RE: [virtio-comment] RE: [PATCH v3 2/2] content: Support enabling virtqueue after DRIVER_OK stage

[Parav Pandit <parav@nvidia.com>]

> From: Michael S. Tsirkin <mst@redhat.com>
> Sent: Thursday, October 26, 2023 11:10 AM

> 
> shorter: i.e. after FEATURES_OK is set and before DRIVER_OK is set.

Ok.

> If VIRTIO_F_RING_DYNAMIC is negotiated,
> the driver can choose to enable some virtqueues after DRIVER_OK is set.
Ok. will take this simpler version.

> 
> 
> >
> > > > > +enable every virtqueue it wants to use before setting the
> > > > > +DRIVER_OK status bit; the driver can choose to enable a
> > > > > +virtqueue even after the driver has set the DRIVER_OK status bit.
> 
> 
> 
> 
> >> The virtqueue enable
> > > > > +mechanism is
> > > > > transport specific.
> 
> 
> 
> 
> I still feel there is no real explanation in a transport agnostic manner what does
> it mean to "enable" virtqueue. Do you maybe just mean "configure virtqueue"?
> 
There is explanation of "re-enabling" a virtqueue in section " Virtqueue Re-enable".

There is no good example of "configured" in generic section either, other than cleanup and mmio section.

> 
> Can we just avoid talking about "enabling"?
> Is it true that you can basically change anything you want about the vq not just
> the enable bit? If so talking about enabling is just confusing I think.
Not really.
Enabling sequence of a virtqueue for pci and mmio transport is:

1. select a q
2. program the addresses
3. set the enable bit

However, as I think more, this one-way interface of driver instructing the device without any error code and without any response is just too limiting interface.
I will be dropping this series that misuses the config registers beyond its intended init time use.

Let me work on more mature interface that solves following challenges.
1. ability to create a virtqueue dynamically after driver_ok
2. ability to create virtqueue with one or multiple physical addresses
3. ability for the device to return error if it cannot create dynamic vq creation

The question is, do we need this generic facility beyond virtio-net?