From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Fri, 28 Jan 2022 10:52:40 -0500 From: "Michael S. Tsirkin" Subject: Re: [PATCH v2 1/4] Add virtio Admin Virtqueue Message-ID: <20220128105012-mutt-send-email-mst@kernel.org> References: <20220124093918.34371-1-mgurtovoy@nvidia.com> <20220124093918.34371-2-mgurtovoy@nvidia.com> <87wnikys4p.fsf@redhat.com> <20220128074613-mutt-send-email-mst@kernel.org> <87tudnzwq9.fsf@redhat.com> MIME-Version: 1.0 In-Reply-To: <87tudnzwq9.fsf@redhat.com> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline To: Cornelia Huck Cc: Max Gurtovoy , virtio-comment@lists.oasis-open.org, virtio-dev@lists.oasis-open.org, jasowang@redhat.com, parav@nvidia.com, shahafs@nvidia.com, oren@nvidia.com, stefanha@redhat.com List-ID: On Fri, Jan 28, 2022 at 04:49:34PM +0100, Cornelia Huck wrote: > On Fri, Jan 28 2022, "Michael S. Tsirkin" wrote: > > > On Fri, Jan 28, 2022 at 01:14:14PM +0100, Cornelia Huck wrote: > >> On Mon, Jan 24 2022, Max Gurtovoy 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