Re: [PATCH V1 vfio 9/9] vfio/virtio: Introduce a vfio driver over virtio devices

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 17 Oct 2023 16:42:17 +0300
Yishai Hadas <yishaih@nvidia.com> wrote:
> +static int virtiovf_pci_probe(struct pci_dev *pdev,
> +			      const struct pci_device_id *id)
> +{
> +	const struct vfio_device_ops *ops = &virtiovf_acc_vfio_pci_ops;
> +	struct virtiovf_pci_core_device *virtvdev;
> +	int ret;
> +
> +	if (pdev->is_virtfn && virtiovf_support_legacy_access(pdev) &&
> +	    !virtiovf_bar0_exists(pdev) && pdev->msix_cap)
> +		ops = &virtiovf_acc_vfio_pci_tran_ops;


This is still an issue for me, it's a very narrow use case where we
have a modern device and want to enable legacy support.  Implementing an
IO BAR and mangling the device ID seems like it should be an opt-in,
not standard behavior for any compatible device.  Users should
generally expect that the device they see in the host is the device
they see in the guest.  They might even rely on that principle.

We can't use the argument that users wanting the default device should
use vfio-pci rather than virtio-vfio-pci because we've already defined
the algorithm by which libvirt should choose a variant driver for a
device.  libvirt will choose this driver for all virtio-net devices.

This driver effectively has the option to expose two different profiles
for the device, native or transitional.  We've discussed profile
support for variant drivers previously as an equivalent functionality
to mdev types, but the only use case for this currently is out-of-tree.
I think this might be the opportunity to define how device profiles are
exposed and selected in a variant driver.

Jason had previously suggested a devlink interface for this, but I
understand that path had been shot down by devlink developers.  Another
obvious option is sysfs, where we might imagine an optional "profiles"
directory, perhaps under vfio-dev.  Attributes of "available" and
"current" could allow discovery and selection of a profile similar to
mdev types.

Is this where we should head with this or are there other options to
confine this transitional behavior?

BTW, what is "acc" in virtiovf_acc_vfio_pci_ops?

> +
> +	virtvdev = vfio_alloc_device(virtiovf_pci_core_device, core_device.vdev,
> +				     &pdev->dev, ops);
> +	if (IS_ERR(virtvdev))
> +		return PTR_ERR(virtvdev);
> +
> +	dev_set_drvdata(&pdev->dev, &virtvdev->core_device);
> +	ret = vfio_pci_core_register_device(&virtvdev->core_device);
> +	if (ret)
> +		goto out;
> +	return 0;
> +out:
> +	vfio_put_device(&virtvdev->core_device.vdev);
> +	return ret;
> +}
> +
> +static void virtiovf_pci_remove(struct pci_dev *pdev)
> +{
> +	struct virtiovf_pci_core_device *virtvdev = dev_get_drvdata(&pdev->dev);
> +
> +	vfio_pci_core_unregister_device(&virtvdev->core_device);
> +	vfio_put_device(&virtvdev->core_device.vdev);
> +}
> +
> +static const struct pci_device_id virtiovf_pci_table[] = {
> +	/* Only virtio-net is supported/tested so far */
> +	{ PCI_DRIVER_OVERRIDE_DEVICE_VFIO(PCI_VENDOR_ID_REDHAT_QUMRANET, 0x1041) },
> +	{}
> +};
> +
> +MODULE_DEVICE_TABLE(pci, virtiovf_pci_table);
> +
> +static struct pci_driver virtiovf_pci_driver = {
> +	.name = KBUILD_MODNAME,
> +	.id_table = virtiovf_pci_table,
> +	.probe = virtiovf_pci_probe,
> +	.remove = virtiovf_pci_remove,
> +	.err_handler = &vfio_pci_core_err_handlers,
> +	.driver_managed_dma = true,
> +};
> +
> +module_pci_driver(virtiovf_pci_driver);
> +
> +MODULE_LICENSE("GPL");
> +MODULE_AUTHOR("Yishai Hadas <yishaih@nvidia.com>");
> +MODULE_DESCRIPTION(
> +	"VIRTIO VFIO PCI - User Level meta-driver for VIRTIO device family");

Not yet "family" per the device table.  Thanks,

Alex