RE: [PATCH v2] vfio/pci: Remove vfio_device_get_from_dev()

[Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>]

> -----Original Message-----
> From: Jason Gunthorpe [mailto:jgg@nvidia.com]
> Sent: 03 May 2022 00:42
> To: Alex Williamson <alex.williamson@redhat.com>; Cornelia Huck
> <cohuck@redhat.com>; kvm@vger.kernel.org; liulongfang
> <liulongfang@huawei.com>; Shameerali Kolothum Thodi
> <shameerali.kolothum.thodi@huawei.com>; Yishai Hadas
> <yishaih@nvidia.com>
> Cc: Kevin Tian <kevin.tian@intel.com>
> Subject: [PATCH v2] vfio/pci: Remove vfio_device_get_from_dev()
> 
> The last user of this function is in PCI callbacks that want to convert
> their struct pci_dev to a vfio_device. Instead of searching use the
> vfio_device available trivially through the drvdata.
> 
> When a callback in the device_driver is called, the caller must hold the
> device_lock() on dev. The purpose of the device_lock is to prevent
> remove() from being called (see __device_release_driver), and allow the
> driver to safely interact with its drvdata without races.
> 
> The PCI core correctly follows this and holds the device_lock() when
> calling error_detected (see report_error_detected) and
> sriov_configure (see sriov_numvfs_store).
> 
> Further, since the drvdata holds a positive refcount on the vfio_device
> any access of the drvdata, under the driver_lock, from a driver callback

device_lock() ? (v1 discussion says it's a typo).

> needs no further protection or refcounting.
> 
> Thus the remark in the vfio_device_get_from_dev() comment does not apply
> here, VFIO PCI drivers all call vfio_unregister_group_dev() from their
> remove callbacks under the driver lock and cannot race with the remaining
> callers.

May be we can also mention the removal of vfio_group_get_from_dev() as well
in the commit.

> Reviewed-by: Kevin Tian <kevin.tian@intel.com>
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

Reviewed-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>

Thanks,
Shameer