Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753271AbcKBMYz (ORCPT ); Wed, 2 Nov 2016 08:24:55 -0400 Received: from mga06.intel.com ([134.134.136.31]:15503 "EHLO mga06.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751494AbcKBMYx (ORCPT ); Wed, 2 Nov 2016 08:24:53 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.31,583,1473145200"; d="scan'208";a="896829897" Message-ID: <5819DA45.9000200@intel.com> Date: Wed, 02 Nov 2016 20:21:25 +0800 From: Jike Song User-Agent: Mozilla/5.0 (X11; Linux i686 on x86_64; rv:17.0) Gecko/20130801 Thunderbird/17.0.8 MIME-Version: 1.0 To: Alexey Kardashevskiy CC: Kirti Wankhede , alex.williamson@redhat.com, pbonzini@redhat.com, kraxel@redhat.com, cjia@nvidia.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, kevin.tian@intel.com, qemu-devel@nongnu.org, bjsdjshi@linux.vnet.ibm.com Subject: Re: [Qemu-devel] [PATCH v9 04/12] vfio iommu: Add support for mediated devices References: <1476739332-4911-1-git-send-email-kwankhede@nvidia.com> <1476739332-4911-5-git-send-email-kwankhede@nvidia.com> <62ade373-6edc-c7f3-c205-200cf4fd211f@nvidia.com> <45b517de-3766-e96b-fec0-2b77da4dcf8d@nvidia.com> <695ca09f-b332-d33a-22fb-073f03dfaebf@ozlabs.ru> In-Reply-To: <695ca09f-b332-d33a-22fb-073f03dfaebf@ozlabs.ru> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3106 Lines: 74 On 11/02/2016 12:09 PM, Alexey Kardashevskiy wrote: > On 02/11/16 14:29, Kirti Wankhede wrote: >> >> >> On 11/2/2016 6:54 AM, Alexey Kardashevskiy wrote: >>> On 02/11/16 01:01, Kirti Wankhede wrote: >>>> >>>> >>>> On 10/28/2016 7:48 AM, Alexey Kardashevskiy wrote: >>>>> On 27/10/16 23:31, Kirti Wankhede wrote: >>>>>> >>>>>> >>>>>> On 10/27/2016 12:50 PM, Alexey Kardashevskiy wrote: >>>>>>> On 18/10/16 08:22, Kirti Wankhede wrote: >>>>>>>> VFIO IOMMU drivers are designed for the devices which are IOMMU capable. >>>>>>>> Mediated device only uses IOMMU APIs, the underlying hardware can be >>>>>>>> managed by an IOMMU domain. >>>>>>>> >>>>>>>> Aim of this change is: >>>>>>>> - To use most of the code of TYPE1 IOMMU driver for mediated devices >>>>>>>> - To support direct assigned device and mediated device in single module >>>>>>>> >>>>>>>> Added two new callback functions to struct vfio_iommu_driver_ops. Backend >>>>>>>> IOMMU module that supports pining and unpinning pages for mdev devices >>>>>>>> should provide these functions. >>>>>>>> Added APIs for pining and unpining pages to VFIO module. These calls back >>>>>>>> into backend iommu module to actually pin and unpin pages. >>>>>>>> >>>>>>>> This change adds pin and unpin support for mediated device to TYPE1 IOMMU >>>>>>>> backend module. More details: >>>>>>>> - When iommu_group of mediated devices is attached, task structure is >>>>>>>> cached which is used later to pin pages and page accounting. >>>>>>> >>>>>>> >>>>>>> For SPAPR TCE IOMMU driver, I ended up caching mm_struct with >>>>>>> atomic_inc(&container->mm->mm_count) (patches are on the way) instead of >>>>>>> using @current or task as the process might be gone while VFIO container is >>>>>>> still alive and @mm might be needed to do proper cleanup; this might not be >>>>>>> an issue with this patchset now but still you seem to only use @mm from >>>>>>> task_struct. >>>>>>> >>>>>> >>>>>> Consider the example of QEMU process which creates VFIO container, QEMU >>>>>> in its teardown path would release the container. How could container be >>>>>> alive when process is gone? >>>>> >>>>> do_exit() in kernel/exit.c calls exit_mm() (which sets NULL to tsk->mm) >>>>> first, and then releases open files by calling exit_files(). So >>>>> container's release() does not have current->mm. >>>>> >>>> >>>> Incrementing usage count (get_task_struct()) while saving task structure >>>> and decementing it (put_task_struct()) from release() should work here. >>>> Updating the patch. >>> >>> I cannot see how the task->usage counter prevents do_exit() from performing >>> the exit, can you? >>> >> >> It will not prevent exit from do_exit(), but that will make sure that we >> don't have stale pointer of task structure. Then we can check whether >> the task is alive and get mm pointer in teardown path as below: > > > Or you could just reference and use @mm as KVM and others do. Or there is > anything else you need from @current than just @mm? > I agree. If @mm is the only thing needed, there is really no reason to refer to the @task :-) -- Thanks, Jike