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Subject: Re: [virtio-dev] Re: [PATCH v2 1/4] Add virtio Admin Virtqueue
On Mon, Jan 31, 2022 at 04:47:29PM +0100, Halil Pasic wrote: > On Mon, 31 Jan 2022 10:16:43 +0100 > Cornelia Huck <cohuck@redhat.com> wrote: > > > On Sun, Jan 30 2022, "Michael S. Tsirkin" <mst@redhat.com> wrote: > > > > > On Sun, Jan 30, 2022 at 05:12:46PM +0200, Max Gurtovoy wrote: > > >> > > >> On 1/30/2022 4:41 PM, Michael S. Tsirkin wrote: > > >> > On Sun, Jan 30, 2022 at 11:56:30AM +0200, Max Gurtovoy wrote: > > >> > > 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 <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. > > >> > > > > > 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. > > >> > > > > 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. > > >> > > So you are suggesting adding a new cfg_type (#define > > >> > > VIRTIO_PCI_CAP_ADMIN_CFG 10) ? > > >> > > > > >> > > that will look something like: > > >> > > > > >> > > struct virtio_pci_admin_cfg { > > >> > > > > >> > > le32 queue_index; /* read only for the driver */ > > >> > > > > >> > > le16 queue_size; /* read-write */ > > >> > > le16 queue_msix_vector; /* read-write */ > > >> > > le16 queue_enable; /* read-write */ > > >> > > le16 queue_notify_off; /* read-only for driver */ > > >> > > le64 queue_desc; /* read-write */ > > >> > > le64 queue_driver; /* read-write */ > > >> > > le64 queue_device; /* read-write */ > > >> > > le16 queue_notify_data; /* read-only for driver */ > > >> > > le16 queue_reset; /* read-write */ > > >> > > > > >> > > }; > > >> > > > > >> > > instead of re-using the struct virtio_pci_common_cfg ? > > >> > > > > >> > > > > >> > > 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. > > >> > > >> OK. > > >> > > >> #define VIRTIO_PCI_CAP_MISC_CFG 10 > > >> > > >> and > > >> > > >> struct virtio_pci_misc_cfg { > > >> le16 admin_queue_index; /* read-only for driver */ > > >> }; > > >> > > >> Is agreed by all for V3 ? instead of the net and blk AQ index definitions. > > > > > > We need to add it to MMIO and CCW I guess too. > > > > That seems ok for pci. > > > > For ccw, I'd do something like > > > > #define CCW_CMD_READ_MISC_CONF 0x82 > > > > struct virtio_misc_conf { > > be16 admin_queue_index; > > }; > > > > bound to revision 3, which gets a payload data containing the length of > > this structure (for future expansions). > > > > Halil, do you think that would work? > > > I think so. But I would like to review the actual proposal :) > > Some of the questions I have in mind are: > * Are fields in this config protected with feature bits > * Do we ever need to write this config? Adding a writable field to pci > is easier than to ccw (no need to invent a new ccw) > * Looks like we will have an addressability mismatch with > ccw compared to pci and mmio: ccw can only read a prefix of this new > config, while mmio and pci have access to the individual fields > * Is this static stuff only? If not, do we need notifications for config > change like for the device specific config? I'd just reuse the generic config change if we ever add something non-static. Donnu about other questions. > > > > For mmio, I'd need to think a bit more. Any mmio experts around? > > > > > > --------------------------------------------------------------------- > > To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org > > For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org > >
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