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

[Jason Wang <jasowang@redhat.com>]

On Tue, Apr 18, 2023 at 12:59âAM Parav Pandit <parav@nvidia.com> wrote:
>
>
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Sunday, April 16, 2023 4:44 PM
> >
> > On Sun, Apr 16, 2023 at 01:41:55PM +0000, Parav Pandit wrote:
> > > > From: virtio-comment@lists.oasis-open.org
> > > > <virtio-comment@lists.oasis- open.org> On Behalf Of Michael S.
> > > > Tsirkin
> > > > Sent: Friday, April 14, 2023 2:57 AM
> > >
> > > > Do you refer to the trick Jason proposed where BAR0 is memory but
> > > > otherwise matches legacy BAR0 exactly? Is this your preferred solution at
> > this point then?
> > >
> > > We look at it again.
> > > Above solution can work reliably only for a very small number of PF and that
> > too with very special hardware circuitry due to the reset flow.
> > >
> > > Therefore, for virtualization below interface is preferred.
> > > a. For transitional device legacy configuration register transport
> > > over AQ,
> >
> > I don't get what this has to do with transitional ...
> >
> Typically, in current wordings, transitional is the device that supports legacy interface.
> So, it doesn't have to be for the transitional.
>
> I just wanted to highlight that a PCI VF device with its parent PCI PF device can transport the legacy interface commands.
>
> > > Notification to utilize transitional device notification area of the BAR.
> >
> > The vq transport does something like this, no?
> >
> Notifications over a queuing interface unlikely can be a performant interface because one is configuration task and other is data path task.

Note that current transport virtqueue only allows the notification via
MMIO. It introduces a command to get the address of the notification
area.

Thanks

>
> > > b. Non legacy interface of transitional and non-transitional PCI device to
> > access direct PCI device without mediation.
> >
> > So VF can either be accessed through AQ of PF, or through direct mapping?
> Right. VF to access legacy registers using AQ of PF and continue non-legacy registers using direct mapping as done today.
>