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Tue, 25 Aug 2026 09:10:49 -0500 Message-ID: <4b253fa1-1d2e-49e4-9d1a-ea4ff81d4486@amd.com> Date: Tue, 25 Aug 2026 10:10:48 -0400 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 v2 1/2] x86/amd_node: Avoid divide by zero on virtualized systems To: Borislav Petkov CC: Dave Hansen , "H. Peter Anvin" , Ingo Molnar , Mario Limonciello , Thomas Gleixner , , Yazen Ghannam , , Penny Zheng , References: <20260814214255.83127-1-jason.andryuk@amd.com> <20260814214255.83127-2-jason.andryuk@amd.com> <20260825040039.GAao0TZ6b8j0kp2RKk@fat_crate.local> Content-Language: en-US From: Jason Andryuk In-Reply-To: <20260825040039.GAao0TZ6b8j0kp2RKk@fat_crate.local> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: BN3PEPF00022BDA:EE_|BL1PR12MB5970:EE_ X-MS-Office365-Filtering-Correlation-Id: 318eba26-517d-439e-c1fa-08df02b2ad0c X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|23010399003|82310400026|1800799024|36860700016|376014|56012099006|10067099003|3023799007|18002099003|22082099003|11063799006|4143699003; X-Microsoft-Antispam-Message-Info: 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 X-Forefront-Antispam-Report: CIP:149.199.90.133;CTRY:US;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:satlexmb07.amd.com;PTR:unknown-90-133.xilinx.com;CAT:NONE;SFS:(13230040)(23010399003)(82310400026)(1800799024)(36860700016)(376014)(56012099006)(10067099003)(3023799007)(18002099003)(22082099003)(11063799006)(4143699003);DIR:OUT;SFP:1101; X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: jU648oDu4BHR8qOE+rzQFLZMw/N0BmOrbf2Vwyl/Df0UquTC22b4rMPDIH0tI7j8SObiacRmVDFl1eND88qTH0SCZzdLulIz3XpYg9D94fmSJMmUMm7eO8gzrtk8A5kvQvLZ4Z1MBWuGlBRZlOsWj3Yu9uL0PTdTq1sNeS8C7s6bFTEUlFl1FD0ZBwbVm6/nbkx/ZaO7H2LgRH+FZOd/nh+BJwq2Jr9gIGW85VNy3IpZ62zCAC2SiJYu6jQzMhrHL7k6QmCVz3y3+Yq4R1M1YbDUcaWP5gJZ37GGSTL6tG5tAC/3wPAXY1bN1WdrWWTCXwATpz8ZUoY02AhbNEhwuVpy7PzYGFCQWPQCI7R1Awe6UrZZBS3ppidwtWA5AJse+iyX13xcInX10+qpk01oTrZqYqTJ6Affo55s8TtXdKFYN1/lWrHY0+S/o1HbaTZG X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 25 Aug 2026 14:10:53.8837 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 318eba26-517d-439e-c1fa-08df02b2ad0c 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: BN3PEPF00022BDA.namprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: BL1PR12MB5970 On 2026-08-25 00:00, Borislav Petkov wrote: > On Fri, Aug 14, 2026 at 05:42:52PM -0400, Jason Andryuk wrote: >> On a virtualized system, the number of nodes does not have a >> relationship to the number of roots. A Xen PVH dom0 can calculate >> roots_per_node as 0, which crashes with a divide by zero in: >> >> if (count++ % roots_per_node) >> >> The issue is seen with Xen, but it could affect other systems. >> >> Change roots_per_node == 0 to 1 to allow the system to boot. Print a >> warning when this is performed for non-virtualized systems. >> >> Fixes: 0a4b61d9c2e4 ("x86/amd_node: Fix AMD root device caching") >> Cc: stable@vger.kernel.org >> Suggested-by: Borislav Petkov >> Signed-off-by: Jason Andryuk >> --- >> v2: >> Print as a warning instead of error >> Use "!" instead of "== 0". >> >> X86_FEATURE_XENPV is only for PV, but this is observed with a PVH dom0. >> --- >> arch/x86/kernel/amd_node.c | 5 +++++ >> 1 file changed, 5 insertions(+) >> >> diff --git a/arch/x86/kernel/amd_node.c b/arch/x86/kernel/amd_node.c >> index 0be01725a2a4..c3e214925d9c 100644 >> --- a/arch/x86/kernel/amd_node.c >> +++ b/arch/x86/kernel/amd_node.c >> @@ -287,6 +287,11 @@ static int __init amd_smn_init(void) >> return -ENOMEM; >> >> roots_per_node = num_roots / num_nodes; >> + if (!roots_per_node) { >> + if (!cpu_feature_enabled(X86_FEATURE_HYPERVISOR)) >> + pr_warn(FW_BUG "Error detecting roots per node."); >> + roots_per_node = 1; >> + } >> >> count = 0; >> node = 0; >> -- > > https://sashiko.dev/#/patchset/20260814214255.83127-1-jason.andryuk%40amd.com > > And this one specifically (the other two are being addressed): > > "Are we missing a newline character at the end of this warning message? > Without a trailing \n in the format string, subsequent kernel messages might > be appended to the same line, resulting in malformed dmesg output." Yes, thanks. I'll fix. Regards, Jason