RE: [virtio] RE: [virtio-comment] proposal: use admin command (and aq) of the device to query config space

[Parav Pandit <parav@nvidia.com>]

> From: Jason Wang <jasowang@redhat.com>
> Sent: Wednesday, August 2, 2023 3:02 PM
> 
> On Wed, Aug 2, 2023 at 5:07âPM Parav Pandit <parav@nvidia.com> wrote:
> >
> >
> > > From: Jason Wang <jasowang@redhat.com>
> > > Sent: Wednesday, August 2, 2023 2:23 PM
> > >
> > > On Tue, Aug 1, 2023 at 3:09âPM Parav Pandit <parav@nvidia.com> wrote:
> > > >
> > > > One line proposal:
> > > > Let's use new admin command and admin q for all device types to
> > > > query
> > > device config space for new fields. (always).
> > >
> > > Before we mandate anything to admin command, we need to first invent
> > > an admin command over MMIO interface otherwise it would always be an
> > > issue for the nesting.
> > >
> > Nesting can be independent requirement in itself.'
> 
> I don't understand here. If you tie new fields to the DMA interface, it basically
> means nesting won't get any new features unless:
> 
> 1) it's a PCI VF
> 2) SR-IOV emulation is done
> 3) admin virtqueue emulation is done
> 
> If you want differ nesting devices from others, it would be a nightmare to
> maintain.
>
New fields for sure are tied to the DMA interface.
And nesting can also get just like how a VQ works in nested mode.
 
> > >
> > > 1) device configuration space is transport independent, some
> > > transport already use DMA to access the device configuration space
> > You can say ccw instead of "some". :)
> 
> Kind of but the transport vq proposal goes in the same way.
>
We debated many times that the wording transport vq is wrong as it is _not_ going to transport driver notifications.
Anyway, there is nothing to discuss here. So focusing on main items below.
 
> >
> > > 2) device configuration space is not read only, we've already had
> > > several examples of using it as write
> > >
> > It is even worse to have writable.
> 
> Well, what I meant is that, it's not necessarily read only and not necessarily a
> register interface.
>
I took PCI as being most common interface and took net, blk as devices who are experiencing high growth on features and device specific config space.

This isnât really a normative part of the spec. The key takeway to have it, for common things it is read only and its register.
 
> >
> > > > It is growing rapidly.
> > > > Some devices may be even multi-functionality device in coming
> > > > future such as
> > > net + clock + rdma device.
> > > > For a PCI transport implementing such ever-growing capabilities,
> > > configuration is burdensome as plain registers.
> > >
> > > We've already fixed size VIRTIO_PCI_CAP_PCI_CFG. What's wrong with that?
> > >
> > The wrong part is: it is still and indirect and slow, sub-optimal register
> interface.
> 
> Do we really care about the performance here? 
When it comes to bulk data transfer, in range of few hundred bytes and looking for 5+ year period, than yes, reading using indirect register is slow.
And after all it is still indirect register which is does not have time variability part.
So it is not fulfilling the requirement at all.

> And if it is one of your major
> concerns, it's better to explain it along with the "ever-growing" concern.
>
I thought it is clear that it is still a register, even slower than current one and still have same issues without it.

> > VQ is decoupled from transport already.
> > So, there is no flexibility broken.
> > And yet you suggested transport dependent VIRTIO_PCI_CAP_PCI_CFG above
> > making it further wrong. :)
> >
> 
> The context here is that you want to mandate any new fields to be DMA.
> DMA is obviously transport specific. There are transports that don't use DMA at
> all (e.g the shared memory).
>

VQ is surprisingly does the DMA without being transport specific.
How net device mandate tx pkts via vq, how a console device mandate receive and transmit queue,
How a crypto device mandate a control vq,

What is proposed here is no different...

Shared memory is not for bulk data transfer in virtio spec.
We donât see "shared memory" as transport in " Virtio Transport Options" section.
 
> > > > 3. A device must be able to choose, starting from which field
> > > > driver must query such configuration via DMA interface. This field
> > > > offset must be
> > > greater than currently defined configuration field.
> 
> [...]
> 
> > > >
> > > > d. Any other option?
> > >
> > > Transport virtqueue on top of admin virtqueue will address this seamlessly.
> > >
> > :)
> >
> > Donât see why one would create few more objects on top of aq when aq or
> cvq itself can fulfil the need.
> > Can you please elaborate?
> 
> If cvq can work, there's no need for any other methods.
Cvq is not present for all the device, at the same time all devices are not experiencing high growth of config space either.
Hence the discussion here.
Michael raised the concern to use some generic scheme for all the device previously, hence the aq.

I think the practical approach is, whichever device has cvq, they should be able to use cvq for config+caps query+set.
And rest can burn AQ to transport the same command.

> If you care about admin
> virtqueue then device configuration space is not the only thing that can be
> "ever growing", common_cfg is another one. 
The idea is to do minimal bootstrap work from the common config space and switch to the queue.
So common config shouldnât be growing either other than minimal bootstrap functionality.
Hence, common config also to be available via dma command.

> Allowing them to be accessed via a
> virtqueue (thus DMA in PCI transport) is basically the idea of the transport
> virtqueue. Ling shan is rebasing transport virtqueue proposals on top of admin
> commands, we can see then.

Current proposal doesnt need yet another new transport vq with/without nesting.
We just need to make choice as _always_admin command or efficiently use cvq when available.