Re: [PATCH v3 1/2] vfio/pci: Have all VFIO PCI drivers store the vfio_pci_core_device in drvdata

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, May 05, 2022 at 12:10:47PM -0600, Alex Williamson wrote:

> > +	/* Drivers must set the vfio_pci_core_device to their drvdata */
> > +	if (WARN_ON(vdev != dev_get_drvdata(&vdev->pdev->dev)))
> > +		return -EINVAL;
> > +
> 
> The ordering seems off, if we only validate in the core enable function
> then we can only guarantee drvdata is correct once the user has opened
> the device.  However, we start invoking power management controls,
> which Abhishek proposes moving to runtime pm, from the core register
> device function.  Therefore we've not validated drvdata for anything we
> might do in the background, not under the direction of the user.

It is just a guard to make it obvious to someone testing the driver
that something has gone wrong, ie in backporting or something. 

It is not intended to be protective against drivers that are actually
wrong and installed in the system.

I added this because I felt a driver could silently be wrong and never
hit a PM or AER callback during some basic testing to catch a crash or
whatever.

> I'd also rather see the variant driver fail to register with the core
> than to see a failure opening the device an arbitrary time later.

It still permits a driver to be wrong, eg all the drivers are like
this today:

	ret = vfio_pci_core_register_device(&hisi_acc_vdev->core_device);
	if (ret)
		goto out_free;
	dev_set_drvdata(&pdev->dev, &hisi_acc_vdev->core_device);

So a WARN_ON inside register_device will not catch the mistake, as
this is the common pattern it isn't as helpful.

Jason