the hvc instructions immediate operand is entirely up to the hypervisor authors. We could have chosen 0 or some random number, we went for the latter because it increases the chances, by some tiny amount, that we won't clash with some other hypervisors ABI which makes supporting "foreign" guests that bit easier should it even come to it. IOW it's arbitrary in the same way that a Linux system calls used to use int 0x80. Ian.