From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Sun, 30 Jan 2022 09:41:40 -0500 From: "Michael S. Tsirkin" Subject: Re: [PATCH v2 1/4] Add virtio Admin Virtqueue Message-ID: <20220130093740-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> <20220130043917-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="iso-8859-1" Content-Disposition: inline Content-Transfer-Encoding: quoted-printable To: Max Gurtovoy Cc: Jason Wang , Cornelia Huck , virtio-comment@lists.oasis-open.org, Virtio-Dev , Parav Pandit , Shahaf Shuler , Oren Duer , Stefan Hajnoczi List-ID: On Sun, Jan 30, 2022 at 11:56:30AM +0200, Max Gurtovoy wrote: >=20 > On 1/30/2022 11:40 AM, Michael S. Tsirkin wrote: > > On Sun, Jan 30, 2022 at 11:13:38AM +0200, Max Gurtovoy wrote: > > > On 1/29/2022 5:53 AM, 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" wrot= e: > > > > > >=20 > > > > > > > On Fri, Jan 28, 2022 at 01:14:14PM +0100, Cornelia Huck wrote: > > > > > > > > On Mon, Jan 24 2022, Max Gurtovoy wr= ote: > > > > > > > > > +\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 vari= ous features, > > > > > > > > > +if possible, of another device within the same group (e.= g. PCI VFs of > > > > > > > > > +a parent PCI PF device are grouped together. These devic= es can be > > > > > > > > > +optionally managed by its parent PCI PF using its admin = virtqueue.). > > > > > > > > > + > > > > > > > > > +Use of Admin virtqueue is negotiated by the VIRTIO_F_ADM= IN_VQ > > > > > > > > > +feature bit. > > > > > > > > > + > > > > > > > > > +Admin virtqueue index may vary among different device ty= pes. > > > > > > > > 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 ad= min vq, it > > > > > > > > also needs to specify where it shows up when the featur= e is negotiated > > > > > > > >=20 > > > > > > > > Do we expect that eventually all device types will need to = support the > > > > > > > > admin vq (if some use case comes along that will require al= l devices to > > > > > > > > participate, for example?) > > > > > > > I suspect yes. And that's one of the reasons why I'd rather w= e had a > > > > > > > device independent way to locate the admin queue. There are l= ess > > > > > > > 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 negotiat= ed? > > > > > > Should be straightforward for the device types that have a fixe= d number > > > > > > of vqs, and doable for those that have a variable amount (two d= evice > > > > > > types are covered by this series anyway.) I think we need to pu= t it with > > > > > > the device types, as otherwise the numbering of virtqueues coul= d 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 fiel= d 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 wa= y, > > > > we can simply use 0 as queue index for admin virtqueue. > > > Or we can use index 0xFFFF for admin virtqueue for compatibility. > > I think I'd prefer a register with the #. For example we might want > > to limit the # of VQs in order to pass extra data with the kick write. >=20 > So you are suggesting adding a new cfg_type (#define > VIRTIO_PCI_CAP_ADMIN_CFG 10) ? >=20 > that will look something like: >=20 > struct virtio_pci_admin_cfg { >=20 > =A0=A0=A0 le32 queue_index; /* read only for the driver */ >=20 > =A0=A0=A0 le16 queue_size; /* read-write */ > =A0=A0=A0 le16 queue_msix_vector; /* read-write */ > =A0=A0=A0 le16 queue_enable; /* read-write */ > =A0=A0=A0 le16 queue_notify_off; /* read-only for driver */ > =A0=A0=A0 le64 queue_desc; /* read-write */ > =A0=A0=A0 le64 queue_driver; /* read-write */ > =A0=A0=A0 le64 queue_device; /* read-write */ > =A0=A0=A0 le16 queue_notify_data; /* read-only for driver */ > =A0=A0=A0 le16 queue_reset; /* read-write */ >=20 > }; >=20 > instead of re-using the struct virtio_pci_common_cfg ? >=20 >=20 > or do you prefer extending the struct virtio_pci_common_cfg with "le16 > admin_queue_index; /* read only for the driver */ ? The later. Other transports will need this too. Cornelia has another idea which is that instead of adding just the admin queue register to all transports, we instead add a misc_config structure to all transports. Working basically like device specific config, but being device independent. For now it will only have a single le16 admin_queue_index register. For PCI we would thus add it with VIRTIO_PCI_CAP_MISC_CFG The point here is that we are making it easier to add more fields just like admin queue index in the future. > >=20 > >=20 > > > > Thanks > > > >=20 > > > > > 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 le= ss > > > > > memory space. > > > > >=20 > > > > > -- > > > > > MST > > > > >=20