linux-kernel - Re: [Linuxarm] [PATCH 1/2] hisi_acc_vfio_pci: Add debugfs to migration driver

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Date:   Tue, 18 Oct 2022 14:55:09 +0800
From:   liulongfang <liulongfang@...wei.com>
To:     Shameerali Kolothum Thodi <shameerali.kolothum.thodi@...wei.com>,
        "John Garry" <john.garry@...wei.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "jgg@...dia.com" <jgg@...dia.com>
CC:     "cohuck@...hat.com" <cohuck@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxarm@...neuler.org" <linuxarm@...neuler.org>
Subject: Re: [Linuxarm] [PATCH 1/2] hisi_acc_vfio_pci: Add debugfs to
 migration driver

On 2022/10/17 21:57, Shameerali Kolothum Thodi Wrote:
> 
> 
>> -----Original Message-----
>> From: liulongfang
>> Sent: 17 October 2022 10:21
>> To: John Garry <john.garry@...wei.com>; alex.williamson@...hat.com;
>> jgg@...dia.com; Shameerali Kolothum Thodi
>> <shameerali.kolothum.thodi@...wei.com>
>> Cc: cohuck@...hat.com; linux-kernel@...r.kernel.org;
>> linuxarm@...neuler.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()? 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.