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mx.microsoft.com 1; spf=pass smtp.mailfrom=citrix.com; dmarc=pass action=none header.from=citrix.com; dkim=pass header.d=citrix.com; arc=none DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=citrix.onmicrosoft.com; s=selector2-citrix-onmicrosoft-com; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck; bh=8Tqwm8Fs2ADZOHdM1i3IdulpDkSnIyXb4xED76D59oY=; b=phXpnzMDkcRtxZeG2KQrf1P3vuO8yw3qLI0DwqIWQa8D/7H9N2kPmNMQZgEjU2JwYuXZ00Hu9kSz3GmpqlZp/4T/gQpgRMMFoZfVNsGt2Bs2c9k+sP4nDru72eyn26wLRHQhBdbHMMjgofa8R7lCYeWMCrjd4ws8tw06w++RqLQ= To: Jan Beulich CC: =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= , Wei Liu , Xen-devel References: <20210426175421.30497-1-andrew.cooper3@citrix.com> <20210426175421.30497-2-andrew.cooper3@citrix.com> <86cf1d97-2034-7791-071a-48208b6ba54b@citrix.com> <28d7b7a9-9dd2-1664-e946-d7e4a330da0f@suse.com> From: Andrew Cooper Subject: Re: [PATCH 1/3] x86/hvm: Introduce experimental guest CET support Message-ID: <3d1847f2-4ba6-416a-7957-3310b5d5124c@citrix.com> Date: Wed, 28 Apr 2021 18:54:22 +0100 User-Agent: Mozilla/5.0 (X11; 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X-MS-Exchange-AntiSpam-MessageData: =?utf-8?B?N09oL3JkQ2lMQU1naS81bDZWc1UyaGpOdXAxdGt3L05KOTJkWU9pSjJSRjVi?= =?utf-8?B?MHBWYURCcEo4ajdPUHRMN1hJSGNSRVF5UlVRWW5PdEhlUEZtMTdNMXVQNFRN?= =?utf-8?B?bm53Y2NjUXFwZjZUL0Z2NFY1cjJZUUwyblpDa1Iydzl2UFA4S0VzOWpnUHZ5?= =?utf-8?B?QkR6SnNTbUVLL1Vza0hFdGdZRDdFSExBN25wMDhmejBQVnVZUTIwVmZSVWpi?= =?utf-8?B?eGVzejJEdTBpOWg3UHJpTUN0TUVFRjdMU1BLU240OWJjUFJIMDVhdkF2eWZk?= =?utf-8?B?UHVQOGU0aVhUSGRNNXBHL1hjYnRtcFhuUm9odE9TcVlkUFJLa0Y5dUIydW5j?= =?utf-8?B?dVBLSmY0bkxwT1UvQyt3c2hLejF5Mk9wVlk0MEZYYTdUU3hPRmlyZGxxdFFn?= =?utf-8?B?SzNoR3J2T3ZiZWFXcE9TNGlaNWd5UVFIZ2NyeW1OekNKZEsrb3kveUZlMUJY?= =?utf-8?B?UkZrZkZuU250K0V0bFIrSFZZNVYzZHBxQUFkUXN4RlVkeXhPMmthaFgwc0Rt?= =?utf-8?B?dk0rUjBoUnh3c29GRFl1Y3JUc2dUOW9JbDdJMUNzcEFtSkVMUWxSYnpmMDd4?= =?utf-8?B?UTlCcDEvK1pWZWkrUkZkak5SbnZHZkRNNHk1L3hrUS80K0tyUElrSEpzaTVx?= =?utf-8?B?cEovdUU5Q3hiTFh6Tk1zODNua0VXdXVDMGZnSmZ6RTlhbHpXdElDSzBTRVM5?= =?utf-8?B?K1pNcVVra3MzYndDOFUvMkhUMHA2TFlCRmoyY1dES2YxVXRuTFpJNUw0L1Yz?= =?utf-8?B?UmJoelloK3pQMEZNbEcvK0pSUXorWkNsRlRWczBVR0MvTjU4WWg4eHdvY2RQ?= =?utf-8?B?ZnQ3YnBVOFFoQnhleUNJbmQrWXpGQktuNFZuWHRCYnQ5L1V5cmxYTUY1UjB0?= =?utf-8?B?MjkzSjNiamlPaEEvd1FnM1lNcFR3Y1NwSFJOaXR6em5leTZORHN2T0d1NlNI?= =?utf-8?B?bk5tZHY5SnBWN1ZUbnFCdyttSE5OczRRc25lWUlqNlpidmVpa25QRVlqb3o5?= =?utf-8?B?TlVBNXpxOGwrYnZKU2wwR3lsOUhlZXRMbzQzUEV1aVdQSk14T1lUcTZOaUxh?= =?utf-8?B?NHJQZ2lnVExoR2lVdUN3dDVPemoxVlUwclY2aE00czFzaDdTOU5NdDEzMDFz?= =?utf-8?B?aWxHUUp4TUl1bFJla0ZVOVYzRmtyMWgxK25CbVR5OHV1VFJ1ckxUUmVuYlFP?= =?utf-8?B?dFB1MS9TdGwxditISHlzNWhIZlN5VU1LYllFK0x2VFB6TW4yOG52bnRVUm5o?= =?utf-8?B?amlPcGUyajcvZFVMdFZqWmxOVWpIaTVLZllTSFZxV1oyTUd5WEZ1V3hiMmlK?= =?utf-8?B?SzZjUUQ1MGNhcXptVmdSODh2dit0b0NTdjZIVjZqUjAyTFVpWnpvWDZMMlUx?= =?utf-8?B?WlVsSDVyR0RGenpQeVVJNGhJcUpqY0R6cmhxTW5CVUpoWlczZzNpOTBSRFph?= =?utf-8?B?dGRkMi9udHhaYUZlWjdoZkdlK0wrRU5DaDVRM294OVRSRVlxNUQvRWJLVktW?= =?utf-8?B?WEd2b01GeXNONXNCZnF4aFREbTI3cnBxTmRVTys5Z3pWNkd1YVhLU0RPQTNr?= =?utf-8?B?NDRqVHVobXo2SFA2ZkZnK3ZwWXVpUXU1VytMdDFDUktRbmZsUGlFdDdGUnFO?= =?utf-8?B?Mk1pRG1pMFhVMFFxNmVOajd2WTFhT1VwNlZ2ZS9ySlRDRFJrWWszU2pPVUJG?= =?utf-8?B?ZVQ5VVBQZS8wY094ZUVIcnNBTUg2Z0VPVHcwYmVlc3hMZW5IUUhaZWJXSHBP?= =?utf-8?Q?BDETPGZ53xr5LCHeAAdWyYTSpVe+UMBwjlEpq36?= X-MS-Exchange-CrossTenant-Network-Message-Id: 63627e6a-3438-47b8-ec3e-08d90a6eabc5 X-MS-Exchange-CrossTenant-AuthSource: BYAPR03MB3623.namprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 28 Apr 2021 17:54:29.5650 (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: oXfeVVjKN7aAXAP5gdqDOeGDGqjCh6sol/wXoL7mRSK2+NsLcxkdV4t3AT6yAm8WYikQF81VaQxy6/r9rvhvRu9jh2dCTB5YU9i6hP9Dqnk= X-MS-Exchange-Transport-CrossTenantHeadersStamped: BY5PR03MB5029 X-OriginatorOrg: citrix.com On 28/04/2021 10:11, Jan Beulich wrote: > On 27.04.2021 19:39, Andrew Cooper wrote: >> On 27/04/2021 16:47, Jan Beulich wrote: >>> On 26.04.2021 19:54, Andrew Cooper wrote: >>>> --- a/xen/include/public/arch-x86/cpufeatureset.h >>>> +++ b/xen/include/public/arch-x86/cpufeatureset.h >>>> @@ -232,7 +232,7 @@ XEN_CPUFEATURE(UMIP, 6*32+ 2) /*S User M= ode Instruction Prevention */ >>>> XEN_CPUFEATURE(PKU, 6*32+ 3) /*H Protection Keys for Users= pace */ >>>> XEN_CPUFEATURE(OSPKE, 6*32+ 4) /*! OS Protection Keys Enable= */ >>>> XEN_CPUFEATURE(AVX512_VBMI2, 6*32+ 6) /*A Additional AVX-512 Vector= Byte Manipulation Instrs */ >>>> -XEN_CPUFEATURE(CET_SS, 6*32+ 7) /* CET - Shadow Stacks */ >>>> +XEN_CPUFEATURE(CET_SS, 6*32+ 7) /*h CET - Shadow Stacks */ >>>> XEN_CPUFEATURE(GFNI, 6*32+ 8) /*A Galois Field Instrs */ >>>> XEN_CPUFEATURE(VAES, 6*32+ 9) /*A Vector AES Instrs */ >>>> XEN_CPUFEATURE(VPCLMULQDQ, 6*32+10) /*A Vector Carry-less Multipl= ication Instrs */ >>>> @@ -267,7 +267,7 @@ XEN_CPUFEATURE(SRBDS_CTRL, 9*32+ 9) /* MSR_MC= U_OPT_CTRL and RNGDS_MITG_DIS. >>>> XEN_CPUFEATURE(MD_CLEAR, 9*32+10) /*A VERW clears microarchitec= tural buffers */ >>>> XEN_CPUFEATURE(TSX_FORCE_ABORT, 9*32+13) /* MSR_TSX_FORCE_ABORT.RTM_A= BORT */ >>>> XEN_CPUFEATURE(SERIALIZE, 9*32+14) /*a SERIALIZE insn */ >>>> -XEN_CPUFEATURE(CET_IBT, 9*32+20) /* CET - Indirect Branch Tra= cking */ >>>> +XEN_CPUFEATURE(CET_IBT, 9*32+20) /*h CET - Indirect Branch Tra= cking */ >>>> XEN_CPUFEATURE(IBRSB, 9*32+26) /*A IBRS and IBPB support (us= ed by Intel) */ >>>> XEN_CPUFEATURE(STIBP, 9*32+27) /*A STIBP */ >>>> XEN_CPUFEATURE(L1D_FLUSH, 9*32+28) /*S MSR_FLUSH_CMD and L1D flu= sh. */ >>> If by the time 4.16 finishes up the various todo items haven't been >>> taken care of, should we take note to undo these markings? I would >>> have suggested allowing them for debug builds only, but that's kind >>> of ugly to achieve in a public header. >> TBH, I still don't think this should be a public header.=C2=A0 If I woul= d >> have forseen how much of a PITA is it, I would have argued harder >> against it. >> >> It is, at best, tools-only (via SYSCTL_get_featureset), and I don't >> intend that interface to survive the tools ABI stabilisation=C2=A0 work,= as >> it has been fully superseded by the cpu_policy interfaces and libx86. > Well - what features we expose is part of the (or at least something > like an) ABI. Yes, but that ABI is called CPUID and MSRs, per Intel/AMD's spec. VM's see it via the real instructions.=C2=A0 Control software sees it via a pair of arrays ({leaf, subleaf, a, b, c, d} and {idx, val}) expressed in terms of the vendors ABI. > The actual numbering of course isn't (or shouldn't be). > I'm in no way opposed to moving the header out of public/ (and until > now I was of the impression that it was you who put it there), but if > we do I think we will want an alternative way of expressing which > extended features we support. I say this in particular because I think > SUPPORT.md doesn't lend itself to documenting support status at this > level of granularity. I'd much rather see that file refer to this > header, even if this may mean some difficulty to non-programmers. We don't need to say or do anything for experimental features. Hitting Tech Preview and supported ought to be in release notes.=C2=A0 I do agree that SUPPORT.md isn't great. >> As for the max markings themselves, I'm not sure they ought to be >> debug-only.=C2=A0 Its important aspect of making guest support "tech pre= view" >> to ensure the logic works irrespective of CONFIG_DEBUG, and I think it >> is entirely fine for an experimental feature to be of status "your VM >> will explode if you enable this right now", even if that spills over >> into 4.17. > For a release I consider this undesirable. If a feature is in such a > state at the point of entering the RC phase, I think such markings > ought to be undone. Well no.=C2=A0 They either don't go in in the first place, or they go in an= d stay.=C2=A0 Putting them in with an expectation to take them out later is a recipe for forgetting to do so. I know we're making up our "how to do complicated experimental features" process as we go here, but nothing *in Xen* will malfunction if a user opts into CET_SS/IBT.=C2=A0 Therefore I think they're fine to go in and sta= y. ~Andrew