RE: [PATCH vfio 10/11] vfio/virtio: Expose admin commands over virtio device

[Parav Pandit via Virtualization]

> From: Yishai Hadas <yish...@nvidia.com>
> Sent: Tuesday, October 10, 2023 9:14 PM
> 
> On 10/10/2023 18:14, Michael S. Tsirkin wrote:
> > On Tue, Oct 10, 2023 at 06:09:44PM +0300, Yishai Hadas wrote:
> >> On 10/10/2023 17:54, Michael S. Tsirkin wrote:
> >>> On Tue, Oct 10, 2023 at 11:08:49AM -0300, Jason Gunthorpe wrote:
> >>>> On Tue, Oct 10, 2023 at 09:56:00AM -0400, Michael S. Tsirkin wrote:
> >>>>
> >>>>>> However - the Intel GPU VFIO driver is such a bad experiance I
> >>>>>> don't want to encourage people to make VFIO drivers, or code that
> >>>>>> is only used by VFIO drivers, that are not under drivers/vfio review.
> >>>>> So if Alex feels it makes sense to add some virtio functionality
> >>>>> to vfio and is happy to maintain or let you maintain the UAPI then
> >>>>> why would I say no? But we never expected devices to have two
> >>>>> drivers like this does, so just exposing device pointer and saying
> >>>>> "use regular virtio APIs for the rest" does not cut it, the new
> >>>>> APIs have to make sense so virtio drivers can develop normally
> >>>>> without fear of stepping on the toes of this admin driver.
> >>>> Please work with Yishai to get something that make sense to you. He
> >>>> can post a v2 with the accumulated comments addressed so far and
> >>>> then go over what the API between the drivers is.
> >>>>
> >>>> Thanks,
> >>>> Jason
> >>> /me shrugs. I pretty much posted suggestions already. Should not be hard.
> >>> Anything unclear - post on list.
> >>>
> >> Yes, this is the plan.
> >>
> >> We are working to address the comments that we got so far in both
> >> VFIO & VIRTIO, retest and send the next version.
> >>
> >> Re the API between the modules, It looks like we have the below
> >> alternatives.
> >>
> >> 1) Proceed with current approach where we exposed a generic API to
> >> execute any admin command, however, make it much more solid inside
> VIRTIO.
> >> 2) Expose extra APIs from VIRTIO for commands that we can consider
> >> future client usage of them as of LIST_QUERY/LIST_USE, however still
> >> have the generic execute admin command for others.
> >> 3) Expose API per command from VIRTIO and fully drop the generic
> >> execute admin command.
> >>
> >> Few notes:
> >> Option #1 looks the most generic one, it drops the need to expose
> >> multiple symbols / APIs per command and for now we have a single
> >> client for them (i.e. VFIO).
> >> Options #2 & #3, may still require to expose the
> >> virtio_pci_vf_get_pf_dev() API to let VFIO get the VIRTIO PF (struct
> >> virtio_device *) from its PCI device, each command will get it as its first
> argument.
> >>
> >> Michael,
> >> What do you suggest here ?
> >>
> >> Thanks,
> >> Yishai
> > I suggest 3 but call it on the VF. commands will switch to PF
> > internally as needed. For example, intel might be interested in
> > exposing admin commands through a memory BAR of VF itself.
> >
> The driver who owns the VF is VFIO, it's not a VIRTIO one.
> 
> The ability to get the VIRTIO PF is from the PCI device (i.e. struct pci_dev).
> 
> In addition,
> virtio_pci_vf_get_pf_dev() was implemented for now in virtio-pci as it worked
> on pci_dev.
> Assuming that we'll put each command inside virtio as the generic layer, we
> won't be able to call/use this API internally to get the PF as of cyclic
> dependencies between the modules, link will fail.
> 
> So in option #3 we may still need to get outside into VFIO the VIRTIO PF and
> give it as pointer to VIRTIO upon each command.
>
I think,
For #3 the virtio level API signature should be,

virtio_admin_legacy_xyz_cmd(struct virtio_device *dev, u64 group_member_id, 
....);

This maintains the right abstraction needed between vfio, generic virtio and 
virtio transport (pci) layer.
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