Re: [virtio-comment] Re: [PATCH 0/2] Selective queue enabling

[Eugenio Perez Martin <eperezma@redhat.com>]

On Tue, Jun 13, 2023 at 2:29âPM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Tue, Jun 13, 2023 at 12:12:43PM +0200, Eugenio Perez Martin wrote:
> > On Tue, Jun 13, 2023 at 9:53âAM Michael S. Tsirkin <mst@redhat.com> wrote:
> > >
> > > On Tue, Jun 13, 2023 at 03:46:35AM -0400, Michael S. Tsirkin wrote:
> > > > On Mon, Jun 12, 2023 at 09:56:57AM +0200, Eugenio Perez Martin wrote:
> > > > > > > Another drawback of that is the long time a device can spend to
> > > > > > > reconfigure its memory maps.
> > > > > >
> > > > > > The map is already used for cvq though, I don't see why it would take
> > > > > > much more with this.
> > > > > >
> > > > >
> > > > > In the case of vDPA HW, CVQ maps may not need to reach the device's or
> > > > > platform IOMMU, so the modification of that mapping is way cheaper.
> > > >
> > > > Oh, I get it. It's setup time, but you are right.
> > > >
> > > > > > > taking that into account, does it make
> > > > > > > sense to send a new version with the bit 1 on enable?
> > > > > > >
> > > > > > > Thanks!
> > > > > >
> > > > > > Current hardware won't support the extra bit though.
> > >
> > >
> > > Thinking more about this, I think the following works:
> > >
> > > - enable all queues before DRIVER_OK
> > > - reset all data queues before DRIVER_OK
> > > - DRIVER_OK
> > > - CVQ commands
> > > - re-enable data queues
> > >
> > >
> > > No?
> > >
> >
> > If it is valid to write to queue_reset before DRIVER_OK, then sure. I
> > asked for that in the past at
> > https://lists.oasis-open.org/archives/virtio-dev/202305/msg00329.html
> > .
>
> Well then Jason said:
>  we should make svq work without RING_RESET
> but now apparently we went full circle and propose
> a feature bit after all?
>

My understanding is that RING_RESET covers more than just delaying the
enabling of net data vqs, like to change the address or properties of
vqs after DRIVER_OK. While the late enabling may be easy to implement
in current hw, vq property changes may be more challenging. There are
current HW that only support late enabling but not reconfigure the
queues. Ideally, just the subset of late enabling is needed.

Does it make sense to change the proposal to something in the line of:
RING_F_LATE_ENABLING: This feature flag is a subset of _F_RING_RESET.
If this feature is negotiated, all vqs start as if the vqs has been
reset before DRIVER_OK. This feature does not allow resetting a queue
once it is live.

Thanks!

>
> > >
> > >
> > > > --
> > > > MST
> > >
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