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

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Date:   Mon, 17 Oct 2022 13:57:30 +0000
From:   Shameerali Kolothum Thodi <shameerali.kolothum.thodi@...wei.com>
To:     liulongfang <liulongfang@...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



> -----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.

Thanks,
Shameer