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Received-SPF: SoftFail (protection.outlook.com: domain of transitioning amd.com discourages use of 149.199.90.133 as permitted sender) Received: from satlexmb07.amd.com (149.199.90.133) by BN3PEPF00022BC6.mail.protection.outlook.com (10.167.248.219) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.360.3 via Frontend Transport; Wed, 26 Aug 2026 23:06:01 +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 15.2.2562.45; Wed, 26 Aug 2026 18:06:00 -0500 Message-ID: <776393b0-f35d-4b08-9089-8599eb3fe514@amd.com> Date: Wed, 26 Aug 2026 18:05:59 -0500 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 02/10] cpu/bugs: Allow forcing Automatic IBRS with SNP active using spectre_v2=eibrs To: Pawan Gupta CC: , , , , Sean Christopherson , Paolo Bonzini , K Prateek Nayak , Nikunj A Dadhania , Tom Lendacky , Michael Roth , "Borislav Petkov" , Borislav Petkov , Naveen Rao , David Kaplan , Dave Hansen , , kernel test robot References: <20260804235611.4053375-1-kim.phillips@amd.com> <20260804235611.4053375-3-kim.phillips@amd.com> Content-Language: en-US From: Kim Phillips In-Reply-To: Content-Type: text/plain; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: sK18jK6AOui4utiSpalzDAZLsdWcKHHcXiV1jHmEIV9khjJfaETkalhmYuA+gaYgvFoU2GzJ2mUFpLkF6ZNdXLwlI1x+lGXTj2+3jYKjEdytppOXy+MoQ0mw03bgNTZxI1yUJGWA+L2Zf9Z0XY1pKedjioQem7clFJmdlQ9f5aQk1JX52kKApjluXzSxIFRUjtA9RfzfQ327mXaK+Xw/NqHgFA3W409pYNDln6v/BatVB5eCJNTrmJZbfhiyj6iA5LXZjDRlKznblx++B40t0nwxNav0X5MkODcJhyqNu7beRVz1dAeLylv2cugfxfltOte9pKvtiRKjJ6Te2QAbyA5ujo7reUki8CqA01nX7C8hYP8N71ZSTxuKuNGkHXdbqAZFgyHqHxp49aNAxS+qz6JBRjIDTx4ZOs0L+0dyU00suPbNAIr9cn39xvQXOaU8 X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 26 Aug 2026 23:06:01.3739 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 0cd106c3-d234-41b1-c51c-08df03c69903 X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=3dd8961f-e488-4e60-8e11-a82d994e183d;Ip=[149.199.90.133];Helo=[satlexmb07.amd.com] X-MS-Exchange-CrossTenant-AuthSource: BN3PEPF00022BC6.namprd05.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DS0PR12MB7996 On 8/18/26 7:12 PM, Pawan Gupta wrote: > On Tue, Aug 04, 2026 at 06:56:03PM -0500, Kim Phillips wrote: > ... >> @@ -2192,7 +2193,12 @@ static void __init spectre_v2_select_mitigation(void) >> break; >> fallthrough; >> case SPECTRE_V2_CMD_FORCE: >> - if (boot_cpu_has(X86_FEATURE_IBRS_ENHANCED)) { >> + /* >> + * Don't use AutoIBRS when SNP is enabled because it degrades >> + * host userspace indirect branch performance. >> + */ >> + if (boot_cpu_has(X86_FEATURE_IBRS_ENHANCED) && >> + !boot_cpu_has(X86_FEATURE_SEV_SNP)) { >> spectre_v2_enabled = SPECTRE_V2_EIBRS; >> break; >> } > Can we also account for retpoline availability here? If both SNP and > retpoline are available, then only avoid AutoIBRS: > > case SPECTRE_V2_CMD_FORCE: > /* > * Prefer retpoline when SNP is enabled because Auto-IBRS > * degrades host userspace indirect branch performance. > */ > if (boot_cpu_has(X86_FEATURE_IBRS_ENHANCED) && > !(boot_cpu_has(X86_FEATURE_SEV_SNP) && > IS_ENABLED(CONFIG_MITIGATION_RETPOLINE))) { > spectre_v2_enabled = SPECTRE_V2_EIBRS; > break; > } > > With this, patch 1 & 3 can be omitted. Also avoids the ping pong: we don't > want AutoIBRS -> We don't have retpoline -> Lets have AutoIBRS. Thank you for your constructive review - I have addressed this comment in v5: https://lore.kernel.org/lkml/20260826223510.3669875-1-kim.phillips@amd.com/ Thanks, Kim