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Fri, 31 Jul 2020 14:36:56 +0000 From: Bertrand Marquis To: Jan Beulich Subject: Re: [PATCH v2] xen/arm: Convert runstate address during hypcall Thread-Topic: [PATCH v2] xen/arm: Convert runstate address during hypcall Thread-Index: AQHWZPcpcJdIAbXhcEqLxJ7JubPXUKkdZ+eAgAC8aQCAAMFpAIAAcn6AgAHoeQCAADumgIAAAacAgABIKQA= Date: Fri, 31 Jul 2020 14:36:56 +0000 Message-ID: References: <4647a019c7b42d40d3c2f5b0a3685954bea7f982.1595948219.git.bertrand.marquis@arm.com> <8d2d7f03-450c-d50c-630b-8608c6d42bb9@suse.com> <1b046f2c-05c8-9276-a91e-fd55ec098bed@suse.com> <1a8bbcc7-9d0c-9669-db7b-e837af279027@suse.com> <73c8ade5-36a3-cc13-80b6-bda89e175cbb@xen.org> <6066b507-f956-8e7a-89f3-b21428b66d65@suse.com> In-Reply-To: <6066b507-f956-8e7a-89f3-b21428b66d65@suse.com> Accept-Language: en-GB, en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: Authentication-Results-Original: suse.com; dkim=none (message not signed) header.d=none;suse.com; dmarc=none action=none header.from=arm.com; x-originating-ip: [90.126.203.125] x-ms-publictraffictype: Email X-MS-Office365-Filtering-HT: Tenant X-MS-Office365-Filtering-Correlation-Id: c0c3a65c-2c61-483d-18cb-08d8355f30db x-ms-traffictypediagnostic: DB8PR08MB5323:|DB8PR08MB5482: X-Microsoft-Antispam-PRVS: x-checkrecipientrouted: true nodisclaimer: true x-ms-oob-tlc-oobclassifiers: OLM:9508;OLM:9508; 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SFS:(4636009)(136003)(376002)(396003)(39860400002)(346002)(46966005)(6512007)(53546011)(4326008)(2906002)(83380400001)(54906003)(6506007)(5660300002)(6486002)(82310400002)(26005)(82740400003)(8676002)(6862004)(70206006)(478600001)(356005)(47076004)(70586007)(316002)(336012)(36906005)(81166007)(186003)(2616005)(107886003)(36756003)(86362001)(33656002)(8936002); DIR:OUT; SFP:1101; X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 31 Jul 2020 14:37:03.0125 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: c0c3a65c-2c61-483d-18cb-08d8355f30db X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d; Ip=[63.35.35.123]; Helo=[64aa7808-outbound-1.mta.getcheckrecipient.com] X-MS-Exchange-CrossTenant-AuthSource: AM5EUR03FT060.eop-EUR03.prod.protection.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DB8PR08MB5482 X-BeenThere: xen-devel@lists.xenproject.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Cc: Stefano Stabellini , Julien Grall , Wei Liu , Andrew Cooper , Ian Jackson , George Dunlap , Xen-devel , nd , Volodymyr Babchuk , =?iso-8859-1?Q?Roger_Pau_Monn=E9?= Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" > On 31 Jul 2020, at 12:18, Jan Beulich wrote: >=20 > On 31.07.2020 12:12, Julien Grall wrote: >> On 31/07/2020 07:39, Jan Beulich wrote: >>> We're fixing other issues without breaking the ABI. Where's the >>> problem of backporting the kernel side change (which I anticipate >>> to not be overly involved)? >> This means you can't take advantage of the runstage on existing Linux=20 >> without any modification. >>=20 >>> If the plan remains to be to make an ABI breaking change, >>=20 >> For a theoritical PoV, this is a ABI breakage. However, I fail to see=20 >> how the restrictions added would affect OSes at least on Arm. >=20 > "OSes" covering what? Just Linux? >=20 >> In particular, you can't change the VA -> PA on Arm without going=20 >> through an invalid mapping. So I wouldn't expect this to happen for the= =20 >> runstate. >>=20 >> The only part that *may* be an issue is if the guest is registering the= =20 >> runstate with an initially invalid VA. Although, I have yet to see that= =20 >> in practice. Maybe you know? >=20 > I'm unaware of any such use, but this means close to nothing. >=20 >>> then I >>> think this will need an explicit vote. >>=20 >> I was under the impression that the two Arm maintainers (Stefano and I)= =20 >> already agreed with the approach here. Therefore, given the ABI breakage= =20 >> is only affecting Arm, why would we need a vote? >=20 > The problem here is of conceptual nature: You're planning to > make the behavior of a common hypercall diverge between > architectures, and in a retroactive fashion. Imo that's nothing > we should do even for new hypercalls, if _at all_ avoidable. If > we allow this here, we'll have a precedent that people later > may (and based on my experience will, sooner or later) reference, > to get their own change justified. After a discussion with Jan, he is proposing to have a guest config setting= to turn on or off the translation of the address during the hypercall and add = a global Xen command line parameter to set the global default behaviour.=20 With this was done on arm could be done on x86 and the current behaviour would be kept by default but possible to modify by configuration. @Jan: please correct me if i said something wrong @others: what is your view on this solution ? Bertrand