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Mon, 7 Sep 2026 14:57:05 -0700 Date: Mon, 7 Sep 2026 14:57:05 -0700 From: Nicolin Chen To: "Kiryl Shutsemau (Meta)" CC: Will Deacon , Robin Murphy , "Joerg Roedel" , Jason Gunthorpe , "Pranjal Shrivastava" , Mostafa Saleh , "Thierry Reding" , Krishna Reddy , Jonathan Hunter , Breno Leitao , Kyle McMartin , Usama Arif , , , , , Subject: Re: [PATCH v5 2/2] iommu/arm-smmu-v3: Default queue depths to one page in a kdump kernel Message-ID: References: <20260907095835.1233352-1-kas@kernel.org> <20260907095835.1233352-3-kas@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <20260907095835.1233352-3-kas@kernel.org> X-NV-OnPremToCloud: ExternallySecured X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SN1PEPF000252A2:EE_|SA1PR12MB6970:EE_ X-MS-Office365-Filtering-Correlation-Id: b66bff7c-c596-4d64-ba99-08df0d2b00ab X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|7416014|376014|36860700016|23010399003|82310400026|3023799007|10067099003|4143699003|56012099006|11063799006|18002099003|22082099003|6133799003; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: seZawaODw08vs1mgU2NXgT4Z/H3VtjvOojG1TJkkEg0vsfmKpvAlz71RWxJmkWur/8PODY8vUhbjs6JaVQ3RX2Q9Dzt9Krwc1EFEYcgEarJXtqouQd7EBU2GRbV72I6jN1aDDzLxm58M7B58mBgdZDqoFaUs/+HXMFKYpUmJpOrPKSgqvzYnqHT0ld3QYqLmfhmk4yBxL2gUE7wEc18adx1ySsCZFsYH+bMAk7obFUhqainuN/RXlUSi/OrXf8Doxb5NM/zYtJUhNNm/nDfZsw/oMU/UmrHEs3xoJVOLWBSFVS4zvUKt2Qdy7/yScJPl0wSNhkj/g2KqQrOQg9p3GabvKQbf9bBGvB7t+aVtIS/nEolD9zkzr/EGN+msNT0/lIpfK3fcd7KFVXAeekfsVfLbCcnvbOOjGa664OTigiq2OB86KWbh4N3oNWp1TCFg X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 07 Sep 2026 21:57:25.4273 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: b66bff7c-c596-4d64-ba99-08df0d2b00ab X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=43083d15-7273-40c1-b7db-39efd9ccc17a;Ip=[216.228.117.160];Helo=[mail.nvidia.com] X-MS-Exchange-CrossTenant-AuthSource: SN1PEPF000252A2.namprd05.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: SA1PR12MB6970 On Mon, Sep 07, 2026 at 10:58:35AM +0100, Kiryl Shutsemau (Meta) wrote: > All three queues are sized from the maxima the hardware advertises in IDR1 > and allocated at probe, up to 4 MB each on a 4K-page kernel. The capture > kernel already disables two of them: arm_smmu_device_reset() drops > CR0_EVTQEN and CR0_PRIQEN. It still allocates both at full size. > > A kdump capture kernel runs from a small crashkernel reservation, and every > SMMUv3 instance pays that cost again, up to 12 MB apiece. It goes to queues > that either serve the handful of devices used to save the dump or are > switched off outright, and it is memory the dump itself needs. > > Default all three depths to one page worth of entries when > is_kdump_kernel(). The queues carry commands and fault records rather than > DMA data, so dump throughput is unaffected. A shallower command queue only > bounds how many commands may be in flight before a sync, which does not > matter for the few devices that save the dump. > > An explicit cmdq_max_entries still wins, so a capture kernel that wants a > deeper command queue can ask for one on the command line. > > Suggested-by: Kyle McMartin > Signed-off-by: Kiryl Shutsemau (Meta) > Assisted-by: Claude-Code:claude-opus-5 Reviewed-by: Nicolin Chen Tested-by: Nicolin Chen