Re: [PATCH v2 0/2] transport-pci: Introduce legacy registers access using AQ

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

On Tue, May 16, 2023 at 11:43:09AM +0800, Jason Wang wrote:
> On Tue, May 16, 2023 at 11:37âAM Jason Wang <jasowang@redhat.com> wrote:
> >
> >
> > å 2023/5/16 02:01, Michael S. Tsirkin åé:
> > > On Mon, May 15, 2023 at 06:00:02PM +0000, Parav Pandit wrote:
> > >>> From: Michael S. Tsirkin <mst@redhat.com>
> > >>> Sent: Monday, May 15, 2023 1:56 PM
> > >>>
> > >>> On Mon, May 15, 2023 at 05:51:05PM +0000, Parav Pandit wrote:
> > >>>>> From: Michael S. Tsirkin <mst@redhat.com>
> > >>>>> Sent: Monday, May 15, 2023 1:45 PM
> > >>>>>
> > >>>>> On Mon, May 15, 2023 at 03:49:44PM +0000, Parav Pandit wrote:
> > >>>>>> All legacy interface via AQ.
> > >>>>>> All modern interface access via PCI or its own transport between
> > >>>>>> driver and
> > >>>>> device.
> > >>>>>
> > >>>>> I am wondering however about the hypervisor notifications.
> > >>>>> Generally these are the most problematic aspect here I feel.  For
> > >>>>> example, how does it interact with VIRTIO_F_NOTIF_CONFIG_DATA?  And
> > >>>>> generally, having both guest and host be allowed to access device's BAR
> > >>> seems problematic.
> > >>>>> Can we reserve a PF BAR region for these things or is that too expensive?
> > >>>> VIRTIO_F_NOTIF_CONFIG_DATA is not present for the legacy.
> > >>> it is not but it just might be required, or it won't be there.
> > >>>
> > >> How can it be required if it not part of it?
> > >> I likely didn't follow your question/comment, can you please explain.
> > > VIRTIO_F_NOTIF_CONFIG_DATA is there presumably for a reason.
> > >
> > >>>> For modern device, guest driver will access the device own BAR directly with
> > >>> its own config_data anyway.
> > >>>> Should we reserve a region in the PF BAR for SF/SIOV device?, should device
> > >>> report which BAR/area to use for the given SF/SIOV device?
> > >>>> May be yes, those are different discussions with tradeoff to consider during
> > >>> SIOV discussions. It is not related to VFs.
> > >>>
> > >>> For SIOV it's a given. But we can do this for SRIOV: reserve a PF region per VF.
> > >> Each VF has its own BAR area for driver notifications.
> > > But it is passed through to guest presumably.
> >
> >
> > Probably not in the case of VIRTIO_F_NOTIF_CONFIG_DATA. Did you see any
> > problem if we do mediation here except for some performance penalty?
> 
> Ok, rethink of this, when using legacy, VIRTIO_F_NOTIF_CONFIG_DATA
> should be assumed to be not negotiated. So I think I agree with Parav,
> it should not be a problem.
> 
> Or do you mean for the devices that have some mandated features?
> 
> Thanks

Exactly. VIRTIO_F_NOTIF_CONFIG_DATA if there is likely to be
required for device to work as opposed to being an optimization.

> >
> > Thanks
> >
> >
> > >