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

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

On Fri, Jun 16, 2023 at 4:40âPM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Thu, Jun 15, 2023 at 10:35:09AM +0200, Eugenio Perez Martin wrote:
> > 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!
>
> Maybe. No idea - I am not a hardware designer. Can some hardware
> vendors chime in and confirm?
> It seems like a very very narrow "make my hack work" use-case.
> And I'm not even sure how well it works.
> For example what if cvq command enables rss? If the queues in
> question are not enabled packets will be dropped.
> So - fix some setups, break others?
>

I don't think it breaks, they are already dropped before setting
DRIVER_OK anyway. If we restore the configuration before DRIVER_OK in
the future, the device is supposed to drop packets meanwhile anyway,
isn't it?

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