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

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Oct 05, 2023 at 01:49:54AM -0700, Christoph Hellwig wrote:

> But for all the augmented vfio use cases it doesn't, for them the
> augmented vfio functionality is an integral part of the core driver.
> That is true for nvme, virtio and I'd argue mlx5 as well.

I don't agree with this. I see the extra functionality as being an
integral part of the VF and VFIO. The PF driver is only providing a
proxied communication channel.

It is a limitation of PCI that the PF must act as a proxy.

> So we need to stop registering separate pci_drivers for this kind
> of functionality, and instead have an interface to the driver to
> switch to certain functionalities.

?? We must bind something to the VF's pci_driver, what do you imagine
that is?

> E.g. for this case there should be no new vfio-virtio device, but
> instead you should be able to switch the virtio device to an
> fake-legacy vfio mode.

Are you aruging about how we reach to vfio_register_XX() and what
directory the file lives?

I don't know what "fake-legacy" even means, VFIO is not legacy.

There is alot of code in VFIO and the VMM side to take a VF and turn
it into a vPCI function. You can't just trivially duplicate VFIO in a
dozen drivers without creating a giant mess.

Further, userspace wants consistent ways to operate this stuff. If we
need a dozen ways to activate VFIO for every kind of driver that is
not a positive direction.

Basically, I don't know what you are suggesting here. We talked about
this before, and my position is still the same. Continuing to have
/dev/vfio/XX be the kernel uAPI for the VMM to work with non-mediated
vPCI functions with live migration is the technically correct thing to
do.

Why wouldn't it be?

> Similarly for nvme.  We'll never accept a separate nvme-live migration
> vfio driver.  This functionality needs to be part of the nvme driver,
> probed there and fully controlled there.

We can debate where to put the files when the standard is done, but
the end of the day it needs to create /dev/vfio/XXX.

Jason