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Received-SPF: Pass (protection.outlook.com: domain of amd.com designates 165.204.84.17 as permitted sender) receiver=protection.outlook.com; client-ip=165.204.84.17; helo=satlexmb07.amd.com; pr=C Received: from satlexmb07.amd.com (165.204.84.17) by SJ1PEPF00002316.mail.protection.outlook.com (10.167.242.170) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.451.8 via Frontend Transport; Thu, 24 Sep 2026 18:45:56 +0000 Received: from [10.236.30.85] (10.180.168.240) by satlexmb07.amd.com (10.181.42.216) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.2562.49; Thu, 24 Sep 2026 13:45:53 -0500 Message-ID: <67fb182c-4baa-4b3e-b59c-64cec20839c2@amd.com> Date: Thu, 24 Sep 2026 13:45:52 -0500 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 1/8] x86/bugs: Allow forcing Automatic IBRS with SNP active using spectre_v2=eibrs To: Borislav Petkov CC: , , , , Sean Christopherson , Paolo Bonzini , K Prateek Nayak , Nikunj A Dadhania , Tom Lendacky , Michael Roth , Naveen Rao , David Kaplan , Pawan Gupta , Dave Hansen , Nathan Fontenot References: <20260826223510.3669875-1-kim.phillips@amd.com> <20260826223510.3669875-2-kim.phillips@amd.com> <20260903040330.GNapjxkgMd2Ey38egR@fat_crate.local> <01b57e61-df45-44a6-848b-c19eae226ad0@amd.com> <20260924014517.GEarSArVM12m_ZshhA@fat_crate.local> Content-Language: en-US From: Kim Phillips In-Reply-To: <20260924014517.GEarSArVM12m_ZshhA@fat_crate.local> Content-Type: text/plain; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: bzE5xzBoAK0Zv9C1OeZorTXfjDyPIU1At/1baXsVkXIdp6V8EBbIY+mFPoyX3vV1o02jEhD6IIyb+WKk6DMsjz1Lch6h4VJlM59uoVlMGgTX1fCwBGxdTci28v5JM9lJROkSR6b/u4zloUyrJmOB/k4418ZGH+hqr8UdRk4Qcunsrx3lk9Cgi3nTyHQ40STOP/IeeeODRSVXg9zK/juf/+YaZjzMLTV+72b7x4yeVCqz6MKsvw+b2HfWZO159gl6TdwqDF5PfCer3VmMNZ69NuVg/P3pGGRKiEiIgXpjl2jrQ/BHaGI96xXDlxPXeo/a9/MS7cYkqhbXlaRmD5DnuZESBkLFVvsSpwc+6FmjoOoYnMBpV3Out8pXwZPeukXUYUZBh978Lh+7R+cTysDaShwY1f4Bm08rjm26f0yNTyPRibMFH0pjGpf5cD6MLqhk X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 24 Sep 2026 18:45:56.0724 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 5a3f8b25-a6dd-4081-d801-08df1a6c117b X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=3dd8961f-e488-4e60-8e11-a82d994e183d;Ip=[165.204.84.17];Helo=[satlexmb07.amd.com] X-MS-Exchange-CrossTenant-AuthSource: SJ1PEPF00002316.namprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: SA1PR12MB6798 On 9/23/26 8:45 PM, Borislav Petkov wrote: > On Fri, Sep 18, 2026 at 06:00:21PM -0500, Kim Phillips wrote: >> That fallback cannot be overridden.  An admin running an SNP host whose >> workload is dominated by guest activity rather than host userspace may >> well prefer AutoIBRS > Why would she/he prefer to do that? Any persuasive arguments to make her/him > do that? On the same silicon with SNP off, the kernel's own default is AutoIBRS, not retpoline. Enabling SNP doesn't make retpoline the better mitigation; it just adds a host-userspace cost to AutoIBRS. So the admin is being pushed off the recommended hardware mitigation onto a software one, with no say in it. What each side actually costs: retpoline taxes every indirect call in the kernel, and a virtualization host lives in the kernel - VM exit handling, networking, storage. AutoIBRS costs nothing on entry/exit and additionally covers indirect branches the kernel never compiled (EFI runtime services, prebuilt out-of-tree modules). Its penalty falls on host userspace, which on a dedicated SNP host is thin and mostly parked in KVM_RUN. For a guest-dominated host that trade plausibly goes the other way, and only the admin knows their deployment. Kim