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Thu, 12 Mar 2026 15:50:20 -0700 Received: from Asurada-Nvidia (10.127.8.14) by mail.nvidia.com (10.126.190.180) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.2562.20 via Frontend Transport; Thu, 12 Mar 2026 15:50:20 -0700 Date: Thu, 12 Mar 2026 15:50:19 -0700 From: Nicolin Chen To: Cheng-Yang Chou CC: Pranjal Shrivastava , , , , , Subject: Re: [PATCH] iommu/arm-smmu-v3: Allocate cmdq_batch on the heap Message-ID: References: <20260311094444.3714302-1-yphbchou0911@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: X-NV-OnPremToCloud: ExternallySecured X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CH2PEPF0000009B:EE_|SJ2PR12MB7944:EE_ X-MS-Office365-Filtering-Correlation-Id: 23f9664c-a757-49de-38a8-08de8089c5d9 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|376014|82310400026|36860700016|56012099003|22082099003|18002099003; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: AtfE4WH6ZpDJWnxsLV8k94GGDWjoGHjTgo0Kvw0Risg4wwMHS9gNfT3z0pycfTcJp6FEFXCJBVuJ7Us8J2FHYMw0HXZg3EtO4We7vG4jJ1bv1oSi2nlhxMPF6Fo48vBN64BkiIuxnbmWtoXe3Y/nBbc0KjsT7V1tIrBM2eCf+Cl3ilJnjfRIBZcEVCb2tryWb4Llsaq9+km/0gtH2EB9K5FF/qpni7ejLrI0CdAJ3jEERww4gAMHfe+QQsl8SlN7MEmFNYGiJID7hJpSD15kcS0dqRFIdj3QAc3MIj3KFeZH/DKgB4d2MgtIfZ/bfw0MCkZid4RMugpkPXuDoS4NlfZCwBwHLE/A3GyXlu5CfNoh74tBLxK2icaHGSUQmF3XNtfLlG801TOdtZomntmr7zYBTuIeNg7ghmlBltnF3RPlXAtqXdoiFn32vTXYICh1 X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 12 Mar 2026 22:50:34.9974 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 23f9664c-a757-49de-38a8-08de8089c5d9 X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=43083d15-7273-40c1-b7db-39efd9ccc17a;Ip=[216.228.118.233];Helo=[mail.nvidia.com] X-MS-Exchange-CrossTenant-AuthSource: CH2PEPF0000009B.namprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: SJ2PR12MB7944 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260312_155040_127945_38F36DA6 X-CRM114-Status: GOOD ( 14.97 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Fri, Mar 13, 2026 at 02:24:09AM +0800, Cheng-Yang Chou wrote: > On Wed, Mar 11, 2026 at 02:22:50PM +0000, Pranjal Shrivastava wrote: > > IMO, if we really want to address these, instead of kmalloc, we could > > potentially consider some pre-allocated per-CPU buffers (that's a lot of > > additional book-keeping though) to keep the data off the stack or > > something similar following a simple rule: The fast path must be > > deterministic- no SLAB allocations and no introducing new failure points > To resolve the stack warnings, I'm considering using per-CPU buffers in v2. > Does this direction sound reasonable, or would you prefer to keep it as-is > to avoid the added complexity? I don't think per-CPU buffers would work here either.. arm_smmu_atc_inv_master() is used in a preemptible context, while arm_smmu_atc_inv_domain() can be called from an irq context. Think of a !SMP case for simplification: we only have one per-CPU buffer, which is not enough if an IRQ preempts the task context. Maybe having a smaller backup array on the stack that can be used when the heap allocation fails? Still, I don't see how to address it elegantly without losing some of the performance optimization. Nicolin