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

Yishai Hadas
2023-10-18T09:02:23Z

On 17/10/2023 23:24, Alex Williamson wrote:
> 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.

Users here mainly refer to cloud operators.

We may assume, I believe, that they will be fine with seeing a
transitional device in the guest as they would like to get the legacy IO
support for their system.

However, we can still consider supplying a configuration knob in the
device layer (e.g. in the DPU side) to let a cloud operator turning off
the legacy capability.

In that case upon probe() of the vfio-virtio driver, we'll just pick-up
the default vfio-pci 'ops' and in the guest we may have the same device
ID as of in the host.

With that approach we may not require a HOST side control (i.e. sysfs,
etc.), but stay with a s device control based on its user manual.

At the end, we don't expect any functional issue nor any compatible
problem with the new driver, both modern and legacy drivers can work in
the guest.

Can that work for you ?

>
> 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.

Referring to the sysfs option,

Do you expect the sysfs data to effect the libvirt decision ? may that
require changes in libvirt ?

In addition,
May that be too late as the sysfs entry will be created upon driver
binding by libvirt or that we have in mind some other option to control
with that ?

Jason,
Can you please comment here as well ?

> 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?

"acc" is just a short-cut to "access", see also here[1] a similar usage.

[1]
https://elixir.bootlin.com/linux/v6.6-rc6/source/drivers/vfio/pci/hisilicon/hisi_acc_vfio_pci.c#L1380

>
>> +
>> + 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,

Right

How about dropping the word "family" and say instead ".. for VIRTIO
devices" as we have in the Kconfig in that patch [1] ?

[1] "This provides support for exposing VIRTIO VF devices .."

Yishai

> Alex
>