From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751137AbdIMNvh (ORCPT ); Wed, 13 Sep 2017 09:51:37 -0400 Received: from aserp1040.oracle.com ([141.146.126.69]:48365 "EHLO aserp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751010AbdIMNvc (ORCPT ); Wed, 13 Sep 2017 09:51:32 -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> <46a17191-916a-e039-7e2f-a197e6783fe4@oracle.com> <258bc8dd-c475-bd5d-dffc-8c77fbe3cca3@suse.com> <9e8e497a-49d1-6299-8101-dd1123f81fee@oracle.com> From: Boris Ostrovsky Message-ID: <0ea7c145-9a86-f002-75d0-bc6b8be72867@oracle.com> Date: Wed, 13 Sep 2017 09:50:15 -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: 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 On 09/13/2017 09:38 AM, Juergen Gross wrote: > On 13/09/17 15:22, Boris Ostrovsky wrote: >> On 09/12/2017 02:18 PM, Juergen Gross wrote: >>> On 12/09/17 18:21, Boris Ostrovsky wrote: >>>> 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. >>> No, without the functions to use them it will be impossible. >> I don't follow this. Which functions? The ones this patch is removing? > Yes, just after having been added one patch earlier. > > Right now the Linux kernel doesn't support grant V2 at all. So there > surely is no driver making use of V2 features right now. > > Ican merge patches 1 and 2 in case you want. I just thought a pure > revert of the former V2 remove patch would be easier to review, > taking into account that the former V2 support was working in > production environments (and even back then there was no user of > sub-page or transient grants). No, I don't have problems with *how* you are doing this (revert fully first and then remove). I am just not sure that removing these functions is the way to go because we are ending up with partial implementation of v2. The fact that noone is/has been using these features is IMO not particularly relevant. If these two were optional features then it would have been reasonable to drop them. -boris