From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Sun, 30 Jan 2022 04:39:10 -0500 From: "Michael S. Tsirkin" Subject: Re: [PATCH v2 1/4] Add virtio Admin Virtqueue Message-ID: <20220130043801-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> <20220128105012-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=us-ascii Content-Disposition: inline To: Jason Wang Cc: Cornelia Huck , Max Gurtovoy , virtio-comment@lists.oasis-open.org, Virtio-Dev , Parav Pandit , Shahaf Shuler , Oren Duer , Stefan Hajnoczi List-ID: On Sat, Jan 29, 2022 at 11:53:06AM +0800, Jason Wang wrote: > On Fri, Jan 28, 2022 at 11:52 PM Michael S. Tsirkin wrote: > > > > 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. > > 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 Problem is 0 and 1 use the same page for notification unless there's a page per vq (which is possible but expensive in memory space usage term). > > 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 > >