Re: [virtio-dev] [PATCH v10 08/10] admin: command list discovery

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Mar 06, 2023 at 01:22:30PM +0100, Jiri Pirko wrote:
> Thu, Mar 02, 2023 at 02:05:22PM CET, mst@redhat.com wrote:
> >Add commands to find out which commands does each group support,
> >as well as enable their use by driver.
> >This will be especially useful once we have multiple group types.
> >
> >An alternative is per-type VQs. This is possible but will
> >require more per-transport work. Discovery through the vq
> >helps keep things contained.
> >
> >Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
> >Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> 
> [...]
> 
> 
> >+
> >+The driver issues the command VIRTIO_ADMIN_CMD_LIST_QUERY to
> >+query the list of commands valid for this group and before sending
> >+any commands for any member of a group.
> >+
> >+The driver then enables use of some of the opcodes by sending to
> >+the device the command VIRTIO_ADMIN_CMD_LIST_USE with a subset
> >+of the list returned by VIRTIO_ADMIN_CMD_LIST_QUERY that is
> >+both understood and used by the driver.
> 
> To my untrained ear, this sounds somewhat similar to the feature
> negotiantion mechanism. Why the fact that device/driver supports some
> command can't be covered by just another feature? Looks like unnecassary
> complexicity to negotiate supported commands like this.

Absolutely, it is similar. The issue is that a single device can
be an owner for multiple group types.
For example, I think an SRIOV PF itself can have subfunctions with SIOV
as well as virtual functions with SRIOV.
And the set of supported commands might differ.

We thus need a command that is kind of like features but per group type.

As a small bonus as we are building this from scratch, I added
something which was requested for a long time from features
which is blocking access to commands not allowed by features.
With standard virtio features we trust the driver not to send
commands before enabling a feature and this was considered
a maintainance/security problem.


-- 
MST