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Received: from CH3PR12MB8659.namprd12.prod.outlook.com (2603:10b6:610:17c::13) by DS0PR12MB7898.namprd12.prod.outlook.com (2603:10b6:8:14c::20) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.8335.16; Mon, 13 Jan 2025 19:12:26 +0000 Received: from CH3PR12MB8659.namprd12.prod.outlook.com ([fe80::6eb6:7d37:7b4b:1732]) by CH3PR12MB8659.namprd12.prod.outlook.com ([fe80::6eb6:7d37:7b4b:1732%5]) with mapi id 15.20.8314.022; Mon, 13 Jan 2025 19:12:26 +0000 Date: Mon, 13 Jan 2025 15:12:25 -0400 From: Jason Gunthorpe To: Nicolin Chen Cc: kevin.tian@intel.com, corbet@lwn.net, will@kernel.org, joro@8bytes.org, suravee.suthikulpanit@amd.com, robin.murphy@arm.com, dwmw2@infradead.org, baolu.lu@linux.intel.com, shuah@kernel.org, linux-kernel@vger.kernel.org, iommu@lists.linux.dev, linux-arm-kernel@lists.infradead.org, linux-kselftest@vger.kernel.org, linux-doc@vger.kernel.org, eric.auger@redhat.com, jean-philippe@linaro.org, mdf@kernel.org, mshavit@google.com, shameerali.kolothum.thodi@huawei.com, smostafa@google.com, ddutile@redhat.com, yi.l.liu@intel.com, patches@lists.linux.dev Subject: Re: [PATCH v5 06/14] iommufd: Add IOMMUFD_OBJ_VEVENTQ and IOMMUFD_CMD_VEVENTQ_ALLOC Message-ID: <20250113191225.GS5556@nvidia.com> References: <86e78625c48d0c8fb815496dbc57708b98a46ec7.1736237481.git.nicolinc@nvidia.com> <20250110174842.GI396083@nvidia.com> <20250110194950.GI5556@nvidia.com> Content-Type: text/plain; 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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, Jan 10, 2025 at 01:58:21PM -0800, Nicolin Chen wrote: > Hmm, it seems that I haven't got your first narrative straight.. > > Would you mind elaborate "copy_to_user() can block while waiting > on a page fault"? When would this happen? copy_to_user() is a sleeping function that sleeps if the user memory is non-present. So userspace can cause copy_to_user to copy to anything, including memory that is non-present and will take along time to page fault in. Eg perhaps by abusing userfaultfd. We should not allow userspace to globally DOS the iommu driver this way. So do not hold locks that are also held by the HW event path across copy_to_user(). Jason