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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Oct 10, 2023 at 06:43:32PM +0300, Yishai Hadas wrote:
> 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.

On pci_dev of vf, yes? So again just move this into each command,
that's all. I.e. pass pci_dev to each.

> 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.
> 
> Does it work for you ?
> 
> Yishai

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