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

[Cornelia Huck <cohuck@redhat.com>]

On Fri, Jan 28 2022, "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. 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.

I only meant to make it the last one _now_ :)

But admin-vq-gets-its-own-page is a good point.

Maybe pci gets a new entry in common_cfg, mmio gets a new register, and
ccw gets a new command? (Although I'd prefer to be conservative with new
commands for ccw, maybe it's time to introduce a "get misc config" type
command that can be reused for other things. There's a generic ccw for
that, but adding new stuff to it would require an s390 architecture
change AFAIK, so I had decided not to go down that path for virtio.)