From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Fri, 28 Jan 2022 11:16:29 -0500 From: "Michael S. Tsirkin" Subject: Re: [PATCH v2 1/4] Add virtio Admin Virtqueue Message-ID: <20220128111514-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> <87r18rzvki.fsf@redhat.com> MIME-Version: 1.0 In-Reply-To: <87r18rzvki.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 05:14:37PM +0100, Cornelia Huck wrote: > On Fri, Jan 28 2022, "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. 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? That's fine. > (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.) You mean a generic "get misc config" for all transports? That's fine by me too. I suspect nvidia guys will need help building something like this though ... -- MST