Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751673AbdILQVs (ORCPT ); Tue, 12 Sep 2017 12:21:48 -0400 Received: from aserp1040.oracle.com ([141.146.126.69]:17178 "EHLO aserp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751426AbdILQVo (ORCPT ); Tue, 12 Sep 2017 12:21:44 -0400 Subject: Re: [PATCH 2/4] xen: limit grant v2 interface to the v1 functionality To: Juergen Gross , linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org References: <20170908144849.2958-1-jgross@suse.com> <20170908144849.2958-3-jgross@suse.com> <8c87dc06-bbc8-5279-d135-d3351032913d@suse.com> <2777c0f8-7555-fbf3-6e18-81cea2fd1a39@oracle.com> <7dd99241-f771-9fa2-8a6a-ce019dbe08b6@suse.com> From: Boris Ostrovsky Message-ID: <46a17191-916a-e039-7e2f-a197e6783fe4@oracle.com> Date: Tue, 12 Sep 2017 12:21:30 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0 MIME-Version: 1.0 In-Reply-To: <7dd99241-f771-9fa2-8a6a-ce019dbe08b6@suse.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit X-Source-IP: userv0022.oracle.com [156.151.31.74] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1330 Lines: 28 On 09/12/2017 12:09 PM, Juergen Gross wrote: > On 12/09/17 18:05, Boris Ostrovsky wrote: >> On 09/12/2017 11:50 AM, Juergen Gross wrote: >>> On 12/09/17 17:44, Boris Ostrovsky wrote: >>>> On 09/08/2017 10:48 AM, Juergen Gross wrote: >>>>> As there is currently no user for sub-page grants or transient grants >>>>> remove that functionality. This at once makes it possible to switch >>>>> from grant v2 to grant v1 without restrictions, as there is no loss of >>>>> functionality other than the limited frame number width related to >>>>> the switch. >>>> But isn't that ABI violation? v2 is expected to support this (XSAs >>>> notwithstanding) >>> No, I don't think so. >>> >>> The hypervisor still supports it, but the domU (or dom0) isn't required >>> to make use of all the features IMHO. Or are you aware of any backend >>> querying the grant version of a frontend and acting in another way if v2 >>> is detected? >> I am not aware of any but that doesn't mean that they don't (or won't) >> exist. > But isn't the frontend the one which is defining what is granted in > which way? How should there be an ABI breakage when the frontend just > isn't using sub-page or transitive grants? People may provide both front and backend drivers and frontends, knowing that v2 is available, could decide to use those features. -boris