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Peter Anvin" , Pu Wen , Paolo Bonzini , Sean Christopherson , "Peter Zijlstra" , Yazen Ghannam , Brijesh Singh , Huang Rui , "Andy Lutomirski" , Kim Phillips , References: <20211001133349.9825-1-jane.malalane@citrix.com> From: Andrew Cooper Subject: Re: [PATCH] x86/cpu: Fix migration safety with X86_BUG_NULL_SEL Message-ID: Date: Wed, 6 Oct 2021 15:15:51 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.14.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Content-Language: en-GB X-ClientProxiedBy: LO2P265CA0390.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:f::18) To BYAPR03MB3623.namprd03.prod.outlook.com (2603:10b6:a02:aa::12) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: aea9918c-bded-4736-469e-08d988d3d258 X-MS-TrafficTypeDiagnostic: SJ0PR03MB5438: X-MS-Exchange-Transport-Forked: True X-Microsoft-Antispam-PRVS: X-MS-Oob-TLC-OOBClassifiers: OLM:9508; X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?utf-8?B?QzM3azdKKzNYckV6djZXNU03TzBETnlhWTBWYjNpZXRjY2dqb0tnWTFVTTV2?= =?utf-8?B?cUVTanhHaWFqVy9UVjNYZHYvdFhtc2JrUHN0VEJMekVuNEhZamhEN1p3N2Uz?= =?utf-8?B?cnFCeS9tbS9lR3d1TlJMaDR2Tm5pRnFzOWpTTGVzeUR6NGZkM214bWdCV3pR?= =?utf-8?B?elYwM1BOSVMyRk1CaHlsVitUcUk2MGZqT1NyNWE5S2JYNmh3QzJON0NQQ0Mw?= =?utf-8?B?cWxzKzVUSVlkWjY3VWhuejRadWNKeTdsYm90bVR4R0hHekptMGQrbVMyU0to?= =?utf-8?B?cnZIcGNNQy9TVWRVNTRDTUZVbDRjYnJZbTdlR05RQk1nTlBsZFNwVzhBUkJX?= =?utf-8?B?YlBacy84TVl3Wks2UDg0K0ovZUMvYTRIR29IWVpoSnRLKzRkZVVxaWVIUFdp?= =?utf-8?B?dzdqMGR0WUFGSkhwcU9kaGRsQVpxWXlDWEc0RUtyV3lpcUxOa3NnbDNibVRZ?= =?utf-8?B?UTV2MndFWUdJcldVUm05bFErNk5KOTRaVmVpVHZUei9FNysrRERJbGhsMEQ0?= =?utf-8?B?N2ZMQjdUa1Z5MjJNTG9qTVlVeGZmWDJZTEVQT1lXM3NxTlB3UHUzc25iZ0gz?= =?utf-8?B?QisxVndsM3ZRMkNRVTFWMjFpcldhUkVDQnRaK29rQmkxSFRHZDRzZFpKL2p0?= =?utf-8?B?NnI3ZlkvNVowa3JoLyszN3JLdkplV2c4SGxDbHUwS3BoRkEwVGhzZ1lJeHEw?= =?utf-8?B?QmlzYkpHekdqcGtvS0xTUzR6ODZkbkdycDRob2kwVXJUVytpL1ZNcjhIdVFJ?= =?utf-8?B?dlVFME5ySHV1K0RIRmJmYXh2UHJIMTRGdjIrT1hBMUtxY0xjVTR3U0VMWFpn?= =?utf-8?B?THNTSmNHNnBwYmM3TnlERWljYmtPK0lhRzlCMnhNZE43SjBsT1BiTy9oaHRV?= =?utf-8?B?OVJHS0JwaDRQR1oxMzJsbGRNU2ZTN1hqWXRyNnRnTUNrMHpKMVVxQm5RMjVM?= =?utf-8?B?MDdudjdCV2cxZUo2NU1tbmo4MU1Ib25NRDYwU1NsNnJuVi8zWC9aaEhWOW45?= =?utf-8?B?WWQ5TWs0YmJKbUZxTW1taUZoS3FqTmlxd2pjcnR1SGdzd09uQUNEYzAvTXBN?= =?utf-8?B?RFNmbms5WGRZQVI1MzFyRWFVY1VZcDBqNWExRURZck1CbkpXRXZBWDBWakNU?= =?utf-8?B?YUJRWU9WZ0tjU2ZEUGhoY0tIT2V2SmxLTUtkbGFkRFZlWUhwUGVlOTN4RHZt?= =?utf-8?B?NjFlTHhLbURRelplMGxqQTNoaFNoTFljQlNRWUNsUXpXNmtnSlR5Nnp1SXRE?= =?utf-8?B?c0NCQ0llM0R2bS9aVXVwam9YS1pIL2VHQWxjQUxuQ2NvdEp4cFBPTlRPZTVM?= =?utf-8?B?d1hsc1ppa0xpWXowVExIUmFnVXcrSnYzZFFhT3A3OEtDTEgvYlBWK0pjc3RZ?= =?utf-8?B?djBzc2J3cFdlUGxKSXpURmVVaWtubGo1OU1EN0NkNm9kRG5aMS9YRnJ5V3FK?= =?utf-8?B?bE5waTlXSlMzbFdlL2FWTFB5WlJIMjNPd1h2WW1tRFQvSmU5dlhpZ2d2akor?= =?utf-8?B?K1o5Wk5hVnlRdzRpWEtpTkdNTFBrWXIyUWNXM1ozWXBaN3NBMkg4Ym1lK2sz?= =?utf-8?B?UTllKzl5dkwvcTVMSW1Ea2o3dUJaWUNUSCtiUkxCR0ZWVGlUdHQ1WWgyVWZh?= =?utf-8?B?M3FoUUFmSUlIekVlUE1SZVhtU1FiTTc2QjZhSi80ZFdXR1NsbSs3ZVlqQTMx?= =?utf-8?B?dG1CSTNjNG8rTzU3LzNLeFZveFllNEJFc0V0UWcvWjluUnFvRVpXOE9TQm9C?= =?utf-8?Q?tcZgKTxTnD/thOGnwvRD+PXTKVB7C68YgckqTDR?= X-MS-Exchange-CrossTenant-Network-Message-Id: aea9918c-bded-4736-469e-08d988d3d258 X-MS-Exchange-CrossTenant-AuthSource: BYAPR03MB3623.namprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 06 Oct 2021 14:15:59.9522 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 335836de-42ef-43a2-b145-348c2ee9ca5b X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: ocFCqUlpQ/xidiuPKT+lRvIFy27JINpka9Qdvn9SRXS74WaCFqZv+iLmJImU68ty0/wH/FVFmueX6uQw6h5EoYTOfh7WxHLU7n7/6A7s4ZE= X-MS-Exchange-Transport-CrossTenantHeadersStamped: SJ0PR03MB5438 X-OriginatorOrg: citrix.com Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/10/2021 15:19, Borislav Petkov wrote: > On Fri, Oct 01, 2021 at 02:33:49PM +0100, Jane Malalane wrote: >> Subject: Re: [PATCH] x86/cpu: Fix migration safety with X86_BUG_NULL_SEL >> ... >> Currently, Linux probes for X86_BUG_NULL_SEL unconditionally which >> makes it unsafe to migrate in a virtualised environment as the >> properties across the migration pool might differ. > Sorry but you need to explain "migration safety" in greater detail - > we're not all virtualizers. The case which goes wrong is this: 1. Zen1 (or earlier) and Zen2 (or later) in a migration pool 2. Linux boots on Zen2, probes and finds the absence of X86_BUG_NULL_SEL 3. Linux is then migrated to Zen1 Linux is now running on a X86_BUG_NULL_SEL-impacted CPU while believing that the bug is fixed. The only way to address the problem is to fully trust the "no longer affected" CPUID bit when virtualised, because in the above case it would be clear deliberately to indicate the fact "you might migrate to somewhere which really is affected". ~Andrew