RE: [PATCH v3 2/3] transport-pci: Introduce legacy registers access commands

[Parav Pandit <parav@nvidia.com>]

> From: Michael S. Tsirkin <mst@redhat.com>
> Sent: Sunday, June 4, 2023 9:22 AM

> > +The driver that may use the driver notifications region of the VF
> > +device returned in this result likely attain higher performance or
> > +the drier may use the VIRTIO_ADMIN_CMD_LREG_WRITE command.
> 
> Obtain I guess ... but how? There's no explanation.
>
Do you suggest to rewrite, above as below?

The driver that MAY use the driver notifications region of the VF likely obtain higher performance.
The driver may use VIRTIO_ADMIN_CMD_LCC_REG_WRITE command for doorbell notifications.

> > +\begin{note}
> > +The PCI VF device should not use PCI BAR 0 when it prefers to support
> > +legacy interface registers access using its group owner PF. This
> > +enables hypervisor software to operate with least complexities to
> > +compose a legacy interface I/O space BAR and passthrough other PCI
> > +BARs and PCI device capabilities to the guest virtual machine without any
> translation.
> > +\end{note}
> 
> Is this related to this last command somehow? what does it mean for PCI VF
> device to use a BAR? not use a BAR? Prefer what to what?

This is no different than v2, not sure why to discuss this in v3.
But ok.

No. It is not related to last command.
What does a PCI Device use BAR for?
As described in the spec, it uses the BAR in struct virtio_pci_cap for exposing various things.

So it means that PCI VF should use other than PCI BAR 0 for various Virtio Structure PCI Capabilities that it exposes.