From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ian Campbell <ian.campbell@citrix.com>
Subject: Re: [PATCH v1 2/4] xen-version: Add third parameter
 (len) to the do_version hypercall.
Date: Fri, 9 Oct 2015 15:48:55 +0100
Message-ID: <1444402135.1410.405.camel@citrix.com>
References: <1444359390-14153-1-git-send-email-konrad.wilk@oracle.com>
	<1444359390-14153-3-git-send-email-konrad.wilk@oracle.com>
	<5617962602000078000A99DE@prv-mh.provo.novell.com>
	<5617B316.7040408@citrix.com> <1444394805.1410.376.camel@citrix.com>
	<5617ED8A02000078000A9C4D@prv-mh.provo.novell.com>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Return-path: <xen-devel-bounces@lists.xen.org>
Received: from mail6.bemta5.messagelabs.com ([195.245.231.135])
	by lists.xen.org with esmtp (Exim 4.72)
	(envelope-from <prvs=717dc9c5a=Ian.Campbell@citrix.com>)
	id 1ZkYyh-00083V-F3
	for xen-devel@lists.xenproject.org; Fri, 09 Oct 2015 14:48:59 +0000
In-Reply-To: <5617ED8A02000078000A9C4D@prv-mh.provo.novell.com>
List-Unsubscribe: <http://lists.xen.org/cgi-bin/mailman/options/xen-devel>,
	<mailto:xen-devel-request@lists.xen.org?subject=unsubscribe>
List-Post: <mailto:xen-devel@lists.xen.org>
List-Help: <mailto:xen-devel-request@lists.xen.org?subject=help>
List-Subscribe: <http://lists.xen.org/cgi-bin/mailman/listinfo/xen-devel>,
	<mailto:xen-devel-request@lists.xen.org?subject=subscribe>
Sender: xen-devel-bounces@lists.xen.org
Errors-To: xen-devel-bounces@lists.xen.org
To: Jan Beulich <JBeulich@suse.com>, Andrew Cooper <andrew.cooper3@citrix.com>, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: mpohlack@amazon.de, ian.jackson@eu.citrix.com, dgdegra@tycho.nsa.gov, wei.liu2@citrix.com, xen-devel@lists.xenproject.org
List-Id: xen-devel@lists.xenproject.org

On Fri, 2015-10-09 at 08:38 -0600, Jan Beulich wrote:
> > > > On 09.10.15 at 14:46, <ian.campbell@citrix.com> wrote:
> > On Fri, 2015-10-09 at 13:29 +0100, Andrew Cooper wrote:
> > > On 09/10/15 09:25, Jan Beulich wrote:
> > > > > > > On 09.10.15 at 04:56, <konrad.wilk@oracle.com> wrote:
> > > > > All existing commands ignore the parameter so this does
> > > > > not break the ABI.
> > > > Does it not? What about the debug mode clobbering of hypercall
> > > > argument registers?
> > > 
> > > That is an implementation detail of the hypervisor.  It is irrelevant
> > > to
> > > guests whether Xen chooses to clobber the spare registers or not.
> > 
> > Or in other words the effect here is to clobber one _less_ register,
> > and
> > the guest cannot have been relying on a register getting so clobbered
> > (if
> > nothing else it doesn't happen in debug=n builds).
> 
> No, the one less register clobbered is in the first clobbering phase,
> where _unused_ inputs get clobbered (for hypervisor internal
> consumption). The second clobbering phase destroys all _used_
> input registers' contents (the guest visible values), and _this_ is
> what results in ABI breakage (because callers assuming the
> hypercall to take two arguments assume that the 3rd argument
> register will retain its contents.

Ah, yes, that's correct. My mistake.

Ian.