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

[Jason Wang <jasowang@redhat.com>]

On Fri, Jan 28, 2022 at 11:52 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> 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.

Technically, there's no need to use the same namespace for admin
virtqueue if it has a dedicated notification area. If we go this way,
we can simply use 0 as queue index for admin virtqueue.

Thanks

> 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
>