RE: [virtio-comment] Re: [PATCH 09/11] transport-pci: Describe PCI MMR dev config registers

[Parav Pandit <parav@nvidia.com>]

> From: Jason Wang <jasowang@redhat.com>
> Sent: Thursday, April 13, 2023 10:37 PM

> I'm not sure I get this. With VIRTIO_NET_F_LEGACY_HEADER, we don't need
> mediation for datapath. For the control path, mediation is a must for legacy and
> it's very easy to keep it work for modern, what's wrong with that?

There is virtio PCI VF.
The user who attaches this VF to the VM, it doesnât know if its guest is to run legacy driver or 1.x
Hence hypervisor doesnât want to run special stack when guest may (likely) be 1.x and device also supports 1.x.

Therefore, MMIO BAR 0 emulation is better solution in this use case without any mediation.
The current specification claim that transitional device "works", hence it is fine to extend BAR0 to be of MMIO type instead of IO type.

In some other cases control path or additional other mediation can be done.