Re: [PATCH v2 1/4] Add virtio Admin Virtqueue

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

On Fri, Jan 28, 2022 at 04:49:34PM +0100, Cornelia Huck wrote:
> On Fri, Jan 28 2022, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Fri, Jan 28, 2022 at 01:14:14PM +0100, Cornelia Huck wrote:
> >> On Mon, Jan 24 2022, Max Gurtovoy <mgurtovoy@nvidia.com> wrote:
> >> > +\section{Admin Virtqueues}\label{sec:Basic Facilities of a Virtio Device / Admin Virtqueues}
> >> > +
> >> > +Admin virtqueue is used to send administrative commands to manipulate
> >> > +various features of the device and/or to manipulate various features,
> >> > +if possible, of another device within the same group (e.g. PCI VFs of
> >> > +a parent PCI PF device are grouped together. These devices can be
> >> > +optionally managed by its parent PCI PF using its admin virtqueue.).
> >> > +
> >> > +Use of Admin virtqueue is negotiated by the VIRTIO_F_ADMIN_VQ
> >> > +feature bit.
> >> > +
> >> > +Admin virtqueue index may vary among different device types.
> >> 
> >> So, my understanding is:
> >> - any device type may or may not support the admin vq
> >> - if the device type wants to be able to accommodate the admin vq, it
> >>   also needs to specify where it shows up when the feature is negotiated
> >> 
> >> Do we expect that eventually all device types will need to support the
> >> admin vq (if some use case comes along that will require all devices to
> >> participate, for example?)
> >
> > I suspect yes. And that's one of the reasons why I'd rather we had a
> > device independent way to locate the admin queue. There are less
> > transports than device types.
> 
> So, do we want to bite the bullet now and simply say that every device
> type has the admin vq as the last vq if the feature is negotiated?
> Should be straightforward for the device types that have a fixed number
> of vqs, and doable for those that have a variable amount (two device
> types are covered by this series anyway.) I think we need to put it with
> the device types, as otherwise the numbering of virtqueues could change
> in unpredictable ways with the admin vq off/on.

Well that only works once. The next thing we'll need we won't be able to
make the last one ;) So I am inclined to add a per-transport field that
gives the admin queue number. Another advantage to this approach is that
we can make sure admin queue gets a page by itself (which can be good if
we want to allow access to regular vqs but not to the admin queue to
guest) even if regular vqs share a page. Will help devices use less
memory space.

-- 
MST