From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id E8BA2C4332F for ; Tue, 18 Oct 2022 07:05:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229587AbiJRHF0 (ORCPT ); Tue, 18 Oct 2022 03:05:26 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48990 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230056AbiJRHFO (ORCPT ); Tue, 18 Oct 2022 03:05:14 -0400 Received: from szxga08-in.huawei.com (szxga08-in.huawei.com [45.249.212.255]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 620A65BC35 for ; Tue, 18 Oct 2022 00:05:09 -0700 (PDT) Received: from dggemv711-chm.china.huawei.com (unknown [172.30.72.57]) by szxga08-in.huawei.com (SkyGuard) with ESMTP id 4Ms4WF1z47z1P75y; Tue, 18 Oct 2022 15:00:25 +0800 (CST) Received: from kwepemm600005.china.huawei.com (7.193.23.191) by dggemv711-chm.china.huawei.com (10.1.198.66) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.31; Tue, 18 Oct 2022 15:05:05 +0800 Received: from [10.67.103.158] (10.67.103.158) by kwepemm600005.china.huawei.com (7.193.23.191) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.31; Tue, 18 Oct 2022 15:05:04 +0800 Subject: Re: [Linuxarm] [PATCH 1/2] hisi_acc_vfio_pci: Add debugfs to migration driver From: liulongfang To: Shameerali Kolothum Thodi , "John Garry" , "alex.williamson@redhat.com" , "jgg@nvidia.com" CC: "cohuck@redhat.com" , "linux-kernel@vger.kernel.org" , "linuxarm@openeuler.org" References: <20221014025757.39415-1-liulongfang@huawei.com> <20221014025757.39415-2-liulongfang@huawei.com> <42d12308-4777-47a7-a1ae-50dfada050cf@huawei.com> <7c041671-71bf-d558-d47a-70296fe5c30e@huawei.com> <43a109b8-7cde-7bc6-f05d-73d322cea3c2@huawei.com> Message-ID: <397c9149-6845-4167-ea2f-a6ab0fcce831@huawei.com> Date: Tue, 18 Oct 2022 15:05:04 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <43a109b8-7cde-7bc6-f05d-73d322cea3c2@huawei.com> Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit X-Originating-IP: [10.67.103.158] X-ClientProxiedBy: dggems703-chm.china.huawei.com (10.3.19.180) To kwepemm600005.china.huawei.com (7.193.23.191) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2022/10/18 14:55, liulongfang Wrote: > On 2022/10/17 21:57, Shameerali Kolothum Thodi Wrote: >> >> >>> -----Original Message----- >>> From: liulongfang >>> Sent: 17 October 2022 10:21 >>> To: John Garry ; alex.williamson@redhat.com; >>> jgg@nvidia.com; Shameerali Kolothum Thodi >>> >>> Cc: cohuck@redhat.com; linux-kernel@vger.kernel.org; >>> linuxarm@openeuler.org >>> Subject: Re: [Linuxarm] [PATCH 1/2] hisi_acc_vfio_pci: Add debugfs to >>> migration driver >>> >>> On 2022/10/14 17:20, John Garry wrote: >>>> On 14/10/2022 03:57, Longfang Liu wrote: >>>>> +static void hisi_acc_vf_debugfs_init(struct hisi_acc_vf_core_device >>> *hisi_acc_vdev) >>>>> +{ >>>>> +    struct pci_dev *vf_pdev = hisi_acc_vdev->vf_dev; >>>>> +    struct device *dev = &vf_pdev->dev; >>>>> +    int ret; >>>>> + >>>>> +    if (!atomic_read(&hisi_acc_root_ref)) >>>>> +        hisi_acc_debugfs_root = debugfs_create_dir("hisi_vfio_acc", >>> NULL); >>>>> +    atomic_inc(&hisi_acc_root_ref); >>>>> + >>>> >>>> This looks totally racy, such that I wonder why even bother using an atomic >>> for hisi_acc_root_ref. >>> >>> >>> When enabling VF, it is possible for multiple VMs to enable VF at the same >>> time. The atomic variable >>> is used to ensure that only one "hisi_vfio_acc" is created. When other VFs >>> are enabled, >>> it will not be created again, but will be used directly. >>> >>> Indeed, why is hisi_acc_debugfs_root not created in the driver module init? >>>> >>> Because the normal function of VF is to perform encryption and decryption >>> services, the live migration >>> function is an auxiliary function, which no need to be used in scenarios >>> where only encryption and >>> decryption services are performed. >>> >>> During module init, it can register ops(hisi_acc_vfio_pci_ops) that only >>> perform encryption and >>> decryption services, and can also register with live migration function >>> ops(hisi_acc_vfio_pci_migrn_ops), >>> and this debugfs only needs to register it when the the ops is >>> hisi_acc_vfio_pci_migrn_ops. >> >> Isn't the ops registration happens at probe()? Both ops are registered in probe(), but if you move hisi_acc_debugfs_root() to probe(), this debugfs will be created regardless of whether you need to perform live migration. In any case, I think you can move >> the hisi_acc_debugfs_root creation to module_init() as suggested above >> and avoid the race. >> > > mig_root_ref uses atomic to prevent race, and another function is for reference counting. > Even if this hisi_acc_debugfs_root() is moved to module_init(), this mig_root_ref is still > needed, because the driver needs to ensure that the debugfs file is created when the first > VF is enabled, destroy it when the last VF have unloaded. > > >> Thanks, >> Shameer >> > Thanks, > Longfang. > Thanks, Longfang.