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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?KwkmT+HL7E6uF/gsg0/QSkWUd/9Uaqla/Xe+rUik0dYPKv73AzWoxNZGeT/V?= =?us-ascii?Q?hcQ2cTVcSGfh+Xaidy27RKytjgnPHszh4S8i50vALsaGG+CpNSASKyOe91St?= =?us-ascii?Q?nFwa+nMKVm1xW7BPYCpd2HMfmu0xlgdXT9BzDMUYcc3nGBglIO+L39zkB46J?= =?us-ascii?Q?LLr937Gq7VUV2XX/1WI0Tf0y5mpl4psoZ85nOXxw6bhZ6XssrD4CuUhdOgk/?= =?us-ascii?Q?DsTVvxKt71AXetsQl9z8DHyGbsvV80x0NFTUgRZW3KPhnPEEesYbYiKSEsGT?= =?us-ascii?Q?cTHLue3uRwgSaSVOw3+dggWg8fIbvR+/dvzuFqjJqmwSwZIV1n4dhBsl99iN?= =?us-ascii?Q?bYhXEdKJvHoXeXClsqwzexfZqjJXeDy37wWoPIBlyL8/eWkvyrbt4arYHeO0?= =?us-ascii?Q?j+csk5Cfzk0qyA1cfKQFMxdsAwc5f5To7DCeJwDCQboO7GA+TfknY5OjcjZs?= =?us-ascii?Q?bKbZzPFH0IuxfACkxgR4PheF6JeNw34YnvoRS4lmqeElf0z1vU6SFuAtu+Y+?= =?us-ascii?Q?yp4rOX2zIbTeOubMxLUafXhr/5tFIYsPVmTE/hCa3IuHfMIvUkEcd7oBoVEy?= =?us-ascii?Q?CZ8TXeB1JQ+MXkeqhRQVn794tU7hNTRObnNHq2u4Gj+Dpn3SUKS4aIqHUKJ1?= =?us-ascii?Q?gQ/c+UBBLp1xVfh9Py96q0v2x+Tzf0wS29TPR1nGktSsMqkv1AnNtqcz6gEA?= =?us-ascii?Q?/4MUYED9ad0plI0UKOcsEk6grOVYGV5Dr2mc9MbZIF96Tza3YuMorrsKTctQ?= =?us-ascii?Q?mWAlO0vh2qPRkvFM0c0NOv/4Na2Xw8RfJqhOVdt20Rp+dqnrnAx1kL/NNQss?= =?us-ascii?Q?VO1/hdGu2T/mK8Dlq3eESQlDgkDeYOa7WqsA7787Z2x9p+szFrYdUDNXqcJ/?= =?us-ascii?Q?YiUkRdaItsQYstOWQKZUyxMMUGbQHr3eccDa+91NpfFQNGqFsBD63lU3dfwF?= =?us-ascii?Q?78oJKvGs+YmhaCbENlFQa3Ha1qiAZUrYtnycutXFf5D7ThlSHe6Xd2I6+JjZ?= =?us-ascii?Q?0Wh2IeNShmTrfGlQBNbjFjGw8a5btoMXDM1t1etcRmRGpBwuPUmIxpGo5n62?= =?us-ascii?Q?zcce91OIJ93k9/N2Vd9QAMsDXHHt8bGmUs6aWxRmXZda88kjuXnwPpA+ZI3u?= =?us-ascii?Q?C4ykURjiYHSBuyofOgJuyd58Z4KaCCfdjMBzbQV5sg3HN2zBtyp1zGnI/hRB?= =?us-ascii?Q?Sbq7zob2Vuq6cuNtDcksZ8YoQd+OEDvmCLH1+WNJuQXOgYCNPE7/kXTQfx8L?= =?us-ascii?Q?9j07fazmTfXSRdOlxknSJ8X4KOB8tzqyfQSMoSsYVPkKs5oyC7EcQVLdI72T?= =?us-ascii?Q?0RPbEta7VW04W4aGItnqksBeYg9g5dPIP3+r3tBOERgN6E5qQ15u5DlyOp0L?= =?us-ascii?Q?61W41AsYKZ0H6gfWE66tSS3ya1UuM6TiLH8MwDB/55ebIPYgAsC6EfjfTFp6?= =?us-ascii?Q?5oRajJihG1LL6NaP/4EaHPZ+1w1VU6AAp1FzG5KJl2hRpoAsXKuhuOe7R2FV?= =?us-ascii?Q?oDpGXfpgkSos6sMYxGLcgHIQvxdKoE7pItnA5SkT8oqKCDNUyHyXNtefihwQ?= =?us-ascii?Q?SynPp/fR3n3FsmL+wwlLTQh+Dl4DyHLZLck2bFp8fzC+1PkBhgrP2e7a7lDe?= =?us-ascii?Q?wVTTpeV6dwpCHKHxFft9ukE9HOjAD6xHgZO0UODAY8ZMZ6x8N0KwD7U2EJZA?= =?us-ascii?Q?dPWirV7taR9p11rNsH1W7BxdBjSS1qyxCRdH2E7yVQEkX2j7wiU/dY6hF+yK?= =?us-ascii?Q?+62ThuteMg=3D=3D?= X-OriginatorOrg: suse.com X-MS-Exchange-CrossTenant-Network-Message-Id: 0716f269-a048-48a9-2c9c-08da3c940a96 X-MS-Exchange-CrossTenant-AuthSource: VE1PR04MB6560.eurprd04.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 23 May 2022 08:12:55.8192 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: f7a17af6-1c5c-4a36-aa8b-f5be247aa4ba X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: e5oyvoRMnStgqVUmPFDgBTEIQ2+26j+YIoz3Urecdzn5eRCN7gevqg1Kmk4HXrDRBvaRziBaaD/WFHf7eeKRfQ== X-MS-Exchange-Transport-CrossTenantHeadersStamped: AM0PR04MB6868 On 20.05.2022 16:58, Andrew Cooper wrote: > On 20/05/2022 15:19, Jan Beulich wrote: >> On 20.05.2022 16:10, Andrew Cooper wrote: >>> On 20/05/2022 14:37, Roger Pau Monne wrote: >>>> --- a/xen/include/public/arch-x86/cpufeatureset.h >>>> +++ b/xen/include/public/arch-x86/cpufeatureset.h >>>> @@ -135,7 +135,7 @@ XEN_CPUFEATURE(SSSE3, 1*32+ 9) /*A Supple= mental Streaming SIMD Extensio >>>> XEN_CPUFEATURE(FMA, 1*32+12) /*A Fused Multiply Add */ >>>> XEN_CPUFEATURE(CX16, 1*32+13) /*A CMPXCHG16B */ >>>> XEN_CPUFEATURE(XTPR, 1*32+14) /* Send Task Priority Messag= es */ >>>> -XEN_CPUFEATURE(PDCM, 1*32+15) /* Perf/Debug Capability MSR= */ >>>> +XEN_CPUFEATURE(PDCM, 1*32+15) /*S Perf/Debug Capability MSR= */ >>> This is the bit which requires more toolstack logic to safely enable.= =C2=A0 >>> Using 's' for off-by-default is fine if we want to get the series in no= w. >>> >>> But before we expose the MSR generally, we need to: >>> >>> 1) Put the configuration in msr_policy so the toolstack can reason abou= t it >>> 2) Reject migration attempts to destinations where the LBR format chang= es >> Since this could be quite restrictive, and since people needing to know >> they need to hide this feature for migration to work, I guess this would >> further want qualifying by "did the guest actually use LBRs so far"? >=20 > In practice, it's every major generation ("tock" on Intel's old model), > so isn't actually limiting the kinds of heterogeneous setups used in > production.=C2=A0 (Migration gets steadily less stable the further apart = the > two CPUs are.) >=20 > As to dynamic, no - that would be a security bug in a cloud scenario, > because there must not be anything the guest can do to interfere with > the manageability. >=20 > Use of LBR is rare, as demonstrated by the fact that noone has > complained about the fact that migrating such a VM will malfunction. >=20 > As we now have a way of reporting "no model-specific LBR", Which only rather new guest kernels will know to look for. Hence ... > I'm tempted > to suggest that VMs get no LBR by default, and someone wanting LBR has > to opt in, which is also an explicit agreement to the migration limitatio= n. ... while in principle I agree with this, I see a practical issue. Jan